A Concise Introduction to GeographicalInformation Systems and Science

M Morad and T Connolly

Kingston Centre for GISKingston University, UK

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Contents

Section 1 An Overview of Main Concepts

Section 2 GIS Terminology

Section 3 Major GIS Acronyms

Section 4 Recommended Further Reading

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Section 1

An Overview of Main Concepts

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GIS and geospatial data

A Geographical Information System is, technically speaking, a computerisedsystem for capturing, processing, enhancing, querying, analysing and visualisingspatially-referenced data. GIS draws on several related disciplines, and its cross-disciplinary nature is illustrated below.

GIS draws upon several related disciplines

What distinguishes GIS from other forms of information systems, such asdatabases and spreadsheets, is that GIS deals with spatial information. GIS has thecapability to relate layers of data for the same points in space, combining,analysing and, finally, mapping out the results. Spatial information uses location,within a coordinate system, as its reference base. The most commonrepresentation of spatial information is a map on which the location of any pointcould be given using latitude and longitude, or local grid references such as theBritish National Grid (BNG) or the New Zealand Map Grid (NZMG). See thediagram below for an example of a special engineering survey grid.

1 2 3 4 50

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4

3

2

1

0

y

x

a

b c

d

e

f g

Location of North Sea wellsWell id x y depth a 1 1 2000 b 2 2 3000 c 4 2 2900 d 3 3 5400 e 2 4 1750 f 1 5 2500 g 3 5 6000

Point data and related attribute information

Some applications of GIS are obvious, for example water supply companies useGIS as a spatial database of pipes and manholes; local governments can use GISto manage and update property boundaries, emergency operations andenvironmental resources. GIS may also be used to map out the provision ofservices, such as health care and primary education, taking into accountpopulation distribution and access to facilities. Increasingly, GIS is being used to

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assist businesses in identifying their potential markets and maintaining a spatialdatabase of their customers.

In broad terms, a Geographic Information System could be defined as a set ofprinciples and techniques employed to achieve one (or both) of the followingobjectives:

1. Finding suitable locations that have the relevant attributes. For example,finding a suitable location where an airport, a commercial forest or a retailoutlet can be established. This is usually achieved through the use ofBoolean (logical) operations, as illustrated schematically in the followingfigure.

=

=

=

=OR

(union)

AND

(intersection)

XOR

NOT

Boolean operations

2. Querying the geographical attributes of a specified location. For example,examining the roads in a particular locality, to check road density or findthe shortest path, and so on. This is often achieved by ‘clicking’ onto thelocation or object of interest, and examining the contents of the databasefor that location or object. The following figure shows examples of spatialquery functions in GIS.

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Some of the basic spatial query functions

GIS data are usually stored in more than one layer in order to overcome thetechnical problems caused by handling very large amounts of information at once.It is easier to work with complex spatial problems one layer at a time, to enablethe revision of the data without having to overhaul the entire information system.This is a fundamental aspect of GIS, and working with layers of geographicalinformation is known as data integration (see the figure on the next page).

For example, a comprehensive GIS of a UK county might consist of several layersof data:

• Layer 1 property boundaries and land use types (area maps)• Layer 2 road and railway networks (line maps)• Layer 3 terrain characteristics (spot-height or contour maps)• Layer 4 hospital and school locations (point maps)

road and railn

n

n

nschools

Pointdata

contours

Surfacedata

Linedata

Areadata

property boundaries

Examples of layers of area, line, surface and point data

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Spatial data may be represented in GIS in one (or both) of the two followingformats:

• Vector model, as geometric objects: points, lines, polygons• Raster model, as image files composed of grid-cells known as pixels

The following figure outlines this idea.

Real world Map

Vector representation

Raster representation

lake

Examples of vector and raster data models for the same object

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The importance of geographical data

The nature of any natural or economic activity with a spatial dimension cannot beproperly understood without reference to its spatial qualities. Spatial data havetwo essential parts: location and attributes.

A GIS requires locational references. Typical locational references are latitude andlongitude and national grid references such as the British National Grid. However,other geospatial codes can also be used to identify location, such as postcodes.Any locality would have a number of characteristics or properties associated withit. These attributes are usually kept in tables, containing such information asvegetation types, population, annual income, and so on.

GIS systems store and process data in two formats, vector and raster (or as thefiled and object views of reality, as some experts would prefer to say).

In the vector data model (see the next Figure), the world is represented as amosaic of points and interconnecting lines representing the location andboundaries of geographical entities. In vector data models, the data are representedas:

• lines (arcs)• polygons (traversed areas)• nodes (intersection points)• points (labelled nodes)

The raster (or grid-cell) data model has arisen from aerial and satellite imagingtechnology, which represents geographical objects as grid-cell structures known aspixels. See the following figure.

point

1,62,5

5,4

7,2

7,105,9

4,7

6,68,6

9,8

line

polygon

2,2

5 10

5

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Geographic entities represented in a vector formatEach data model has particular strengths and weaknesses, and the type of modelused is determined by the nature of the work being undertaken and the dataavailable.

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The main advantage of the vector data format is that it permits a neatrepresentation of points, boundaries, and linear features. This makes it particularlyuseful for analysis tasks that require accurate positioning, for example inengineering or cadastral boundary databases. It is also possible in a vector-basedGIS to define the spatial relationship (ie the connectivity and adjacency) betweencoverage features. This aspect of GIS is known as topology, and is important forsuch purposes as network analysis (for example to find an optimal path betweentwo nodes in a complex transport network).

By contrast, raster-based GIS defines the position of features in terms of (x, y) co-ordinates where topological associations are more difficult to identify. However,the main disadvantage of vector data is that the boundaries of the resulting mappolygons are discrete (enclosed by well-defined boundary lines), whereas inreality, assuming maps scales are not an issue, the map polygons may representcontinuous gradation or gradual change, as in soil maps.

Utilitydiagram

Rivernetwork

Cadastralboundaries

Soil

Thematic

Vegetation

Advantages of Vector and raster data

Grid Data Models use a raster matrix (a grid of image cells) to representinformation. Grid data are also known as raster data. The resolution (visualdefinition) of the raster depends on its pixel (cell) size. In other words, pixelresolution represents the size of the ground area covered by each pixel in theimage. The smaller the cell size, the higher the resolution. The raster data modelis, therefore, good for representing indistinct boundaries, such as thematicinformation on soil types, soil moisture, vegetation, ground temperatures, and soon. Furthermore, as reconnaissance satellites and aerial surveys use raster-basedscanners, the information (ie scanned images) can be directly incorporated intoGIS programmes capable of working with raster data. However, the higher thegrid resolution, the larger the data file is going to be. This is the main limitation ofraster based GIS.

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50 m pixel(grid 20 x 16 = 320 pixels)

25 m pixel(grid 40 x 32 = 1280 pixels)

Raster resolution

The question of which data model to use in GIS depends on the nature andobjective of the GIS project. Primarily the model type will depend on the nature ofthe data. Issues of concern are the volume of the data generated, ease of analysisand accuracy. Generally, vector data sets are economical in terms of file size (iedo not take up much memory space) and have a high level of positional precision,but are relatively difficult to use in mathematical computations. On the other hand,grid data sets tend to take up more file space and have a coarser resolution, but areeasier to work with mathematically (ie easier to process computationally).

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Geospatial data processing

There are several techniques for capturing, processing and analysing geospatialdata in GIS. Some of these are explained below.

Data capture. A GIS cannot analyse the information in a map, if the data are notalready in digital form which the computer can recognise. Maps can be digitised(hand-traced with computer mouse) to collect the coordinates of the map features.Electronic scanning devices can also be used to convert map lines and points todigital information.

PC or Workstation

Digitising tabletconnected via serial port

A typical digitiser

Information retrieval. With a GIS we can point at a location, object, or area onthe screen and retrieve recorded information about it from the DatabaseManagement System (DBMS) which holds the information abut the map’sfeatures.

Attribute data (DBMS)Record Area Value OwnerNumber100 109 3900 Scott101 117 2300 Digby102 101 2750 Taylor103 113 3800 Lloyd104 97 4700 Turner105 122 3550 Jones

Spatial data

105

100

101102

103104

Spatial and attribute

Spatial calculations. GIS makes spatial measurements easy to perform. Spatialmeasurements can be the distance between two points, the area of a polygon or thelength of a line or boundary. Calculations can be of a simple nature, such asmeasuring areas on one map, or more complex, such as measuring overlappingareas on two or more maps.

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Distance Area Perimeter

5 kilometres3 hectares4 miles

Measurements

Overlays (data integration). Overlaying is an important procedure in GISanalysis. Overlaying involves superimposing two or more map layers to produce anew map layer. Consider the following example: a new variety of wheat growswell in dry environments, with long growing seasons and alkaline soils. Given theavailability of data on the length of the growing season, moisture regime and soilalkalinity, where is the best place to plant the wheat? The answer can be found byoverlaying (superimposing) several maps showing (separately) water-budget,growing season length, soil pH, sodium content, and so on. The GIS analysis canestablish the locations where all the favourable soil conditions coincide, as theplaces where the wheat will grow best.

Layer 1 Layer 2 Layer 3 Result

+ + =

Data integration

Spatial interpolation. A GIS can be used to estimate the characteristics ofterrain or ecological conditions from a limited number of field measurements. Forexample, a rainfall map can be created from the interpolation of rainfall pointmeasurements taken at a number of different locations on a map.

Spatial interpolation

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Buffering and corridors. Buffers are used when the distance from a point orline is important. For example a classic use of GIS in earth sciences involvesestimating the damage caused by an earthquake, in terms of distance belts awayfrom the epicentre. If the influence of the earthquake is grouped into four classesthen the buffers around the epicentre might look like a map of semi-concentriczones of earthquake damage. Figure 9 shows an example of buffering, involving araster model and a vector model.

Raster representation

5 mile buffer around an airport

5 miles

Vector representation

5 mile buffer around a school

5 miles

Buffer or proximity measurements

Network analysis. GIS can handle complex network problems, such as roadnetwork analysis. A GIS can work out travel times and the shortest path from A toB. This facility can be built into more complicated models that might requireestimates of travel time, accessibility or impedance along a route system. Anexample is how a road network can be used to calculate the risks of accidents.There are, of course, other types of network analysis, involving stream networks.

For example, GIS could be used to model the flow of water through a riversystem, to plan a flood warning system. Real time data would be transmitted byflood warning monitors, such as rain gauges and river height alarms, which couldbe received and passed to a GIS system to assess the hazard. If the amount andintensity of rain exceeds a certain limit, determined by the GIS flood model for thearea, a flood protection plan could be put into operation with computer-generatedmaps demarcating the vulnerable areas at any point in time.

River network

+ =

Rain gauges Flood model

Network analysis

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Registration and edge-matching. Before they can be used in GIS, maps andimages should be geometrically rectified. The process begins by registering theraw maps and images to known (control) co-ordinates. The registration record isthen used or georectify the map or image in need of rectification. The latter step isalso known as rubber-sheeting. The rectification of raster data may involve "re-sampling" the original data, which means re-interpolating the grid cell values ofthe raster by filling in new ones in some places and eliminating others.

Corrected image

Simple (linear) and polynomial (non-linear) image correction

TopologyTopological relationships define the manner in which geographical objects arerelated spatially. In particular, topology defines two aspects of spatial relationship:� Adjacency/contiguity (the identification of adjacent polygon, by recording the

left-hand and right-hand polygons for each line).� Connectivity (the identification of linkage points for each line).

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5

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Topological construction of geographical areas

Map projection. Map projection is a fundamental aspect of mapmaking. A mapprojection is essentially a geometric model that transforms the locations of

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features on the Earth's surface to corresponding locations on a two-dimensionalmap. It is impossible to project every attribute of a spheroid perfectly onto a plane;but some projections can preserve shape, while others can preserve area, distance,or direction.

Different projections are used for different types of maps because each projectionis particularly appropriate for certain uses. For example, a projection thatpreserves the shape of the continents may distort their relative size. The diagrambelow demonstrates how three map projections can be constructed.

Normal Transverse Oblique

Map projections(Note: transverse and oblique projections are less common!)

Digital terrain analysis. GIS can build three dimensional models, where thetopography of a geographical location can be represented with an x, y, z datamodel known as Digital Terrain (or Elevation) Model (DTM/DEM). The x and ydimensions of a DTM represent the horizontal plane, and z represents spot heightsfor the respective (x,y) co-ordinates. As illustrated in the following Figure, thedata represented by a DEM array (grid cells) or a Triangulated Irregular Network(TIN). The latter consists of a networking of intersecting lines that make upirregular triangles.

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DTM array

9 8 7 8

10 7 5 9

9 4 15 10

7 5 10 10

8

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TIN triangles

An illustration of TIN and DEM representation

The data sets derived from a Digital Terrain Model can be used to analyseenvironmental phenomena or engineering projects that are influenced byelevation, aspect or slope, as in forestry and stream sedimentation studies. Thevisualisation (display) power of the computer allows the terrain data to bevisualised in three- dimensional form, often from any angle of view (this is knownas point-of-view analysis).

For example, using a DTM, civil engineers can assess how well a new structure(say, an open-cast mine) is screened by the surrounding terrain, and estimate howmuch additional screening might be needed to conceal the structure or reducenoise levels.

Digital terrain analysis

Global Positioning System (GPS) and GISThe GPS is a system of Earth-orbiting satellites, each providing precise time andposition information which enables portable GPS receiving devices to computepositions on the Earth. Signals must be received from at least three satellites inorder to establish the receiver’s position in latitude and longitude (or a nationalcoordinate system provided with the equipment). Measurement from a fourthsatellite is required to calculate altitude (height) position.

GPS configuration consists of three parts:

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1. Twenty four US Navy satellites orbiting the earth.2. Portable mobile receiver (aerial plus portable console).3. Base station receiver positioned at a known geographic position, in order

to perform differential correction of the raw satellite signals.

Global positioning system

Each satellite in orbit carries a computer and an accurate (atomic) clock. Thecomputer onboard calculates the satellite's orbit a week or so into the future, andpredicts ionospheric conditions over that period. By consulting its clock andcomputer-generated data, the satellite can tell where it is in the sky at any givenmicrosecond, and transmits its position and the current time continuously.

Remote Sensing and GIS

Remote sensing is the study of phenomena sensed from air or satellites. The use ofremote sensing provides scientists with an opportunity to interpret data as seenfrom space. This facility is particularly useful in surveying very large areas atrelatively little cost. Remote sensing is therefore widely used in such locations asthe arctic zones, deserts, forested areas, marshlands and mountainous regions,where small-scale ("magnified") maps of an adequate quality can be reproducedrelatively cheaply from satellite images. In general, remote sensing is suitable forstudying extensive areas with difficult or inhospitable terrain, or whereconventional surveys may be very costly.

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Remote sensing systemsSun (source of solar energy)

Earth (source ofthermal energy)

Absorption and transmission through Earth surface features

lake

Satellite and aerial remote sensing provide an almost infinite pool of information.They provide the experienced interpreter with knowledge of ground conditions,including geomorphology, vegetation, soils and mineral composition. A great dealof time and cost can be saved, through remote sensing, by reducing the necessaryamount of field work. Taken at regular intervals and over a long period of time,satellite remote sensing can also help scientists to identify the extent of change inground conditions over the seasons, such as seasonal soil moisture changes. Thisis particularly useful in mapping areas that witness frequent changes in land cover,like agricultural lands, tidal areas and marshlands.

Stereoscopy (three-dimensional view), which is important for such purposes astopographic and forest canopy studies, is possible through remotely sensedimages. Stereoscopy is made possible by the overlap of adjacent images(neighbouring images which cover common areas). This can be achieved throughthe overlap of orbit paths, or the tilting of the sensor to get a sideways view ofadjacent areas. Images taken by the French satellite SPOT, for example, areespecially popular with land resource scientists using remote sensing techniques,because they combine fairly high resolution (maximum 10 metres) with someoverlap.

Visible light (the colours sensed by our eyes) constitutes only a narrow band(wavelength range) within the vast electromagnetic spectrum. One distinctadvantage of remote sensing is that it offers the opportunity to use many invisiblebands outside the spectrum of visible light. There are many natural phenomenawhich could only be sensed from distance by using invisible bands in theelectromagnetic spectrum.

For example, satellites can identify geothermal reserves close to the Earth’ssurface by sensing the heat emanating from such spots. The procedure is similar,in principle, to the use of infrared (night-vision) security cameras to detectintruders in the dark. The results of sensing invisible phenomena can be developed

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for human use by assigning false colours to the picture (or false grey shades inblack-and-white prints). The next diagram shows the wavelength spectrum.

In land use applications, several bands of the electromagnetic spectrum are oftenused to sense the same place at once. This is useful because each band isespecially sensitive to a given phenomenon: showing vegetation boundaries in onecase, soil-moisture differences in another, and so on. Therefore, by usingmultiband images, more information can be obtained as a result of having morecolour combinations available for analysis. The following diagram demonstratesthe spectral reflectance for three different spectral responses.

Wavelength

Spectral reflectance curves for vegetation, soil and water

Sensors can be divided into two groups: passive and active. Passive sensingresembles the way our eyes operate, by detecting the radiation reflected by thesensed objects from the sun (or another source of illumination). Active sensing isachieved in the reverse manner: the energy is transmitted by the sensor itself,bounces back on impact with the object, giving a ‘backscatter’ echo which isregistered by the sensor. In remote sensing, microwave radar (LiDAR: LightDetection And Ranging) operates by active sensing.

Because of Rdar's ability to penetrate clouds, water, snow and (thin) vegetation, aradar image offers the possibility of obtaining a view of the examined phenomenathrough such obstacles. The technique is especially suitable for surveying areaswith frequent cloud cover or under grass.

Radar is also very useful because it accentuates surface roughness on remotesensing images. This occurs because relative height of sensed objects increaseswhen the wavelength of the sensing radiation becomes longer. Relative height alsoincreases when the angle of incidence (the degree of camera tilt from verticalposition) is increased. To the soil scientist, this facility can be very useful whenstudying such phenomena as stoniness, where relative height must be accentuatedin order to improve detection. An image of a stony soil surface, taken by a side-looking radar sensor, can show a distinctly mottled texture.

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Before any remote sensing image can be made available for mapping, it must becleared of errors caused by geometric displacement or atmospheric interference.The common sources of error in satellite imagery are:

• orbit and flight errors,• Earth curvature,• Earth rotation• surface roughness• atmospheric interference (cloud, fog and smog).

Normal Rolling Yawing Pitching Georectified

However, as shown above image correction is not generallyrecommended in soil and vegetation inventories, because it may givefalse data (as image corrections usually involve pixel-valuemanipulations).Images marred by significant interference from atmosphericphenomena (such as clouds) should be avoided. Atmospheric errorsare especially difficult to correct, often making guess-work inevitable.Similarly, images marred by 'noise' (communication errors, whichoccur during the transmission of images from satellites to thereceiving stations) should also be avoided.

Digital correction of image errors is made possible by computerisedimage processing techniques. Images are normally kept in digitisedform (recorded in binary code). Correcting the geometric errors ofindividual images is usually undertaken by giving the computer thecorrect coordinates of a number of control points (whose position isknown exactly) on each image. The image is then automaticallystretched (rescaled and rotated) to produce a geographically-correctedimage map.

Section 2

GIS Terminology

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Accuracy: is the degree to which information on a map or in a digitaldatabase matches control values. Accuracy is not the same asprecision. See Precision.

Address geocoding: See Geocoding.

Address Point™: A product created by the Ordnance Survey tolocate all known address locations e.g. houses, office blocks, factories.

Address matching: A mechanism for relating two files using(street or postal) address as the common reference.

Adjacency: Where two objects share a common boundary or side.(These properties do not change when geometry is altered throughtransformation).

Aggregated data: The grouping together of data to form largerunits of measurement. Aggregates are often ordered into a basehierarchy eg UK census enumeration districts which can be aggregatedto ward boundary areas (census blocks and tracts in the US). See alsoDisaggregated data

Algorithm: A procedure used to solve a mathematical orcomputational problem, or to address a data processing issue.Algorithms usually consist of a set of rules written in a computerlanguage.

American National Standards Institute (ANSI): ANSIstandards have been established for many elements of GIS andcomputer systems. See BSI.

American Standard Code for Information Interchange(ASCII): A widely used industry standard code for transferringalphanumeric codes between computers.

Analogue: A continuously varying electronic signal (contrast withbinary – see Binary codes and binary files) The term is also used todescribe traditional paper mapping products and aerial photographs.

Application programme interface (API): Computer softwaredesigned to access services from programmes across a network.

Application: In computing terminology, application is the technicalname for a computer programme.

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Arc: A line connecting a set of points that can form one side of apolygon. In ARC/INFO, the vertices at either endpoint of an arc arecalled nodes. In a topological GIS system (see Topology), arcs arelinked to nodes (arc-node topology) and to polygons (polygon-arctopology).

Area: Any object or field that has length and breadth. A fundamentalunit of geographical information, defined by a continuous, closedboundary. Also known as polygon. Examples include fields, counties,lakes, local authority boundaries, school districts and censusenumeration districts.

Array: Data values that are held in the form of a grid. (Arrays canalso be one-dimensional (row or column). See also Grid.

Aspect: The geographical direction toward which a slope faces,measured in degrees from north, in a clockwise direction.

Attribute data: Descriptive information about features or elementsof a database, listed as numbers or characters. For a database featurelike census districts, attributes might include demographic facts suchas population, average income, gender and age. Each row represents ageographical feature, and each column represents one attributevariable.

Automated cartography: The procedure of drawing maps with theaid of computer-based display devices such as plotters and visualdisplay units (monitors).

Automated Mapping / Facilities Management (AM/FM): Thesystems used by public and private utilities, local governments, andother organisations that manage very large capital equipment, stock, orfacilities. AM/FM most commonly describes databases designed tomanage networks of road, pipe, or cable assets.

Azimuth: The horizontal direction of a vector, measured clockwise indegrees of rotation from the positive y-axis. For example, degrees on acompass measured from north.

Band: A class of multispectral data for an interval within theelectromagnetic spectrum (such as light or infrared energy). Satelliteimagery such as Landsat and SPOT provide multispectral images ofthe Earth, containing several bands.

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Band separate: An image format that stores each band of data in aseparate file. The original data are usually collected by multispectralscanners, onboard satellites or aeroplanes.

Bandwidth: A measure of the volume of data that can flow through acommunications link (cable). Also known as throughput.

Base map: A map containing geographical features used forpositional (map grid) reference. Property boundaries are commonlyused as base maps because they are accurately referenced. Base Mapsprovide the background on which other data layers are overlaid andanalysed.

Basic Land and Property Unit (BLPU): In the UK, a referencenumber used in the UK address gazetteer system known as BS7666.See Gazetteer and UPRN.

Baud rate: The speed of data transmission between computer andother devices, measured in bits per second.

Benchmark Tests: Procedures for comparing the performance ofcompeting hardware and software. Specific benchmark tests can bedeveloped by GIS managers to test new equipment or software copeunder conditions close to those which will be encountered in day-to-day use. See Sensitivity analysis.

Binary code and binary files: Digital information and commandsstored and used by hardware and software, as sets of on-off (1-0)signals. Most systems of binary encoding in GIS are specific toparticular hardware and software vendors. Binary data are usually themost compact means of storing information. However, binary files arenot easy to transfer between computer systems which use differentbinary configurations. Binary files are therefore usually translated intoASCII (text) form for transfer between computers. See AmericanStandard Code for Information Interchange (ASCII).

Binary Large Object (BLOB): The data type of a column in anRDBMS (Relational Database Management System) which can storelarge images or textual data.

Bit: The smallest unit of information that a computer can store andprocess digitally. A bit has two possible values, 1 or 0, whichcorrespond to yes and no (or on and off). When several bits arearranged according to an agreed number to the base 2, they make up abyte. See byte.

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Boolean operator: A keyword that specifies how to combinesimple logical expressions into complex expressions. Booleanoperators negate a predicate (NOT), specify a combination ofpredicates (AND), or specify a list of alternative predicates (OR). Forexample: slope > 100 AND vegetation = forest.

Breakline: A linear feature that defines the areal extent and controlsthe surface interpolation of a digital elevation model. The term is usedas a command in ARC/INFO’s TIN (Triangular Irregular Network)module. Terrain features containing shorelines are often clipped asbreakline features, otherwise sea-level surfaces could be erroneouslyinterpolated as hills or valleys. See Digital Terrain Model (DTM).

British Standards Institute (BSI): The UK equivalent of theAmerican National Standards Institute (ANSI).

Buffer: Enclosed polygon created around points, lines or areas at anequal distance in all directions. The results represent areas at setdistances from the original object. For example, the creation of bufferzones around a polluted industrial site may represent the varyingextent of pollution measured at specified distances from the source ofthe contamination. Buffers are therefore useful for proximity analysisand environmental impact assessment.

Bundled GIS: A reference to the way software and hardware aresold together or separately. Some GIS vendors offer a bundledpackage of hardware and software, at a discount negotiated with thesoftware developers and hardware manufacturers.

Byte: A computer-memory and data storage unit composed ofcontiguous bits, usually 8. File sizes are measured in bytes ormegabytes (one million bytes). Bytes consist of values of 0 to 255 (ie0, 1, 2…255) and a collection of bytes (often 4 or 8 bytes) representsreal numbers and integers larger than 255. A Megabyte is a unit ofcomputer storage made up of one million bytes.

Cadastre: A cadastral survey involves the mapping, tracing andrecording of private and public land resources. The term cadastre isFrench in origin, meaning a record of the ownership, extent andmarket value of a property, for tax and legal purposes.

Cartesian coordinate system: A two-dimensional, planarcoordinate system in which x and y represent distances from a point oforigin, and where each point on the plane is defined by an (x, y)coordinate. Locations in the coordinate system can be established

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using any unit of measurement such as metres or yards. Relativemeasures of distance, area, and direction are uniform throughout aCartesian coordinate plane.

Cartography: The geographical discipline concerned with mappreparation and communicating geographical information.

CD-ROM: Common abbreviation for Compact Disk-Read OnlyMemory. A CD-ROM disk can hold about 650 megabytes ofinformation, as text, maps, tables or images.

Cell: The basic element of spatial information in the raster (grid)description of spatial features. See Pixel.

Census: The official count of the population, which in the UK iscarried out on a 10 year basis by the Office for National Statistics.

Centroid: A measure of central tendency for a polygon (i.e. there aremany calculation techniques and many centroids). In the case of anirregularly shaped polygon, the centroid is derived mathematically andis roughly the equivalent of its centre of mass. Centroids are oftenused in GIS to index the polygon within which they are located (ie, thepolygon’s attributes are referenced to its centroid).

Computer Graphics Metafile (CGM): A graphic image exchangestandard.

Chain: A sequence of coordinates defining a line or boundary.

Character: An alphanumerical (alphabetical or numerical) value thatrepresents a single unit of data

Chorley Report: The committee of inquiry into the handling ofgeographic information, completed in 1987. Recognised as animportant milestone of the development of GIS in the UK.

Choropleth map: A map in which areas of different value aredemarcated by clearly defined boundaries. The values of theunderlying data (eg, soil moisture) are represented by different colouror shading intensities, and the map legend acts as a look-up key toexplain the values shown on the map.

Classification function: The ability to analyse informationaccording to a group of classes, often according to common attributevalues.

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Client/server architecture: A computer system consisting of a fileserver software that handles requests from multiple end users (clients)on the network. A file server is usually a high performance PersonalComputer (PC) with a large hard drive that is not used exclusively byany individual on the network. Instead, it serves all the other "client"PCs on the network by providing a common place to store data.

Clip: A polygon which defines the boundaries of features in a map, bycutting the lines off its edge. Clips are used to restrict the extent ofdata processing or querying done in GIS. See Breakline.

Column: The vertical dimension of a table, often used torepresenting the attributes of the geographical features listed in therows. Also know as field.

Command line interface: The software interface which allows theGIS operator to type in commands at a prompt. Command lineinstructions are faster to execute, but typing errors can cause delays.Most modern GIS are Windows-based, but also allow the user to workfrom a command line interface if desired.

Completeness: Datasets do not always cover an entire geographicalarea of interest. They may not, therefore, be complete. This alsorelates to the amount of information recorded within a dataset.

Computer Aided Design (CAD): CAD systems are used to createmaps and plans at architectural scales. They are closely related to GISsystems, as many GIS systems, such as Intergraph and AutoCAD,have their origin in CAD software.

Concurrency management: A database management procedurefor preserving a system’s security against accidental or deliberate dataloss, which supports simultaneous access by multiple users. This typeof access regime allows only one user at a time to change the contentof the database (ie, have write-access), while remaining users willhave read-only access. The next user wanting write-access to thedatabase will have to wait until the first person has completed theirtransaction. All database changes are logged, showing the time oftransaction and the name of the operator.

Conditional operator: An SQL (Structured Query Language)keyword specifying how to compare values. Conditional operators areused to query a database. Examples include = , >, LIKE andCONTAINING.

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Connectivity: A topological property relating to how geographicalfeatures relate to one another spatially. The topological identificationof connected lines are established by recording the ‘from’ and ‘to’nodes for each line. Lines that share a common node are said to betopologically linked. Connectivity is useful in network analysis.

Contiguity: The topological definition of adjacent polygons, byrecording the left and right polygons of each edge of the enclosedpolygon.

Continuous surface data: A surface for which each location has aspecified or derivable value, such as DEM's (Digital ElevationModels), as distinct from continuous attribute data.

Contour: A line on a surface model representing points of equalheight value.

Contouring: The interpolation of contour lines from height data.

Contour interval: The difference in surface values betweensuccessive contours.

Conversion: The process of transforming data derived from existingrecords and maps to a digital database, or from one digital form toanother.

Coordinate Geometry (COGO): A data input method similar tothe way land surveyors enter their survey data. Input begins at a point,moves along a given bearing for a set distance, and continues in thesame fashion until the geographical feature (such as a pond) iscompletely outlined. Unlike the error-prone process of tabletdigitisation, COGO data entry generally establishes more accuratelocations and boundaries.

Coordinate: A set of numbers that designate a location in a givengeographical reference system, such as (x, y) in a two-dimensionalcoordinate system or (x, y, z) in three-dimensional terrain models.

Coordinate system: A reference system used to measure horizontaland vertical distances on a planimetric (two-dimensional) map. Acommon coordinate system (such as the Ordnance Survey’s NationalGrid) is used to reference the (x, y) location of geographical objects.

Copyright: The exclusive right to make copies and to controloriginal work, granted by law for a specific number of years.

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Currency: The period of time when a dataset is valid. This relates tothe date when the data was originally collected and/or updated.

Dangle: A line with a loose end: a line with one end (or both) thatdoes not connect to any other feature in the map. A dangle occurswhen the end of the line undershoots or overshoots the map feature towhich it is meant to be attached.

Data Access Language (DAL): The standard adopted by Applecomputers to facilitate access to relational databases.

Data integrity: Generally speaking, the term data integrity is used torefer to the internal consistency of the data kept in a database. Forexample, the presence of characters in a column that is meant to holdonly numbers indicates poor data integrity.

Data model: In GIS terms, a data model is a user-defined, abstractrepresentation of data describing the behaviour of the geographicalentities represented by the data. For example, the terrain of ageographical location can be represented with an (x, y, z) data modelknown as Digital Terrain Model (DTM), where x and y represent thehorizontal plane and z represents spot heights at the respective x, y co-ordinates.

Data set: A collection of logically related information arranged in aspecified way.

Data type: The quantitative or alphanumeric characteristics ofvariables that define the type of data used. Examples includecharacter, integer and floating point (numbers with decimals, or realnumbers).

DataBase Management System (DBMS): Computer softwareused to enter, store, organise, retrieve and query information in adatabase. A DBMS with limited GIS capability is said to be equippedwith a spatial component (or a "spatial cartridge" in the case of themajor DBMS Oracle™).

Database: A collection of organised information, usually stored onmagnetic tape or disk. A GIS database includes data about the locationand the attributes of geographical features that have been coded aspoints, lines, polygons or grid cells (pixels).

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Data dictionary: A database in which information is stored aboutobjects (and their relationships) in a GIS.

Datum: The singular for data. Specifically, the term datum is oftenused in GIS circles to refer to geodetic datum. See Geodetic datum.

De facto standard: These are standards which are accepted due totheir common usage in a particular applications context. They have nostanding in law.

De jure standard: These are standards which have some basis inlaw either in some States in the US, or internationally.

Demography: The study of population characteristics. In a GIScontext this usually entails statistical analysis.

Disaggregated data: Data collected about individuals or objects.See also Aggregated data.

Dissolve operations: The process of merging two or more adjacentareas by removing common boundaries. See also Merge.

Digital Chart of the World (DCW): A widely available1:1,000,000 digital base map of the world, containing vector datadigitised by the US Defense Mapping Agency.

Digital Elevation Model (DEM): See Digital Terrain Model(DTM).

Digital Terrain Model (DTM): A database with (x, y, z)coordinates, used to represent landform digitally. Also known asDigital Elevation Model. See Data model.

Digital: See Binary code and binary files.

Digitisation: The process of converting analogue map data intodigital codes stored and processed by computers. Digitising commonlyinvolves tracing map features into a computer using a digitising tabletor mouse, or a scanner.

Directory: A partitioned set of data files, stored on computer disk.Operating systems use directories and subdirectories to organise data.The location of a directory is specified with a pathname, for exampleC:\LIS\PROPERTIES\. Under Windows-based systems, directoriesare visually represented with a folder icon.

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Discrete data: Geographical features containing well-definedrepresented by points, lines or polygons boundaries. Unlike raster(grid cell) GIS, vector GIS represent geographical features as discrete(sharply defined) features.

Dots Per Inch (DPI): Measure of the resolution of graphic displaysand printers, representing the number of pixels per inch.

Drape: A perspective visualisation of surface features superimposedon a Digital Elevation/Terrain Model. See Digital Terrain Model(DTM).

Drawing Exchange Format (DXF): File exchange formatdeveloped by Autodesk. DXF files are ASCII (text) records of theobjects in a drawing file. DXF is widely used for exchanging mapfiles.

Dynamic segmentation: In ARC/INFO, the term refers toanalysing events on network routes, based on ‘event tables’ for whichdistance and impedance attributes are listed. An ESRI (Arc/Info)specific term.

Edge Matching: Unmatched polygons often occur whenneighbouring map tiles are misaligned as they are laid edge-to-edge,producing a "sliver". These misalignment errors occur when featuresrun across the boundaries of their respective map sheets. Sliverpolygons are not always a consequence of this. The errors oftenoriginate during digitisation, generalisation of map features, or mapprojection. Warping (rubber-sheet correction) and the automaticelimination of (user-defined) tiny sliver polygons are two majormethods of mitigating edge-matching errors. See Rubber sheeting.

Element: Basic geographical unit of information, such as apoint/node, line/arc, area/polygon or pixel/cell. Also known as mapentity.

Ellipsoid: An elliptic form of geodetic model of the Earth. SeeGeodetic datum.

Embedded SQL: SQL statements that are embedded in aprogramming language (such as AML: ARC/INFO Macro Language).See Structured Query Language (SQL).

Encapsulated PostScript (EPS): See Postscript.

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Entity-relationship model (ER model): A method developed inthe 1990's to describe real world relationships in a logical form.Generally takes the form of a diagram and can be used toconceptualise elements of a proposed database.

Enumeration district: In the UK, the basic area or unit ofapproximately 150 households used by the Office for NationalStatistics for the planning and carrying out of population counts andsurveys.

Environment: In computing jargon, the system’s configurationwhich defines the various display, editing, and data manipulationprocesses handled by GIS.

Ethernet: A local-area network (LAN) protocol used for high-speedcommunication between linked up computers.

Event: An additional geographical feature occurring on or along alinear feature, such as a route.

Extended character set: Extended character sets supportadditional languages which require double-byte characters, such asArabic or Greek.

Field: In its most popular use in GIS, a field is a common synonymfor column in a database.

File Transfer Protocol (FTP): A common protocol for copying ormoving data across a computer network or the Internet.

File: A collection of related data (textual or graphical) in a computer.Files are the basic units managed by the computer's operating system.

Filter: A grid of weighted pixel values, used to remove superfluous(noise) features, or to generalise spatial patterns from a raster data set.

Format: The method in which data are organised and stored in acomputer, for transmission between computers or between a computerand a device. Most GIS have proprietary formats used to store andprocess geographical information.

Fuzzy boundaries: Indeterminate loosely-defined boundaries ofgeographical objects, such as soil groups.

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Gazetteer: A work of geographical reference that supplies placenames and location information. In the UK, an address standardknown as BS7666 has been adopted since 1984, based on two sets ofidentifiers known as BLPU (Basic Land and Property Unit) and UPRN(Unique Property and Reference Number). See BLPU and URPN.

Generalisation: In its basic form, the process simplifying a shape byreducing the number of vertices, according to a pre-specified tolerancelevel or algorithm.

Geocoding: The process of identifying or designating thecoordinates of a geographical object, given its address. Also known asaddress geocoding.

Geodesy: The study of the size and shape of the Earth, and thedetermination of exact longitude and latitude positions on it.

Geodetic Datum: A three dimensional (more specificallyellipsoidal) model used to represent the shape of the Earth in aspecified region. A geodetic datum is the basis for the geographicalcoordinate system adopted by a country. For example, the NationalGrid of the United Kingdom is based on a geodetic datum representingthe shape of the Earth in the UK. A national geodetic datum gives amore accurate representation of the shape of Earth at local level,because global models are quite generalised.

Geodetic framework: A spatial framework of points whosepositions have been precisely determined on the surface of the Earth.The UK’s National Grid is the British geodetic framework. Alsoknown as geodetic network. See Geodetic datum.

Geographical Resource Analysis Support System(GRASS): A public-domain raster GIS developed by the US ArmyCorp of Engineers Research Laboratory (USACERL).

Geomatics: A description of Geography and computing functionsand facilities, usually used in relation to land information systems andlocation-aware applications.

Georelational database: Geometry stored using a numericidentifier as an attribute in a database.

Georeferencing: Establishing the location of a given geographicalobject, according to an agreed system of map coordinates such as theNational Grid.

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GIS Output: This can take the form of maps, tables, reports or digitalinformation. (The first three can be instances of the last.)

Global Positioning System (GPS): A system of Earth-orbitingsatellites, each providing precise time and position information whichenables GPS receiving devices to compute positions on the Earth.According to the US-based system which is most widely in usenowadays, signals must be received from at least three satellites inorder to establish the receiver’s position in latitude and longitude, oraccording to a national coordinate system provided by the equipment.Measurements from a fourth satellite are required to calculate altitude(height) position.

Graphical User Interface (GUI): A monitor, menu-based methodof controlling how a user interacts with a computer. Instead of issuingcommands at a command-line prompt, the user performs the requiredtasks by using a mouse, or selecting a menu item. See Command lineinterface.

Graphics display terminal: A computer unit used to view andmanipulate graphic information.

Gravity model: An analytical technique used in geographicalresearch to analyse the geographical pattern of economic behaviour.The underlying assumption of the model is that the influence ofpopulation density (or any other quantifiable economic parameter) onone another is inversely proportional to the distance (or resistanceeffect) between them. The term gravitational was borrowed fromNewton’s theory of gravitation.

Grey scales: Levels of brightness used in displaying information onmonochrome display devices, or on non-colour printers.

Grid cell: A unit that represents a level of resolution, or scale, on anarray of equally sized (square) cells arranged in rows and columns.Each grid cell is referenced by its geographical (x, y) location. Alsoknown as pixel in raster GIS.

Grid Reference: A precise description of geographical position interms of the planar co-ordinate system. Often associated with map co-ordinates. Some countries such as the UK have developed their ownNational Grid system. See also National Grid and Planar co-ordinates.

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Grid: A regular set of lines used to depict the planar co-ordinatesystem. See also Co-ordinate and Array.

Hard-copy: A paper copy.

Hardware: The physical components of a computer system, such asthe computer and its attached devices: digitisers, plotters, printers, etc.

Heuristic: A computational method that uses trial and error toapproximate a solution, for problems that would otherwise beintractable.

Image analysis: The processing and interpretation of rasterinformation that are held in digital form (such as satellite images). Animage is usually stored as values which represent the intensity ofreflected light, or other values in the electromagnetic spectrum.

Image resampling: See Rectification.

Index coverage/layer: The polygon coverage that is used as anindex (reference layer) for the structure of other layers of geographicalfeatures overlaid onto the index layer.

Information: Data that has undergone some form of evaluation.Maps are information because they are cartographic interpretations ofsurvey data. However, data can also be mapped without addition ofinterpretation.

INFORMIX: A corporate (organisation-wide) relational DBMS.

Infrastructure: In its simplest form, a reference to the basic utilitystructures that support a local economy, such as roads, electricitypylons, water and drain pipes, etc. GIS are used to hold informationabout these structures, and map out their location.

INGRES: A corporate relational DBMS.

Input device: A hardware component for data entry, such as adigitiser, keyboard, scanner, mouse, disk drive, etc.

Institute of Electrical and Electronics Engineers (IEEE). Animportant US organisation, which has played a major role in settingstandards for many engineering and GIS related applications.

Integer: A whole number (a number without decimals).

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Interactive Graphics Design Software (IGDS): An IntergraphGIS file format.

Inter-Application Communication (IAC): The capability ofdifferent computer software systems to communicate with oneanother. With IAC, several computer programs can executecommands simultaneously, share data, and make requests of eachother. See Interoperability.

Interface: A hardware and software link that connects two computersystems, or a computer and its peripherals. Also used as a reference toa Graphic User Interface (GUI).

Internet: An international consortium of Wide Area Networks(WAN's) that operate using a standard set of communicationprotocols. The Internet grew out of a US Defense Advanced ResearchProjects Agency (DARPA) research project in the early 1970s. Privateand public networks have joined the Internet since then.

Internet GIS: Internet-based GIS is a growing area of softwaredevelopment and application. The technology enables Internet users tosee and query a GIS map, using a Web browser such as NetscapeNavigator or Internet Explorer. Major GIS developers now providespecial Internet Server software that allows a GIS (such as ARC/INFOor Intergraph) to be browsed over the Internet by remote users.

Interoperability: The ability of different GIS software and dataformat to work with one another, without having to standardise theformat of the data which any one system uses. Interoperability hasreceived greater momentum with the advent of Open Systems. In thepast, emphasis was placed mainly on standardising GIS data, butincreasing GIS software conforming to Open Systems standards cannow work with different data formats without having to translate themfirst into a standard format. See Open Systems GIS.

Interpolation: Estimating the value of a geographical object (eg apoint, contour or area) from measurements made at surroundingpoints. A basic example is the estimation of z (height) values of asurface model, using the known z values of surrounding points.

Intersection: The topological integration of two spatial data setswithin the area common to both data sets.

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Intranet: An web based networked facility operating mainly withinan organisation. It may be accessible in different geographicallocations but remains as an internal resource to a particularorganisation. Usually it is not connected to the Internet.

Isoline: A line on a surface connecting points of equal value.

Isopleth map: A map showing the distribution of data as linesconnecting points of equal value.

Item indexing: A means of accelerating logical queries by creatingan index for key terms in the database table. The benefits of thisapproach are similar to finding information quickly in a book byconsulting its index.

Item: A column of information in an attribute table.

Land Information System (LIS): A GIS-based system concernedwith capturing, storing, analysing and retrieving land and cadastralinformation. See Cadastre.

Land line : Property boundary maps, in digital form, created by theOrdnance Survey.

Land parcel: A piece of land representing a legal estate. SeeCadastre.

Large scale: A large scale map shows small areas e.g. 1:20, 1:500.At a large scale it would be possible, for example, to identify theindividual buildings of Kingston University.

Latitude: Angular distance, expressed in degrees, along a line that isparallel to the Equator. Also known as parallel.

Latitude-longitude: A global coordinate system used to measurelocations on the Earth's surface. Latitude and longitude are anglesmeasured from the Earth's centre to locations on the Earth's surface.The Greenwich Meridian (passing through Greenwich in England)marks the prime (zero) longitude; while the Equator marks the prime(zero) latitude circumventing the Earth. Latitude measures angles in anorth-south direction. Longitude measures angles in an east-westdirection.

Lattice: A surface representation with a rectangular array of gridpoints spaced at constant sampling intervals in the x and y directions.

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Layer: A thematic plane of GIS features containing geographicallyand logically related data (such as vegetation types). Layers are thebasic components of overlay operations in GIS.

Least-cost path: The path between two points which has the lowesttravel cost, where cost is a function of time, distance, or otherimpedance factors.

Legend: The part of the drawn map explaining the meaning of thesymbols and colours used to encode the geographical elements. Alsoknown as map key.

Lifestyle questionnaire: Created by geodemographic companies,these are designed to collect information about our lifestyles orconsumer habits. Questions may vary from choice of car to ourshopping preferences (say favoured retails outlet, frequency ofpurchases, spending patterns).

Line: A basic geographical element, defined by two or more pointswith known (x, y) coordinates. Examples include motorways, streamsand cable paths.

Lineage: The history of a dataset including its origin and theprocesses by which it was derived from that origin.

Line on polygon analysis: A key operation where lines in onelayer or coverage are intersected with polygons in another layer orcoverage.

Liveware: GIS personnel. People are an important component of anyGIS. See also hardware, software.

Local Area Network (LAN): A network arrangement forconnecting computers, within one site. The networked computers on aLAN can share data and peripheral devices, such as magnetic storagedevices, printers and plotters.

Logical consistency: This term is associated with the relationshipbetween data elements within a dataset. It also relates to themaintenance of those relationships.

Logical operator: One of several words, including AND, OR, NOTand XOR, used to build complex logical expressions in a query. Alsoknown as Boolean operator.

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Logical order: The order in which operations are performed.Conventionally operations in brackets are performed first.

Longitude: The angular distance east or west from a standardmeridian (such as the Greenwich line) expressed in degrees.

Look-up table: A set of data values that can be accessed by acomputer programme to convert data from one form to another (forexample from numerical values to colours or symbols).

Macro: A script (text) file containing a sequence of commands thatcan be executed as one command. Macros can be built to performfrequently used or complex operations.

Many-to-one relate: A relational database arrangement in whichmany records in a table are related by a single record in another table.

Map extent: The rectangular limits of a map, in terms of monitor orprinter coordinates.

Map generalisation: The process of reducing detail on a map as aconsequence of reducing the map scale. See Generalisation.

Map projection: A geometric model that transforms the locations offeatures on the Earth's surface to corresponding locations on a two-dimensional surface. It is impossible to project a spheroid perfectlyonto a plane; but some projections can preserve shape, while otherspreserve area, distance, or direction.

Map scale: The relationship between distance on a map and thecorresponding distance on the Earth's surface. Map scale is oftenexpressed as a fraction of distance, such as 1:50,000 (where one unitof distance on the map represents 50,000 of the same units of distanceon Earth surface). Map scale can also be expressed as a statement ofequivalence using different units, for example, 1 metre = 3 kilometres.

Map: An abstract representation of geographical features usingrecognisable symbols. Maps often highlight (generalise) features,depending on what they are intended to illustrate. For example, a roadatlas may not show navigable canals, and a motorway map may notshow all side roads.

Merge operations: The process of aggregating areas by removingcommon boundaries. See also Dissolve.

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Meridian: A line running vertically from the North to the South Pole.The Prime Meridian runs through Greenwich, England, wheremeasures of longitude are negative to the west and positive to the eastof Greenwich. See Latitude and Longitude.

Metadata: Data that describe the nature of a data set, in terms ofaccuracy, extent, copyright and restrictions. There is growing interestin metadata, because of increasing interest in data quality.

Metes and Bounds: A method of land surveying in which theboundary lines of land parcels are entered in terms of relative bearingand distance (from one station to the next). See Coordinate Geometry(COGO).

Model: An simplified representation of reality used to describe,analyse, predict or simulate a process, using a set of rules andprocedures.

Module: A distinct piece of software that can be called from otherparts of a computer program. Modules are designed to reduce the bulkof large software systems, and reduce running times.

Mouse: A hand-controlled computer device for interacting with acomputer terminal or entering data.

Multispectral scanner (MSS): A scanning system on somesatellites used for imaging the earth. A MSS image will have datarecorded by the scanner from several wavebands of theelectromagnetic spectrum.

National Grid: 1. a superimpsoed grid, with a known origin,developed for particular regions or countries. It allows the precisedescription of any position in it. See also Grid. 2. A term used todescribe the electricity transmission lines in a country.

National Transfer Format (NTF): British Standard BS 7567,which defines an exchange format for the transfer of vector data. NTFis the main format used by the Ordinance Survey.

Network: A system of connected geographical features, such asroads, pipes, or cables. A network structure enables the calculation ofoptimal routes or the simulation of flow through rivers or pipes.

Network analysis: A set of techniques to study the properties ofnetworks in order to simulate flows of traffic or water. Network

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analysis is often employed to calculate shortest (or lowest cost) pathsbetween two points in a network.

Node: The location at which two or more lines connect. Nodes cancarry information about the topology of a network or adjacentpolygons.

Normalisation: A conceptual database task designed to avoid dataredundancy. A table in a relational database is said to be in normalform if it satisfies certain constraints. There are five generallyaccepted steps of normalisation. First Normal Form eliminatesrepeating groups by putting each into a separate table and connectingthem with a one-to-many relationship. Second Normal Formeliminates functional dependencies on a partial key by putting thefields (columns) in a separate table from those that are dependent onthe whole key. Third Normal Form eliminates functional dependencieson non-key fields by putting them in a separate table. At this stage, allnon-key fields are dependent on the key, the whole key and nothingbut the key. Fourth Normal Form separates independent multi valuedfacts stored in one table into separate tables. Fifth Normal Formbreaks out data redundancy that is not covered by any of the previousnormal forms. See Redundancy.

Null value: The equivalent of saying that there is no value specified.

Object Linking and Embedding (OLE): A Microsoft standard,which allows objects from one application to be embedded withinanother (for example, taking a Map Object and embedding it into aWindows application like Excel).

Object Oriented: A data created approach to database modellingbased on objects and object classes. Object oriented data areencapsulated with operations and commands. See also State.

One-to-many: A relate in which one record in a table is related tomany records in another table, within a relational DBMS.

Online access: Direct access to data that does not involve filetransfer. In practice, this means real-time interaction between a serverand end user, or between a computer and a device (such as a printer)on the network.

Open Database Communication (ODBC): A Microsoft API(Application Programme Interface) which enables communicationbetween different database systems.

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Open Systems GIS. See Interoperability.

Open Systems GIS (OGIS): The Open Systems GIS Consortium(OSGC) is an association of GIS developers, academics andgovernment organisations, set up to define and encourage opensystems standards for the GIS industry. The consortium’s mainobjective is to facilitate OGIS (Open Geodata InteroperabilitySpecification) between GIS software and data formats. SeeInteroperability.

Open Look: A GUI (Graphical User Interface) for the X Windowssystem developed by Sun Microsystems. The interface is used mainlyto emulate UNIX-based work on PC’s. See X Windows.

Open Software Foundation (OSF): An international consortiumwhich promotes the UNIX operating system, and defines its technicalstandards.

Operating system (OS): Computer software designed to allowcommunication between the computer and the user, and between thecomputer and the devices attached to it. The operating system controlsthe organisation of files and directories, the flow of data between thecomputer and other hardware and the execution of commands andsoftware. DOS (Disk Operating System) and Windows are the twomost popular operating systems for PC's. Other operating systemsinclude VM and MVS (for IBM mainframes), VMS (for DECmachines), UNIX (including AIX, Ultrix, and UNICOS), OS/2 (forPC's), and Apple OS.

Optical Character Recognition (OCR): The automaticrecognition of text, using scanning devices.

Optical disk: A storage technology that uses optical media to storedata. Optical disks are slower than magnetic disks, but store more dataand cost less per unit of stored data. Optical disks are often used tostore very large amounts of data.

ORACLE: A popular, corporate relational DBMS.

Ordnance Survey (OS): Great Britain's national public mappingagency.

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Overlay: Another name for a layer or coverage. More specifically, anoverlay refers to an additional layer superimposed on the base layer.See Layer.

Overlaying: The process of stacking data layers on top of each other,so that geographical position in one theme layer can be analysed inrelation to other themes for the same position.

Packed binary: A method of data indexing and compression usedwith raster data. Another description of run length encoded.

Panning: Moving the viewable portion of a displayed map aroundthe screen, in order to display map areas which lie outside the viewingwindow.

Parcel: A basic cadastral (property) unit. See Cadastre.

Path: A continuous set of network links and nodes.

Pathfinding: The process of finding a path between an origin anddestination, on a network.

Peripheral device: A hardware device that is not part of the maincomputer unit, such as digitisers, printers, zip drives and externalmodems.

Photogrammetry: The use of stereo pairs of aerial photographs (orsatellite images) to produce geometrically correct maps.

Pixel: A shortened word for picture element, the basic grid-cell unitin raster image files (images). See Grid cell.

Planar co-ordinate system: Two dimensional (Cartesian)representation of spatial data.

Platform: A synonym for computer, especially powerful computerscapable of complex processing.

Plotter: A printing device used to print out maps with high geometricprecision in terms of coordinates, especially for engineering purposes.

Point: A geographical object, with a single (x, y) coordinate, used todesignate such features as schools, wells, trees, etc.

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Point in polygon analysis: A key operation in GIS wherecalculation involves assessing whether points fall within polygons.

Polygon on polygon analysis: A key operation in GIS wherepolygons in one layer or coverage are intersected with polygons inanother layer or coverage. The calculation is made using geometrictechniques.

Polygon: A feature used to represent areas defined by a boundaryand an associated identifier located within the polygon. Attributes maybe related to the identifier.

Positional accuracy: This refers to the closeness of locationalinformation (usually co-ordinates) to their real world position.

Precision: A measure of the tendency of digitised or mappedgeographical objects to cluster, measured by root mean square error(RMSE) or the standard deviation (SD). Note that high precision doesnot necessarily indicate high accuracy, because precisely repeatedspatial errors would not improve accuracy no matter how often theyhave recurred. The term precision is also used to refer to the numberof significant digits used to store numbers and map coordinate values.See Accuracy

Projection: See Map projection.

Protocol: A set of rules or standards which control communicationbetween computers and between computers and peripherals.

Quadtree: A compact and efficient spatial indexing and datacompression system based upon the recursive division of cells intoquadrats.

Query: A statistical or logical expression used to highlight or analysea set of data. See Logical operator.

Raster: A regular grid of cells covering an area, usually in the formof a scanned image (such as a satellite image or scanner-capturedaerial photograph).

Raster-vector conversion: The process of converting a rasterimage into a vector map.

RDBMS: See Relational Database Management System.

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Real number: A numbers with decimal digits. See Integer.

Real time systems: GIS can be used to display specific locations asthey occur at current points in time. A GPS could be fitted to avehicle, for example, and the location of that vehicle could berecorded in the GIS as it moved on its journey. See also GPS.

Record: An individual or set of related data entries within a database.Attribute tables are made up of a number of data records.

Rectification: The process of geometrically correcting an image,according to a set of real-world coordinates. Rectification usuallyinvolves the rotation and re-scaling of the original image. The processis also known as image resampling.

Redundancy: The (unnecessary) duplication of data in a database.

Reference ellipsoid: See Geodetic datum.

Relate key: The common attribute (column) used to relate twodatabases.

Relate: An operation that establishes a connection betweencorresponding records in two tables, in a relational database.

Relational DataBase Management System (RDBMS): Adatabase management system based on using several tables, connectedthrough one or more common columns in the tables to be linked.Therefore, the data in a relational database must be structured ascollections of tables that are logically associated with each other byshared attributes. A relational database arrangement allows separatetables to be linked as and when the need arises, thus eliminating thenecessity of storing all attribute data in one very large table.

Remote sensing: Acquiring information about an object at adistance, using aerial photography, radar, and satellite imagery.

Resampling: See Rectification, Rubber sheeting.

Resolution: The size of the smallest feature that can be detected ormapped on a scanned or digitised image (or monitor display). Thelarger the map scale of the image, the higher the resolution. Alsoknown as visual definition.

Root mean square error (RMSE): See Precision.

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Route: A continuous set of links in a network.

Rubber sheeting: A computer-based geometric process used toedge match mis-aligned image or map tiles. The term is also used torefer to the warping or resampling of images. See Rectification.

Run Length Encoding (RLE): A method of data index andcompression used in raster GIS. Data are stored in ‘runs’ of identicalcell values. See also Packed Binary.

Satellite data: Data gathered by space borne sensors that detect theEarth’s surface reflectance across the electro-magnetic spectrum. Seealso LANDSAT, SPOT.

Scale: The relationship between the size of an object on a map orimage, and its size in the real world.

Scanner: A data capture device for converting images or vectorshapes into digital form automatically. Some scanners are also able toconvert raster data into vector data.

Scanning: The process of converting images into digital formautomatically, using a scanner. It is usually part of the digitisingprocess.

Segment: Part of a line that is described by a set of co-ordinates. Seealso Vertex.

Sensitivity analysis: A set of procedures used in a prototype GISproject to estimate the level of accuracy which can be achieved by aGIS, relative to a set of test objectives. The term is also used in adifferent context in exploratory data analysis.

Shape file: A proprietary ARC/INFO file format.

Single precision: A level of accuracy based on the number ofsignificant digits that can be stored for each coordinate. Single-precision numbers store up to 7 significant digits for each data entry.

Sliver polygon: A small overlap area, or gap, between the bordersof polygons, which results from errors in overlaying or edge-matchingseveral map tiles.

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Slope: A measure of change in surface value over distance, expressedin degrees or as a percentage.

Small scale: A small scale map shows large areas e.g. 1: 625,000. At a small scale Kingston University buildings would not be visible,rather the town of Kingston-upon-Thames would be represented(probably by a symbol).

Snapping: The process of getting two geographical features, whichare meant to be linked (such as a node and a line), to touch oneanother.

Spaghetti data: A series of lines and points which are nottopologically structured or organised as objects. No relationships arerecorded. See also Unlinked data.

Spatial data: Data about the location of geographical features.Postcode is an example of spatial data.

Spatial query: See Map query.

Spike: An overshot line, or an anomalous data point, createderroneously during digitisation.

Spline: A mathematically calculated curve to smooth an abruptchange in the direction of a continuous line.

SPOT: A common abbreviation for Système Probatoire del’Observation de la Terre, a reconnaissance satellite system with high(10-30 metre) resolution launched by France in 1986.

State: A term used in Object Oriented GIS to describe an object. Thestate may contain information about an object’s local variables as wellas a set of operations or methods that are associated with that object.

String: A series of alphanumeric characters.

Structured Query Language (SQL): A computing language fordefining and manipulating data from a relational database, which hasbecome an industry standard for querying in most DBMS.

Surface data: Geographical data can also represent surfaceinformation. Usually it is recorded as continuous data using a modelbuilt from regular or irregular spaced sampling points. See also TIN,DEM.

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Surface model: See Digital Elevation/Terrain Model.

SYBASE: A commercial relational DBMS.

Symbol: A graphic pattern used to represent a feature in GIS andcartography.

Table: A set of data elements organised into rows and columns,within a DBMS.

TCP/IP: The common acronym for Transmission Control Protocol /Internet Protocol, which are the communication protocols that enablecomputers to send and receive data across a network (including theInternet).

Temporal GIS: This describes a GIS that has been used to measureor record information over time. See also Real time GIS.

Terminal: A computer unit acting as part of a network that is servedby a more powerful (server) computer.

Terrain analysis: See Digital Terrain Model (DTM), Spatialanalysis/modelling.

Tessellation. The process of splitting a map region into several(matching) tiles.

The Object Management Group (OMG): A computing industrygroup interested in promoting object-oriented interoperability amongdifferent computing systems. OMG has also developed thespecifications required to make object-oriented interoperability work,known as Common Object Request Broker Architecture (CORBA).

Thematic map: A map displaying information relating to specificgeographical themes, such as soil types, population density, etc.

Thiessen polygons: The boundaries of Thiessen polygons aredefined by the perpendicular bisectors of the lines between allneighbouring points. Thiessen polygons are generated from a set ofirregular points, and are designed to interpolate a continuous surfacefrom an incomplete set of spatial data.

TIFF: The common abbreviation for Tagged Interchange File Format,an industry standard graphics data format.

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Tile: A subset of a GIS database or map that contains informationabout one subset (usually, one sub-area) of the entire map, designed tosave access and processing time.

Tolerance: A distance used, generally, to adjust data errors within aGIS. See also Undershoot, Overshoot errors.

Topographic map: A terrain (relief) map, often in the form of acontour map.

Topology: The spatial relationship between geographical objects.Topological properties include order, connectivity and neighbourhood(adjacency). Topological relationships are built from simple elementsinto complex elements: points (simplest elements), lines (sets ofconnected points), polygons (areas enclosed by connected lines), androutes (sets of route sections). Topology is an important prerequisitefor sophisticated geographical analysis, such as network analysis andcontiguity analysis.

Total station: A survey instrument able to measure distances andangles using infrared beams bounced off reflectors that can store thedata in a digital format (eg as points and line co-ordinates).

Transformation: The process of converting coordinates from onecoordinate system to another, which usually entails changing theorientation, scale or projection of a mapped area.

Translation: The process of converting data or commands from oneGIS format to another, for example from DXF to shapefile.

Triangulated Irregular Network (TIN): A surface representationderived from irregularly spaced sample points and features, marked byThiessen polygons. See Thiessen polygons.

Tuple: A row in a relational database.

Undershoot: A digitised line that does not intersect with anothernode or line, because it is too short. See Dangle.

Union: An overlay of two data layers, where all features from bothlayers are retained.

Unique Property Reference Number (UPRN): A subset of theof BLPU (Basic Land and Property Unit) reference system, used in

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the address gazetteer adopted in the UK since 1984 (known asBS7666). See Gazetteer, Basic Land and Property Unit (BLPU).

Universal Transverse Mercator (UTM): Global coordinateSystem and map projection which provides locational descriptionsaccurate to 1 metre (except the north and south poles).

Unix: A general-purpose, multi-user computer operating system.

Unlinked data: Vector data stored as a collection of lines and pointswith no recorded relationships. See also Spaghetti data.

USGS: United States Geological Survey, one of the US’ mainmapping agencies.

USGS digital elevation models: Surface models produced by theSurvey Branch of the United States Department of the Interior.

Vector: A coordinate (x,y) based data structure commonly used torepresent linear geographical features. In vector format, spatial objectsare defined by points and lines. See Discrete data.

Vertex: One part ,or segment, of a line usually recorded as a set ofco-ordinates. See also Segment.

Viewshed: This can be used by a GIS visualise a given area from aparticular place, or point, on the digital map.

Visibility analysis: The process of creating viewsheds orrepresentations of surface data for the purposes of visibility. Usedextensively in telecommunication and military organisations.

Warping: See Rectification.

Wide area network (WAN): Computer network that connectscomputers at remote sites (not in the same location), often throughtelephone links.

Wide-Area Information Server (WAIS): A networkedclient/server software system that provides sophisticated file indexingbased on all text entries in each file. Searching can include Booleanexpressions.

Workspace: A GIS directory containing path references to the set ofmaps and tables that a user is working on.

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World Wide Web: A global computer-based network, developed bythe European Laboratory for Particle Physics (CERN) in Switzerland.See Internet.

X Windows/Motif: A system that allows applications to be sharedamong different workstations and terminals (irrespective of theoperating system used on the individual computers). Also known asOSF (after the Open Systems Foundation).

Zooming: Enlarging a visual display on a computer monitor.

Z-value: Usually a reference to the value of surface height in aDigital Elevation/Terrain Model at a particular (x, y) location.

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Section 3

Major GIS Acronyms

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ACSM: American Congress on Surveying and MappingAGI: Association for Geographic InformationAI: Artificial/Amplified IntelligenceAM/FM: Automatic Mapping / Facilities ManagementAML: Arc Macro LanguageANSI: American National Standards InstituteASCII: American Standard Code for Information InterchangeAV: ArcView (ESRI software)AVL: Automatic Vehicle Location

BIOS: Basic Input / Output SystemBLOB: Binary Large OBjectBLPU: Basic Land and Property UnitBM: Bench MarkBMP: BitMaPBSI: British Standards Institute

CAD: Computer Aided Drafting, Design, or DeliveryCAL: Computer Aided LearningCAM: Computer Aided Mapping/ManufacturingCASE: Computer Aided Software EngineeringCCT: Computer Compatible TapeCD: Compact DiskCD-ROM: Compact Disk-Read Only MemoryCEN: Comité European NormalisationCGI: Common Gateway InterfaceCLIENT: A software application which interacts with a servercomputerCOBOL: COmmon Business Oriented LanguageCOGO: COordinate GeOmetryCOORDS: x,y coordinates (or x,y,z for three dimensional models)CPU: Central Processing Unit

DB: DataBaseDBMS: Database Management SystemDCM: Digital Cartographic ModelDDE: Dynamic Data ExchangeDEI: Data Extraction InterfaceDEM: Digital Elevation ModelDIGEST: Digital Geographic Information ExchangeDIME: Dual Independent Map Encoding SystemDIR: DIRectoryDLG: Digital Line GraphDLL: Dynamically Linked Library

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DLM: Digital Landscape ModelDN: Digital NumberDOS: Disk Operating SystemDPI: Dot Per InchDTM: Digital Terrain Model

ED: Enumeration DistrictEIA: Environmental Impact AssessmentEIS: Environmental Impact StatementEISA: Extended Industry Standard ArchitectureEOS: Earth Observation SatelliteEPS: Encapsulated PostScriptES: Expert SystemESRI: Environmental Systems Research Institute Inc (developers ofARC/INFO)EXE: EXEcutable file

FORTRAN: FORmula TRANslation (programming language)FTP: File Transfer Protocol

GB: GigaByteGIF: Graphics Interchange FormatGIS: Geographical Information SystemsGPS: Global Positioning SystemGUI: Graphical User Interface

HMLR: Her Majesty’s Land RegistryHP: Hewlett-PackardHTML: Hyper Text Markup LanguageHTTP: Hyper Text Transport ProtocolHW: HardWare

I/O: Input/OutputIE: Internet Explorer (Microsoft browser)IEEE: Institute of Electrical and Electronics EngineersIMS: Information Management SystemIP: Internet ProtocolISDN: Integrated Services Digital NetworkISO: International Standards InstituteISPRS: International Society of Photogrammetry and RemoteSensing

KB: Kilo-ByteKCGIS: Kingston Centre for GIS

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LAN: Local Area NetworkLANDSAT: Land SatelliteLIS: Land Information SystemLUT: Look Up Table

MB: Mega-ByteMHz: Mega-HertzMIS: Management Information SystemMO: MapObjectMODEM: MOdulator-DEMolulatorMOIMS: MapObjects Internet Map Server

NA: Network AnalysisNG: National GridNC: Network ComputerNDVI: Normalised Difference Vegetation IndexNET: NETworkNIC: Network Interface CardNFS: Network File SystemNLIS: National Land Information ServiceNOAA: National Oceanographic and Atmospheric AdministrationNSF: National Science FoundationNT: New Technology (in Windows NT)NTF: National Transfer Format (UK)

OCR: Optical Character RecognitionODBC: Open DataBase ConnectivityOGIS: Open Geodata Interoperability SpecificationOLE: Object Linking and EmbeddingOLTP: OnLine Transaction ProcessingONS: Office for National Statistics (UK)OO: Object OrientedOODBMS: Object Oriented Data Base Management SystemOS: Operating System, Ordnance Survey (UK)

PAF: Postcode Address File (UK)PC: Personal ComputerPERL: Practical Report and Extraction Language (in UNIX)PIXEL: Picture elementPS: PostScript

RAM: Random Access MemoryRDBMS: Relational DataBase Management SystemRGS: Royal Geographical Society (UK)RIMS: Road Information Management System

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RGB: Red, Green, BlueRISC: Reduced Instruction Set Computing

SAS: Statistical Analysis SystemSCSI: Small Computer System InterfaceSDE: Spatial Database EngineSIMS: Survey Information Management SystemSIS: Spatial Information SystemSPOT: Système Probatoire d'Observation de la Terre. Also, SocietéPour l’Observation de la TerreSPSS: Statistical Package for the Social SciencesSQL: Structured Query LanguageSTORET: The Storage and Retrieval SystemSVGA: Super Video Graphics AdapterSW: SoftWare

TCP/IP: Transport Control Protocol / Internet ProtocolTELNET: TELetype NETworkTIFF: Tagged Image File FormatTIGER: Topologically Integrated Geographic Encoding andReferencing (US)TM: Thematic Map(ping) Also LANDSAT Thematic MapperUI: User InterfaceUPRN: Unique Property Reference Number (UK)URL: Uniform Resource LocatorUTM: Universal Transverse Mercator

VB: Visual BasicVGA: Video Graphics AdapterVR: Virtual RealityVRML: Virtual Reality Modelling LanguageWAN: Wide Area NetworkWWW: World Wide Web

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Section 4

Recommended Further Reading

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Paul A. Longley, David J. Maguire, Michael F. Goodchild, David W.Rhind (Editors) (1999). Geographical Information Systems :Principles, Techniques, Management and Applications. Wiley, John &Sons, Incorporated.

The second edition of the definitive GIS reference work. Thehighly-successful first edition has been completely restructuredand rewritten by nearly 100 world authorities from academic,commercial, and government backgrounds. This book takes acontemporary look at the key issues in GIS, identifying andtackling the important challenges in implementing GIS andapplying it in the real world.

D. Ian Heywood Sarah Cornelius Steve Carver Ian Heywood (1998).An Introduction to Geographical Information Systems. Prentice Hall

An up-to-date introduction to the world of GeographicalInformation Systems. It is designed to be easy to use forstudents at all levels, from undergraduates to professionalsretraining in GIS. The book focuses on the practicalapplications of GIS, and considers how the technology works.Each chapter addresses a specific theme and is backed up withexamples, case studies, directed reading and additional. Thebook can be used to give the reader a quick tour through theworld of GIS, to help the reader develop a thoroughunderstanding of GIS or as a source of reference information.Introduction to Geographical Information Systems is an idealtext for undergraduates, postgraduates and professionalsfollowing courses in GIS.

Molenaar, Martien (1998). An Introduction to the Theory of SpatialObject Modelling for GIS. Taylor & Francis

This book provides a background view of the object approach,related geometric aspects, the different level of data modellingand the role of geo-information theory. The rest of the bookbuilds upon these concepts describing the applicability ofbroader aspects such as topology, thematic object classes andclass hierarchies, object aggregation and generalisation anduncertainty.

Jan Van Sickle (1996). GPS for Land Surveyors. Sleeping Bear Press

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An introduction to the Global Positioning System (GPS) and apractical guide to the techniques used in a GPS survey. VanSickele, a surveyor himself, simplifies the mathematics whilestill presenting the most important elements of the GPS designand technology, covering biases and solutions, receivers andmethods, coordinates, planning a survey, observation, andpostmission processing.

Karen Steede-Terry, Jack Dangermond (2000). Integrating GIS andthe Global Positioning System with CD-ROM. EnvironmentalSystems Research Institute (ESRI)

The integration of Global Positioning System (GPS) withGeographic Information System (GIS) is an informationrevolution that gives unprecedented accuracy in measurementand completeness of coverage to anyone who works withgeographic data of any kind. This book examines the questionsinvolved in combining these two systems. It providesinformation about how GPS works, the pros and cons ofimplementing GPS use, what is the best GPS system forspecific applications, and how to integrate the technology andequipment needed to use GPS with existing GIS systems.Real-life case studies give users the opportunity to considerevery aspect of systems integration before the process isstarted. Includes CD-ROM.

Bon A. DeWitt Paul R. Wolf (2000). Elements of Photogrammetry(with Applications in GIS). McGraw-Hill Higher Education

This book incorporates recent changes on the subject ofstreamlining from advances in computers. Their everincreasing speed and storage capabilities have directly led toan entire new approach in photogrammetric mapping known as“Soft-Copy” photogrammetry. Digital Imaging systems,including those used in modern satellite programs, scanners fordigitising photographic images, and digital image processingtechniques are new topics to be covered that are fundamentalto soft copy photogrammetry.

Andrew Skidmore, Hendrik Prins (Editors) (2001). EnvironmentalModeling with GIS and Remote Sensing. Routledge

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This book derives from a training course run by ITC (Holland)for professionals and managers in the environmental sciences,detailing the applications of remote sensing and GIS forenvironmental monitoring, modelling and assessment. It setsout current research results and provides operational methodsfor environmental mapping and monitoring.

J. Stillwell, S. Geertman, S. Openshaw (Editors) (1999).Geographical Information and Planning. Springer-Verlag

This book examines developments in the use of information systemsand modelling methods in different spatial/geographical planningcontexts. It provides a review of the 1990s and offers detailed insightsinto the application of geographical information technology in urban,physical, environmental and socio-economic planning. Importantadvances in the use of the Internet for access to data and to GIS forplanning are also considered. Perspectives are provided by a widerange of contributors from academia, planning practice and thebusiness world.