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OVERVIEW OF GIS
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INTRODUCTION
Geographic Information System (GIS) is a computer based information system
used to digitally represent and analyse the geographic features present on the Earth'
surface and the events (non-spatial attributes linked to the geography under study) that
taking place on it. The meaning to represent digitally is to convert analog (smooth
line) into a digital form.
"Every object present on the Earth can be geo-referenced", is the fundamental
key of associating any database to GIS. Here, term 'database' is a collection of
information about things and their relationship to each other, and 'geo-referencing'
refers to the location of a layer or coverage in space defined by the co-ordinate
referencing system.
Work on GIS began in late 1950s, but first GIS software came only in late
1970s from the lab of the ESRI. Canada was the pioneer in the development of GIS as
a result of innovations dating back to early 1960s. Much of the credit for the early
development of GIS goes to Roger Tomilson. Evolution of GIS has transformed and
revolutionized the ways in which planners, engineers, managers etc. conduct the
database management and analysis.
DEFINING GIS
A GIS is an information system designed to work with data referenced by
spatial / geographical coordinates. In other words, GIS is both a database system with
specific capabilities for spatially referenced data as well as a set of operations forworking with the data. It may also be considered as a higher order map.
GIS technology integrates common database operations such as query and
statistical analysis with the unique visualization and geographic analysis benefits
offered by maps. These abilities distinguish GIS from other information systems and
make it valuable to a wide range of public and private enterprises for explaining
events, predicting outcomes, and planning strategies.
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A Geographic Information System is a computer based system which is used
to digitally reproduce and analyse the feature present on earth surface and the events
that take place on it. In the light of the fact that almost 70% of the data has
geographical reference as it's denominator, it becomes imperative to underline the
importance of a system which can represent the given data geographically.
A typical GIS can be understood by the help of various definitions given below:-
A geographic information system (GIS) is a computer-based tool for mapping
and analyzing things that exist and events that happen on Earth
Burrough in 1986 defined GIS as, "Set of tools for collecting, storing,
retrieving at will, transforming and displaying spatial data from the real world for a
particular set of purposes"
Arnoff in 1989 defines GIS as, "a computer based system that provides four
sets of capabilities to handle geo-referenced data :
data input
data management (data storage and retrieval)
manipulation and analysis
data output. "
Hence GIS is looked upon as a tool to assist in decision-making and management of
attributes that needs to be analysed spatially.
Answers GIS can give
Till now GIS has been described in two ways:
1. Through formal definitions, and
2. Through technology's ability to carry out spatial operations, linking data sets
together.
However there is another way to describe GIS by listing the type of questions the
technology can (or should be able to) answer. Location, Condition, Trends, patterns,
Modelling, Aspatial questions, Spatial questions. There are five type of questions that
a sophisticated GIS can answer:
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Location What is at.?
The first of these questions seeks to find out what exists at a particular location. A
location can be described in many ways, using, for example place name, post code, or
geographic reference such as longitude/latitude or x/y.
Condition Where is it.?
The second question is the converse of the first and requires spatial data to answer.
Instead of identifying what exists at a given location, one may wish to find location(s)
where certain conditions are satisfied (e.g., an unforested section of at-least 2000
square meters in size, within 100 meters of road, and with soils suitable for supporting
buildings)
Trends What has changed since..?
The third question might involve both the first two and seeks to find the differences
(e.g. in land use or elevation) over time.
Patterns What spatial patterns exists..?
This question is more sophisticated. One might ask this question to determine whether
landslides are mostly occurring near streams. It might be just as important to know
how many anomalies there are that do not fit the pattern and where they are located.
Modelling What if..?
"What if" questions are posed to determine what happens, for example, if a new
road is added to a network or if a toxic substance seeps into the local ground water
supply. Answering this type of question requires both geographic and other
information (as well as specific models). GIS permits spatial operation.
Aspatial Questions
"What's the average number of people working with GIS in each location?" is an
aspatial question - the answer to which does not require the stored value of latitude
and longitude; nor does it describe where the places are in relation with each other.
Spatial Questions
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" How many people work with GIS in the major centres of Delhi" OR " Which centres
lie within 10 Kms. of each other? ", OR " What is the shortest route passing through
all these centres". These are spatial questions that can only be answered using latitude
and longitude data and other information such as the radius of earth. Geographic
Information Systems can answer such questions.
Need of GIS?
Many professionals, such as foresters, urban planners, and geologists, have
recognized the importance of spatial dimensions in organising & analysing
information. Whether a discipline is concerned with the very practical aspects of
business, or is concerned with purely academic research, geographic information
system can introduce a perspective, which can provide valuable insights as
1. 70% of the information has geographic location as it's denominator making
spatial analysis an essential tool.
2.
Ability to assimilate divergent sources of data both spatial and non-spatial
(attribute data).
3. Visualization Impact
4.
Analytical Capability
5.
Sharing of Information
Factors Aiding the rise of GIS.
Revolution in Information Technology.
Computer Technology.
Remote Sensing.
Global Positioning System.
Communication Technology.
Rapidly declining cost of Computer Hardware, and at the same time,
exponential growth of operational speed of computers.
Enhanced functionality of software and their user-friendliness.
Visualizing impact of GIS corroborating the Chinese proverb "a picture is
worth a thousand words."
Geographical feature and data describing it are part of our everyday lives &
most of our everyday decisions are influenced by some facet of Geography.
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Philosophy of GIS
The proliferation of GIS is explained by its unique ability to assimilate data from
widely divergent sources, to analyse trends over time, and to spatially evaluate
impacts caused by development.
For an experienced analyst, GIS is an extension one's own analytical thinking. The
system has no in-built solutions for any spatial problems; it depends upon the analyst.
The importance of different factors of GIS in decreasing order is as under:
Spatial Analysis
Database
Software
Hardware
GIS involves complete understanding about patterns, space, and processes or
methodology needed to approach a problem. It is a tool acting as a means to attain
certain objective quickly and efficiently. Its applicability is realized when the user
fully understands the overall spatial concept under which a particular GIS is
established and analyses his specific application in the light of those established
parameters.
Before the GIS implementation is considered the objectives, both immediate and long
term, have to be considered. Since the effectiveness and efficiency (i.e. benefit against
cost) of the GIS will depend largely on the quality of initial field data captured,
organizational design has to be decided upon to maintain this data continuously. This
initial data capture is most important.
Advantages of GIS
The Geographic Information System has been an effective tool for implementation
and monitoring of municipal infrastructure. The use of GIS has been in vogue
primarily due to the advantage mentioned below:
Planning of project
Make better decisions
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Visual Analysis
Improve Organizational Integration
Planning Of Project
Advantage of GIS is often found in detailed planning of project having a large spatial
component, where analysis of the problem is a pre requisite at the start of the project.
Thematic maps generation is possible on one or more than one base maps, example:
the generation of a land use map on the basis of a soil composition, vegetation and
topography. The unique combination of certain features facilitates the creation of such
thematic maps. With the various modules within GIS it is possible to calculate
surface, length, width and distance.
Making Decisions
The adage "better information leads to better decisions" is as true for GIS as it is for
other information systems. A GIS, however, is not an automated decision making
system but a tool to query, analyze, and map data in support of the decision making
process. GIS technology has been used to assist in tasks such as presenting
information at planning inquiries, helping resolve territorial disputes, and siting
pylons in such a way as to minimize visual intrusion.
Visual Analysis
Digital Terrain Modeling (DTM) is an important utility of GIS. Using DTM/3D
modeling, landscape can be better visualized, leading to a better understanding of
certain relations in the landscape. Many relevant calculations, such as (potential) lakes
and water volumes, soil erosion volume (Example: landslides), quantities of earth to
be moved (channels, dams, roads, embankments, land leveling) and hydrologicalmodeling becomes easier.
Not only in the previously mentioned fields but also in the social sciences GIS can
prove extremely useful. Besides the process of formulating scenarios for an
Environmental Impact Assessment, GIS can be a valuable tool for sociologists to
analyze administrative data such as population distribution, market localization and
other related features.
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Improving Organizational Integration
Many organizations that have implemented a GIS have found that one of its main
benefits is improved management of their own organization and resources. Because
GIS has the ability to link data sets together by geography, it facilitates
interdepartmental information sharing and communication. By creating a shared
database one department can benefit from the work of another--data can be collected
once and used many times.
As communication increases among individuals and departments, redundancy is
reduced, productivity is enhanced, and overall organizational efficiency is improved.
Thus, in a utility company the customer and infrastructure databases can be integrated
so that when there is planned maintenance, affected people can be informed by
computer-generated letters.
Components of GIS
GIS constitutes of five key components:
Hardware
Software
Data
People
Method
Hardware
It consists of the computer system on which the GIS software will run. The choice of
hardware system range from 300MHz Personal Computers to Super Computers
having capability in Tera FLOPS. The computer forms the backbone of the GIS
hardware, which gets it's input through the Scanner or a digitizer board. Scanner
converts a picture into a digital image for further processing. The output of scanner
can be stored in many formats e.g. TIFF, BMP, JPG etc. A digitizer board is flat
board used for vectorisation of a given map objects. Printers and plotters are the most
common output devices for a GIS hardware setup.
Software
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GIS software provides the functions and tools needed to store, analyze, and display
geographic information. GIS softwares in use are MapInfo, ARC/Info, AutoCAD
Map, etc. The software available can be said to be application specific. When the low
cost GIS work is to be carried out desktop MapInfo is the suitable option. It is easy to
use and supports many GIS feature. If the user intends to carry out extensive analysis
on GIS, ARC/Info is the preferred option. For the people using AutoCAD and willing
to step into GIS, AutoCAD Map is a good option.
Data
Geographic data and related tabular data can be collected in-house or purchased from
a commercial data provider. The digital map forms the basic data input for GIS.
Tabular data related to the map objects can also be attached to the digital data. A GIS
will integrate spatial data with other data resources and can even use a DBMS, used
by most organization to maintain their data, to manage spatial data.
People
GIS users range from technical specialists who design and maintain the system to
those who use it to help them perform their everyday work. The people who useGIS
can be broadly classified into two classes. The CAD/GIS operator, whose work is to
vectorise the map objects. The use of this vectorised data to perform query, analysis
or any other work is the responsibility of a GIS engineer/user.
Method
And above all a successful GIS operates according to a well-designed plan and
business rules, which are the models and operating practices unique to each
organization. There are various techniques used for map creation and further usage for
any project. The map creation can either be automated raster to vector creator or it can
be manually vectorised using the scanned images. The source of these digital maps
can be either map prepared by any survey agency or satellite imagery.
GIS Applications
Computerized mapping and spatial analysis have been developed simultaneously in
several related fields. The present status would not have been achieved without close
interaction between various fields such as utility networks, cadastral mapping,
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topographic mapping, thematic cartography, surveying and photogrammetery remote
sensing, image processing, computer science, rural and urban planning, earth science,
and geography.
The GIS technology is rapidly becoming a standard tool for management of natural
resources. The effective use of large spatial data volumes is dependent upon the
existence of an efficient geographic handling and processing system to transform this
data into usable information.
The GIS technology is used to assist decision-makers by indicating various
alternatives in development and conservation planning and by modelling the potential
outcomes of a series of scenarios. It should be noted that any task begins and ends
with the real world. Data are collected about the real world. Of necessity, the product
is an abstraction; it is not possible (and not desired) to handle every last detail. After
the data are analysed, information is compiled for decision-makers. Based on this
information, actions are taken and plans implemented in the real world.
Major areas of application
Different streams of planning
Urban planning, housing, transportation planning architectural conservation, urban
design, landscape.
Street Network Based Application
It is an addressed matched application, vehicle routing and scheduling: location and
site selection and disaster planning.
Natural Resource Based Application Management and environmental impact analysis of wild and scenic recreational
resources, flood plain, wetlands, acquifers, forests, and wildlife.
View Shed Analysis
Hazardous or toxic factories siting and ground water modelling. Wild life habitat
study and migrational route planning.
Land Parcel Based
Zoning, sub-division plans review, land acquisition, environment impact analysis,
nature quality management and maintenance etc.
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Facilities Management
Can locate underground pipes and cables for maintenance, planning, tracking energy
use.
Application of GIS
GIS in agriculture
GIS is used in a variety of agricultural applications such as managing crop
yields, monitoring crop rotation techniques, and projecting soil loss forindividual farms or entire agricultural regions.
GIS in business
A GIS is a tool for managing business information of any kind according to
where it's located. You can keep track of where customers are, site businesses,
target marketing campaigns, optimize sales territories, and model retail
spending patterns. A GIS gives you that extra advantage to make you and your
company more competitive and successful.
A GIS enables you to better understand and evaluate your data by creating graphic
displays using information stored in your database. With a GIS, you can change the
display of your geographic data by changing the symbols, colors, or values in the
database tables.
GIS in electric/gas utilities
Cities and utilities use GIS every day to help them map and inventory systems,
track maintenance, monitor regulatory compliance, or model distribution
analysis, transformer analysis, and load analysis.
GIS in the environment
GIS is used every day to help protect the environment. As an environmental
professional, you can use GIS to produce maps, inventory species, measure
environmental impact, or trace pollutants. The environmental applications for
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GIS are almost endless.
GIS in forestry
Today, managing forests is becoming a more complex and demanding challenge.
With GIS, foresters can easily see the forest as an ecosystem and manage it
responsibly.
GIS in geology
Geologists use GIS every day in a wide variety of applications. You too can use
GIS to study geologic features, analyze soils and strata, assess seismic
information, or create 3-dimensional displays of geographic features.
GIS in hydrology
You can use GIS to study drainage systems, assess groundwater, and visualize
watersheds, and in many other hydrologic applications.
GIS in land use planning
People use GIS to help visualize and plan the land use needs of cities, regions, or
even national governments.
GIS in local government
People in local government use GIS every day to help them solve problems.
Often the data collected and used by one agency or department can be used by
another.
GIS in mapping
Mapping is an essential function of a GIS. People in a variety of professions are
using GIS to help others understand geographic data. You don't have to be a
skilled cartographer to make maps with a GIS.
GIS in the military
Military analysts and cartographers use GIS in a variety of applications such as
creating basemaps, assessing terrain, and aiding in tactical decisions.
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GIS in risk management
A GIS can help with risk management and analysis by showing you which areas
will be prone to natural or man-made disasters. Once identified, preventive
measures can be developed that deal with the different scenarios.
GIS in Site Planning
People around the world use GIS to help them locate sites for new facilities or
locate alternate sites for existing facilities.
GIS in transportation
GIS can be used to help you manage transportation infrastructure or help you
manage your logistical problems. Whether monitoring rail systems and road
conditions or finding the best way to deliver your goods or services, GIS can help
you.
GIS in the water/wastewater industry
People in the water/wastewater industry use GIS with the planning, engineering,
operations, maintenance, finance, and administration functions of their
water/wastewater networks.
Fundamentals of GIS
Mapping Concepts, Features & Properties
A map represents geographic features or other spatial phenomena by graphically
conveying information about locations and attributes. Locational information
describes the position of particular geographic features on the Earth's surface, as well
as the spatial relationship between features, such as the shortest path from a fire
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station to a library, the proximity of competing businesses, and so on. Attribute
information describes characteristics of the geographic features represented, such as
the feature type, its name or number and quantitative information such as its area or
length.
Thus the basic objective of mapping is to provide
descriptions of geographic phenomenon
spatial and non spatial information
map features like Point, Line, & Polygon.
Map FeaturesLocational information is usually represented by points for features such as wells and
telephone pole locations, lines for features such as streams, pipelines and contour
lines and areas for features such as lakes, counties and census tracts.
Point feature
A point feature represents as single location. It defines a map object too small to show
as a line or area feature. A special symbol of label usually depicts a point location.
Line feature
A line feature is a set of connected, ordered coordinates representing the linear shape
of a map object that may be too narrow to display as an area such as a road or feature
with no width such as a contour line.
Area feature
An area feature is a closed figure whose boundary encloses a homogeneous area, such
as a state country soil type or lake.
Map Characteristics
In addition to feature locations and their attributes, the other technical characteristics
that define maps and their use includes:
Map Scale
Map Accuracy
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Map Extent and
Data Base Extent
Scale
To show a portion of the Earth's surface on a map, the scale must be sufficiently
adjusted to cover the objective. Map scale or the extent of reduction is expressed as a
ratio. The unit on the left indicates distance on the map and the number on the right
indicates distance on the ground. The following three statements show the same scale.
1 inch = 2.000 feet => 1 inch = 24.000 inches => 1:24.000
The latter is known as a representative fraction (RF) because the amounts on either
side of the colon are equivalent: that is 1:24.000 means 1inch equals 24.000 inchesor1 foot equals 24.000 feet or 1 meter equals 24.000 meters and so on.
Map scale indicates how much the given area has been reduced. For the same size
map, features on a small-scale map (1:1,000,0000) will be smaller than those on a
large-scale map (1:1,200).
A map with less detail is said to be of a smaller scale than one with more detail.
Cartographers often divide scales into three different categories.
Small-scale maps have scales smaller than 1 : 1,000,000 and are used for maps of
wide areas where not much detail is required.
Medium-scale maps have scales between 1 : 75,000 and 1 : 1,000,000.
Large-scale maps have scales larger than 1 : 75,000. They are used in applications
where detailed map features are required.
So each scale represents a different tradeoff. With a small-scale map, you'll be able to
show a large area without much detail. On a large-scale map, you'll be able to show a
lot of detail but not for a large area. The small-scale map can show a large area
because it reduces the area so much that the large-scale map can only show a portion
of one street, but in such detail that you can see shapes of the houses.
To convert this statement to a representative fraction, the units of measure on both the
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sides being compared must be the same. For this example, both measurements will be
in meters.
To do this:
1. Convert 1.6 inches into meters
1.6 inches x 0.0254 meters/inch = 0.04 meters
2. Let us suppose that
0.04 units on the map = 10,000 units on the ground
Then, you can now state the scale as a representative fraction (RF): 0.04:10,000
Though it is a valid statement of scale, most cartographers may find it clumsy.
Traditionally, the first number in the representative fraction is made equal to 1:
0.04 / 0.04 = 1 units on the map = 10,000 / 0.04 units on the ground
1 unit on the map = 250,000 units on the ground
Scale in Digital Maps
With digital maps, the traditional concept of scale in terms of distance does not apply
because digital maps do not remain fixed in size. They can be displayed or plotted at
any possible magnification. Yet we still speak of the scale of a digital map.
In digital mapping, the term scale is used to indicate the scale of the materials from
which the map was made. For example, if a digital map is said to have a scale of
1:100,000, it was made from a 1:100,000-scale paper map.
However, a digital map's scale still allows you to make some educated guesses about
its contents because, generally, digital maps retain the same accuracy and
characteristics as their source maps. So it is still true that a large-scale digital map will
usually be more accurate and less general than a small-scale digital map.
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Because the display size of a computer-based map is not fixed, users are often
tempted to blow up maps to very large sizes. For example, a 1:100,000-scale map can
easily be plotted at a size of 1:24,000 or even 1:2,000-but it usually is not a good idea
to do so. It encourages the user to make measurements that the underlying data does
not support. You cannot measure positions to the nearest foot if your map is only
accurate to the nearest mile. You will end up looking for information that does not
exist.
Map Resolution
Map resolution refers to how accurately the location and shape of map features can be
depicted for a given map scale. Scale affects resolution. In a larger-scale map, the
resolution of features more closely matches real-world features because the extent of
reduction from ground to map is less. As map scale decrease, the map resolution
diminishes because features must be smoothed and simplified, or not shown at all.
Map Accuracy
Many factors besides resolution, influence how accurately features can be depicted,
including the quality of source data, the map scale, your drafting skill and the width of
lines drawn on the ground. A fine drafting pen will draw line's 1/100 of an inch wide.
Such a line represents a corridor on the ground, which is almost 53 feet wide.
In addition to this, human drafting errors will occur and can be compounded by the
quality of your source maps and materials. A map accurate for one purpose is often
inaccurate for others since accuracy is determined by the needs of the project as much
as it is by the map itself.
Some measurements of a map's accuracy are discussed below.
Absolute accuracy of a map refers to the relationship between a geographic
position on a map (a street corner, for instance) and its real-world position measured
on the surface of the earth. Absolute accuracy is primarily important for complex data
requirements such as those for surveying and engineering-based applications.
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Relative accuracy refers to the displacement between two points on a map
(both distance and angle), compared to the displacement of those same points in the
real world. Relative accuracy is often more important and easier to obtain than
absolute accuracy because users rarely need to know absolute positions. More often,
they need to find a position relative to some known landmark, which is what relative
accuracy provides. Users with simple data requirements generally need only relative
accuracy.
Attribute accuracy refers to the precision of the attribute database linked to the
map's features. For example, if the map shows road classifications, are they correct? If
it shows street addresses, how accurate are they? Attribute accuracy is most important
to users with complex data requirements.
A map's Currency refers to how up-to-date it is. Currency is usually expressed
in terms of a revision date, but this information is not always easy to find.
A map is Complete if it includes all the features a user would expect it to
contain. For example, does a street map contain all the streets? Completeness and
currency usually are related because a map becomes less complete as it gets older.
The most important issue to remember about map accuracy is that the more accurate
the map, the more it costs in time and money to develop. For example, digital maps
with coordinate accuracy of about 100 feet can be purchased inexpensively. If 1-foot
accuracy is required, a custom survey is often the only way to get it, which drives up
data-acquisition costs by many orders of magnitude and can significantly delay
project implementation - by months or even years.
Therefore, too much accuracy can be as detrimental to the success of a GIS project as
too little. Rather than focusing on the project's benefits, a sponsoring organizationmay focus on the costs that result from a level of accuracy not justified for the project.
Project support inevitably erodes when its original objectives are forgotten in a flurry
of cost analyses.
A far better strategy is to start the project with whatever data is readily available and
sufficient to support initial objectives. Once the GIS is up and running, producing
useful results, project scope can be expanded. The quality of its data can be improved
as required.
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Even though no maps are entirely accurate, they are still useful for decision-making
and analysis. How ever, it is important to consider map accuracy to ensure that your
data is not used inappropriately.
Any number of factors can cause error. Note these sources can have at cumulative
effect.
E = f(f) + f(1) + f(e) + f(d) + f(a) + f(m) + f(rms) + f(mp) + u
Where,
f = flattening the round Earth onto a two - dimensional surface (transformation from
spherical to planar geometry)
I = accurately measuring location on Earth (correct project and datum information)
c = cartographic interpretation (correct interpretation of features)
d = drafting error (accuracy in tracing of features and width of drafting pen)
a = analog to digital conversion (digitizing board calibration)
m = media stability (warping and stretching, folding. Wrinkling of map)
p = digitizing processor error (accuracy of cursor placement)
rms = Root Mean Square (registration accuracy of ties)
mp = machine precision (coordinate rounding by computer in storing and
transforming)
u = additional unexplained source error
Map Extent
The aerial extent of map is the area on the Earth's surface represented on the map. It is
the limit of the area covered, usually defined by rectangle just large enough to include
all mapped features. The size of the study area depends on the map scale. The smaller
the scale the larger the area covered.
Database Extent
A critical first step in building a geographic database is defining its extent. The aerial
extent of a database is the limit of the area of interest for your GIS project. This
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usually includes the areas directly affected by your organization's responsibility (such
as assigned administrative units) as well as surrounding areas that either influence or
are influenced by relevant activities in the administrative area.
Data Automation
Map features are logically organized into a set of layers or themes of information. A
base map can be organized into layers such as streams, soils, wells or boundaries.
Map data, regardless of how a spatial database will be applied, is collected, automated
and updated as series of adjacent map sheets or aerial photograph. Here each sheet is
mounted on the digitizer and digitized, one sheet at a time. In order to be able to
combine these smaller sheets into larger units or study areas, the co-ordinates of
coverage must be transformed into a single common co-ordinate system. Once in a
common co-ordinate system, attributes are associated with features. Then as needed
map sheets for layer are edge matched and joined into a single coverage for your
study area.
Types of Information in a Digital Map
Any digital map is capable of storing much more information than a paper map of the
same area, but it's generally not clear at first glance just what sort of information the
map includes. For example, more information is usually available in a digital map
than what you see on-screen. And evaluating a given data set simply by looking at the
screen can be difficult: What part of the image is contained in the data and what part
is created by the GIS program's interpretation of the data? You must understand the
types of data in your map so you can use it appropriately.
Three general types of information can be included in digital maps:
Geographic information, which provides the position and shapes of specific
geographic features.
Attribute information, which provides additional non-graphic information
about each feature.
Display information, which describes how the features will appear on the
screen.
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Some digital maps do not contain all three types of information. For example,
raster maps usually do not include attribute information, and many vector data sources
do not include display information.
Geographic Information
The geographic information in a digital map provides the position and shape of each
map feature. For example, a road map's geographic information is the location of each
road on the map.
In a vector map, a feature's position is normally expressed as sets of X, Y pairs or X,
Y, Z triples, using the coordinate system defined for the map (see the discussion of
coordinate systems, below). Most vector geographic information systems support
three fundamental geometric objects:
Point: A single pair of coordinates.
Line: Two or more points in a specific sequence.
Polygon: An area enclosed by a line.
Some systems also support more complex entities, such as regions, circles, ellipses,
arcs, and curves.
Attribute Information
Attribute data describes specific map features but is not inherently graphic. For
example, an attribute associated with a road might be its name or the date it was last
paved. Attributes are often stored in database files kept separately from the graphic
portion of the map. Attributes pertain only to vector maps; they are seldom associated
with raster images.
GIS software packages maintain internal links tying each graphical map entity to its
attribute information. The nature of these links varies widely across systems. In some,
the link is implicit, and the user has no control over it. Other systems have explicit
links that the user can modify. Links in these systems take the form of database keys.
Each map feature has a key value stored with it; the key identifies the specific
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database record that contains the feature's attribute information.
Display Information
The display information in a digital-map data set describes how the map is to be
displayed or plotted. Common display information includes feature colours, line
widths and line types (solid, dashed, dotted, single, or double); how the names of
roads and other features are shown on the map; and whether or not lakes, parks, or
other area features are colour coded.
However, many users do not consider the quality of display information when they
evaluate a data set. Yet map display strongly affects the information you and your
audience can obtain from the map - no matter how simple or complex the project. A
technically flawless, but unattractive or hard-to-read map will not achieve the goal of
conveying information easily to the user.
Cartographic Appeal
Clearly, how a map looks - especially if it is being used in a presentation - determines
its effectiveness. Appropriate color choices, linetypes, and so on add the professional
look you want and make the map easier to interpret. Since display information often is
not included in the source data set or is filtered out by conversion software, you may
need to add it yourself or purchase the map from a vendor who does it for you. Map
display information should convey the meaning of its underlying attribute data.
Various enhancements will increase a map's usefulness and cartographic appeal.
Feature Colors and Linetypes. Colors and line representations should bechosen to make the map's meaning clear. For example, using double-line roads can be
quite helpful. Many GIS data sets only include road centerline information. Actual
road width is not given. So maps with centerlines only can look like spider webs,
which is visually unappealing. Some software and conversion systems can draw roads
as double lines, with distance between lines varying according to road type.
Centerlines can be included, if necessary. Double-line maps are appropriate for
detailed studies of small areas, such as subdivisions, or maps where right-of-way
information is important.
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Naming Roads. Naming, or labeling, roads are important for proper map
interpretation. This information should be legible, positioned in the center of the road
or offset from the center, and drawn at intervals suited to the scale of the final map or
its purpose.
Landmark Symbols. A good set of symbols should be used to indicate
landmarks, such as hospitals, schools, churches, and cemeteries. The symbols should
be sized appropriately in relation to map scale.
Polygon Fills. Polygon features, such as lakes or parks, should be filled with
an appropriate color or hatch pattern.
Zoom Layer Control. If the GIS software platform permits, map layers should
be set up so that detailed, high-density information only appears when the user zooms
in for a close-up of part of the map. For example, when a large area is displayed, only
the major roads should appear; for a smaller area, both major and minor roads should
appear.
Layering
Most GIS software has a system of layers, which can be used to divide a large map
into manageable pieces. For example, all roads could be on one layer and all
hydrographic features on another. Major layers can be further classified into sub-
layers, such as different types of roads - highways, city streets, and so on. Layer
names are particularly important in CAD-based mapping and GIS programs, which
have excellent tools for handling them.
Some digital maps are layered according to the numeric feature-classification codes
found in their source data sets. For example, a major road might be on the 170-201
layer. However, this type of system is not very useful. A well-thought-out layeringscheme can make any data set much easier to use because it allows the user to control
the features with which you want to work. A good layering standard has layer names
that are mnemonic (suggest their meanings) and hierarchical (have a structured
classification scheme that makes it easy to choose general or specific classes).
For example, a map could have its roads on a layer called RD, its railroads on a layer
called RR, its road bridges on a layer called RD-BRIDGE, and its railroad bridges on
a layer called RR-BRIDGE. This scheme is mnemonic because it is easy to tell a
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layer's contents from its name, and it's hierarchical because the user can easily select
all the roads, railroads, bridges, road bridges, or railroad bridges.
Maps and Map Analysis
Automated Mapping
Computer Aided Mapping has its limitations. Goal of GIS is not only to prepare a
good map but also perform map analysis. Maps are the main source of data for GIS.
GIS, though an accurate mapping tool, requires error management.
MAP is a representation on a medium of a selected material or abstract material in
relation to the surface of the earth (defined by Cartographic association). Maps
originated from mathematics. The term Map is often used in mathematics to convey
the motion of transferring the information from one form to another just as
Cartographers transfer information from the surface of the earth to a sheet of paper.
Map is used in a loose fashion to refer to any manual display of information
particularly if it is abstract, generalised or schematic.
Process involved in the production of Maps:
Selection of few features of the real world.
Classification of selected features in to groups eg. Railway in to different
lines. Classification depends upon the purpose.
Simplification of jaggered lines like the coast lines.
Exaggeration of features.
Symbolisation to represent different classes of features.
Drawing Digitization of Maps.
Maps can be broadly classified in to two groups:
1. Topographical maps
2.
Thematic maps
Topographical Maps
It is a reference map showing the outline of selected man-made and natural features of
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the earth. It often acts as a frame for other features Topography refers to the shape of
surface represented by contours or shading. It also shows lands, railway and other
prominent features.
Thematic maps
Thematic maps are an important source of GIS information. These are tools to
communicate geographical concepts such as Density of population, Climate,
movement of goods and people, land use etc. It has many classifications.
Geographical Data Sets
Geographic Data Types
Although the two terms, data and information, are often used indiscriminately, they
both have a specific meaning. Data can be described as different observations, which
are collected and stored. Information is that data, which is useful in answering queries
or solving a problem. Digitizing a large number of maps provides a large amount of
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data after hours of painstaking works, but the data can only render useful information
if it is used in analysis.
Spatial and Non-spatial data
Geographic data are organised in a geographic database. This database can be
considered as a collection of spatially referenced data that acts as a model of reality.
There are two important components of this geographic database: its geographic
position and its attributes or properties. In other words, spatial data (where is it?) and
attribute data (what is it?)
Attribute Data
The attributes refer to the properties of spatial entities. They are often referred to as
non-spatial data since they do not in themselves represent location information.
District Name Area Population
Noida 395 sq. Km. 6,75,341
Ghaziabad 385 sq. Km. 2,57,086
Mirzapur 119 sq. Km. 1,72,952
Spatial data
Geographic position refers to the fact that each feature has a location that must be
specified in a unique way. To specify the position in an absolute way a coordinate
system is used. For small areas, the simplest coordinate system is the regular square
grid. For larger areas, certain approved cartographic projections are commonly used.
Internationally there are many different coordinate systems in use.
Geographic object can be shown by FOUR type of representation viz., points, lines,
areas, and continuous surfaces.
Point Data
Points are the simplest type of spatial data. They are-zero dimensional objects withonly a position in space but no length.
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Line Data
Lines (also termed segments or arcs) are one-dimensional spatial objects. Besides
having a position in space, they also have a length.
Area Data
Areas (also termed polygons) are two-dimensional spatial objects with not only a
position in space and a length but also a width (in other words they have an area).
Continuous Surface
Continuous surfaces are three-dimensional spatial objects with not only a position in
space, a length and a width, but also a depth or height (in other words they have a
volume). These spatial objects have not been discussed further because most GIS do
not include real volumetric spatial data.
Geographic Data -- Linkages and Matching
Linkages
A GIS typically links different sets. Suppose you want to know the mortality rate to
cancer among children under 10 years of age in each country. If you have one file that
contains the number of children in this age group, and another that contains the
mortality rate from cancer, you must first combine or link the two data files. Once this
is done, you can divide one figure by the other to obtain the desired answer.
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Exact Matching
Exact matching occurs when you have information in one computer file about many
geographic features (e.g., towns) and additional information in another file about the
same set of features. The operation to bring them together is easily achieved by using
a key common to both files -- in this case, the town name. Thus, the record in each
file with the same town name is extracted, and the two are joined and stored in
another file.
Name Populaiton
A 4038
B 7030
C 10777
D 5798
E 5606
Name Avg. housing Cost
A 30,500
B 22,000
C 100,000
D 24,000
E 24,000
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Name Population Avg. Housing Cost
A 4038 30,500
B 7030 22,000
C 10777 100,100
D 5798 24,000
E 5606 24,000
Hierarchical Matching
Some types of information, however, are collected in more detail and less frequently
than other types of information. For example, financial and unemployment data
covering a large area are collected quite frequently. On the other hand, population
data are collected in small areas but at less frequent intervals. If the smaller areas nest
(i.e., fit exactly) within the larger ones, then the way to make the data match of the
same area is to use hierarchical matching -- add the data for the small areas together
until the grouped areas match the bigger ones and then match them exactly.
The hierarchical structure illustrated in the chart shows that this city is composed of
several tracts. To obtain meaningful values for the city, the tract values must be added
together.
Tract Town Population
101 P 60,000
102 Q 45,000
103 R 35,000
104 S 36,000
105 T 57,000
106 Nakkhu 25,000
107 Kupondole 58,000
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Tract 101
Tract 102
Tract 103
Tract 104
Tract 105
Tract 107
Tract 106
Fuzzy Matching
On many occasions, the boundaries of the smaller areas do not match those of the
larger ones. This occurs often while dealing with environmental data. For example,
crop boundaries, usually defined by field edges, rarely match the boundaries between
the soil types. If you want to determine the most productive soil for a particular crop,
you need to overlay the two sets and compute crop productivity for each and every
soil type. In principle, this is like laying one map over another and noting the
combinations of soil and productivity.
A GIS can carry out all these operations because it uses geography, as a common key
between the data sets. Information is linked only if it relates to the same geographical
area.
Why is data linkage so important? Consider a situation where you have two data sets
for a given area, such as yearly income by county and average cost of housing for the
same area. Each data might be analysed and/or mapped individually. Alternatively,they may be combined. With two data sets, only one valid combination exists. Even if
your data sets may be meaningful for a single query you will still be able to answer
many more questions than if the data sets were kept separate. By bringing them
together, you add value to the database. To do this, you need GIS.
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Figure 2
Principal Functions of GIS
Data Capture
Data used in GIS often come from many types, and are stored in different ways. A
GIS provides tools and a method for the integration of different data into a format to
be compared and analysed. Data sources are mainly obtained from manual digitization
and scanning of aerial photographs, paper maps, and existing digital data sets.Remote-sensing satellite imagery and GPS are promising data input sources for GIS.
Database Management and Update
After data are collected and integrated, the GIS must provide facilities, which can
store and maintain data. Effective data management has many definitions but should
include all of the following aspects: data security, data integrity, data storage and
retrieval, and data maintenance abilities.
Geographic Analysis
Data integration and conversion are only a part of the input phase of GIS. What is
required next is the ability to interpret and to analyze the collected information
quantitatively and qualitatively. For example, satellite image can assist an agricultural
scientist to project crop yield per hectare for a particular region. For the same region,
the scientist also has the rainfall data for the past six months collected through
weather station observations. The scientists also have a map of the soils for the region
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which shows fertility and suitability for agriculture. These point data can be
interpolated and what you get is a thematic map showing isohyets or contour lines of
rainfall.
Presenting Results
One of the most exciting aspects of GIS technology is the variety of different ways in
which the information can be presented once it has been processed by GIS.
Traditional methods of tabulating and graphing data can be supplemented by maps
and three dimensional images. Visual communication is one of the most fascinating
aspects of GIS technology and is available in a diverse range of output options.
Data Capture an Introduction
The functionality of GIS relies on the quality of data available, which, in most
developing countries, is either redundant or inaccurate. Although GIS are being used
widely, effective and efficient means of data collection have yet to be systematically
established. The true value of GIS can only be realized if the proper tools to collect
spatial data and integrate them with attribute data are available.
Manual Digitization
Manual Digitizing still is the most common method for entering maps into GIS. The
map to be digitized is affixed to a digitizing table, and a pointing device (called the
digitizing cursor or mouse) is used to trace the features of the map. These features can
be boundary lines between mapping units, other linear features (rivers, roads, etc.) or
point features (sampling points, rainfall stations, etc.) The digitizing table
electronically encodes the position of the cursor with the precision of a fraction of a
millimeter. The most common digitizing table uses a fine grid of wires, embedded in
the table. The vertical wires will record the Y-coordinates, and the horizontal ones,
the X-coordinates.
The range of digitized coordinates depends upon the density of the wires (called
digitizing resolution) and the settings of the digitizing software. A digitizing table is
normally a rectangular area in the middle, separated from the outer boundary of the
table by a small rim. Outside of this so-called active area of the digitizing table, no
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coordinates are recorded. The lower left corner of the active area will have the
coordinates x = 0 and y = 0. Therefore, make sure that the (part of the) map that you
want to digitize is always fixed within the active area.
Scanning System
The second method of obtaining vector data is with the use of scanners. Scanning (or
scan digitizing) provides a quicker means of data entry than manual digitizing. In
scanning, a digital image of the map is produced by moving an electronic detector
across the map surface. The output of a scanner is a digital raster image, consisting of
a large number of individual cells ordered in rows and columns. For the Conversion to
vector format, two types of raster image can be used.
In the case of Chloropleth maps or thematic maps, such as geological maps, the
individual mapping units can be separated by the scanner according to their different
colours or grey tones. The resulting images will be in colours or grey tone images.
In the case of scanned line maps, such as topographic maps, the result is a black-
and-white image. Black lines are converted to a value of 1, and the white areas in
between lines will obtain a value of 0 in the scanned image. These images, with only
two possibilities (1 or 0) are also called binary images.
The raster image is processed by a computer to improve the image quality and is then
edited and checked by an operator. It is then converted into vector format by special
computer programmes, which are different for colour/grey tone images and binary
images.
Scanning works best with maps that are very clean, simple, relate to one feature only,
and do not contain extraneous information, such as text or graphic symbols. For
example, a contour map should only contain the contour line, without height
indication, drainage network, or infrastructure. In most cases, such maps will not be
available, and should be drawn especially for the purpose of scanning. Scanning and
conversion to vector is therefore, only beneficial in large organizations, where a large
number of complex maps are entered. In most cases, however, manual digitizing will
be the only useful method for entering spatial data in vector format.
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Figure 3
Data Conversion
While manipulating and analyzing data, the same format should be used for all data.
This Scanning System implies that, when different layers are to be used
simultaneously, they should all be in vector or all in raster format. Usually the
conversion is from vector to raster, because the biggest part of the analysis is done in
the raster domain. Vector data are transformed to raster data by overlaying a grid with
a user-defined cell size.
Sometimes the data in the raster format are converted into vector format. This is the
case especially if one wants to achieve data reduction because the data storage needed
for raster data is much larger than for vector data.
A digital data file with spatial and attribute data might already exist in some way or
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another. There might be a national database or specific databases from ministries,
projects, or companies. In some cases a conversion is necessary before these data can
be downloaded into the desired database.
The commonly used attribute databases are dBase and Oracle. Sometimes spreadsheet
programmes like Lotus, Quattro, or Excel are used, although these cannot be regarded
as real database softwares.
Remote-sensing images are digital datasets recorded by satellite operating agencies
and stored in their own image database. They usually have to be converted into the
format of the spatial (raster) database before they can be downloaded.Spatial Data
Management
Geo-Relational Data Model
All spatial data files will be geo-referenced. Geo-referencing refers to the location of a
layer or coverage in space defined by the coordinate referencing system. The geo
relational approach involves abstracting geographic information into a series of
independent layers or coverages, each representing a selected set of closely associated
geographic features (e.g., roads, land use, river, settlement, etc). Each layer has the
theme of a geographic feature and the database is organized in the thematic layers.
With this approach users can combine simple feature sets representing complex
relationships in the real world. This approach borrows heavily on the concepts of
relational DBMS, and it is typically closely integrated with such systems. This is
fundamental to database organization in GIS.
Topological Data Structure.
Topology is the spatial relationship between connecting and adjacent coverage
features (e.g., arc, nodes, polygons, and points). For instance, the topology of an arc
includes from and to nodes (beginning of the arc and ending of the arc representing
direction) and its left and right polygon. Topological relationships are built from
simple elements into complex elements: points (simplest elements), arcs (sets of
connected points), and areas (sets of connected arcs). Topological data structure, in
fact, adds intelligence to the GIS database.
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Attribute Data Management
All Data within a GIS (spatial data as well as attribute data) are stored within
databases. A database is a collection of information about things and their
relationships to each other. For example, you can have an engineering geological
database, containing information about soil and rock types, field observations and
measurements, and laboratory results. This is interesting data, but not very useful if
the laboratory data, for example, cannot be related to soil and rock types.
The objective of collecting and maintaining information in a database is to relate facts
and situations that were previously separate.
The principle characteristics of a DBMS are: -
Centralized control over the database is possible, allowing for better quality
management and operator-defined access to parts of the database;
Data can be shared effectively by different applications;
The access to the data is much easier, due to the use of a user-interface and the user-
views (especially designed formula for entering and consulting the database);
Data redundancy (storage of the same data in more than one place in the database) can
be avoided as much as possible; redundancy or unnecessary duplication of data are an
annoyance, since this makes updating the database much more difficult; one can
easily overlook changing redundant information whenever it occurs; and
The creation of new applications is much easier with DBMS.
The disadvantages relate to the higher cost of purchasing the software, the increased
complexity of management, and the higher risk, as data are centrally managed.
Relational Database -- Concepts & Model
The relational data model is conceived as a series of tables, with no hierarchy nor any
predefined relations. The relation between the various tables should be made by the
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user. This is done by identifying a common field in two tables, which is assigned as
the flexibility than in the other two data models. However, accessing the database is
slower than with the other two models. Due to its greater flexibility, the relational data
model is used by nearly all GIS systems
Choosing geographic data
The main purpose of purchasing a geographic information system (GIS)* is to
produce results for your organization. Choosing the right GIS/mapping data will help
you produce those results effectively.
The role of base-map data in your GIS,
The common characteristics of geographic data,
The commonly available data sources
Guidelines for evaluating the suitability of any data set for your project.
The world of GIS data is complex, by choosing the right data set, you can save
significant amounts of money and, even more importantly, quickly begin your GIS
project.
Data: The Core of Your Mapping / GIS Project
When most people begin a GIS project, their immediate concern is with purchasing
computer hardware and software. They enter into lengthy discussions with vendors
about the merits of various components and carefully budget for acquisitions. Yet
they often give little thought to the core of the system, the data that goes inside it.
They fail to recognize that the choice of an initial data set has a tremendous influence
on the ultimate success of their GIS project.
Data, the core of any GIS project, must be accurate - but accuracy is not enough.
Having the appropriate level of accuracy is vital. Since an increase in data accuracy
increases acquisition and maintenance costs, data that is too detailed for your needs
can hurt a project just as surely as inaccurate data can. All any GIS project needs is
data accurate enough to accomplish its objectives and no more. For example, you
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would not purchase an engineering workstation to run a simple word-processing
application. Similarly, you would not need third-order survey accuracy for a GIS-
based population study whose smallest unit of measurement is a county. Purchasing
such data would be too costly and inappropriate for the project at hand. Even more
critically, collecting overly complex data could be so time-consuming that the GIS
project might lose support within the organization.
Even so, many people argue that, since GIS data can far outlast the hardware and
software on which it runs, no expense should be spared in its creation. Perfection,
however, is relative. Projects and data requirements evolve. Rather than overinvest in
data, invest reasonably in a well-documented, well-understood data foundation that
meets today's needs and provides a path for future enhancements. This approach is a
key to successful GIS project implementation.
Are Your Data Needs Simple or Complex?
Before you start your project, take some time to consider your objectives and your
GIS data needs. Ask yourself, "Are my data needs complex or simple?"
*Italicized words can be found in the Glossary at the end of this document except for
words used for emphasis or words italicized for reasons of copyediting convention or
layout.
If you just need a map as a backdrop for other information, your data requirements are
simple. You are building a map for your specific project, and you are primarily
interested in displaying the necessary information, not in the map itself. You do not
need highly accurate measurements of distances or areas or to combine maps from
different sources. Nor do you want to edit or add to the map's basic geographic
information.
An example of simple data requirements is a map for a newspaper story that shows
the location of a fire. Good presentation is important; absolute accuracy is not.
If you have simple data needs, read this paper to get the overall picture of what GIS
data is and how it fits into your project. A project with simple data requirements can
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be started with inexpensive maps. Your primary interests will be quality graphic-
display characteristics and finding maps that are easy to use with your software. You
need not be as concerned with technical mapping issues. However, basic knowledge
of concepts such as coordinate systems, absolute accuracy, and file formats will help
you understand your choices and help you make informed decisions when it's time to
add to your system.
What issues suggest more complex GIS data needs?
Building a GIS to be used by many people over a long period of time.
Storing and maintaining database information about geographic features.
Making accurate engineering measurements from the map.
Editing or adding to the map.
Combining a variety of information from different sources.
An example of a system requiring complex data would be a GIS built to manage
infrastructure for an electric utility.
If your data requirements are complex, you ought to pay particular attention to the
sections of this paper that discuss data accuracy, coordinate systems, layering, file
formats, and the issues involved in combining data from different sources.
Also keep in mind that projects evolve, and simple data needs expand into complex
ones as your project moves beyond its original objectives. If you understand the
basics of your data set, you will make better decisions as your project grows.
Basics of Digital Mapping
Vector vs. Raster Maps
The most fundamental concept to grasp about any type of graphic data is making the
distinction between vector data and raster data. These two data types are as different
as night and day, yet they can look the same. For example, a question that commonly
comes up is "How can I convert my TIFF files into DXF files?" The answer is "With
difficulty," because TIFF is a raster data format and DXF (data interchange file) is
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a vector format. And converting from raster to vector is not simple. Raster maps are
best suited to some applications while vector maps are suited to others.
Figure 4
Raster data represents a graphic object as a pattern of dots, whereas vector data
represents the object as a set of lines drawn between specific points. Consider a line
drawn diagonally on a piece of paper. A raster file would represent this image by
subdividing the paper into a matrix of small rectangles-similar to a sheet of graph
paper-called cells (figure 1). Each cell is assigned a position in the data file and given
a value based on the color at that position. White cells could be given the value 0;black cells, the value 1; grays would fall in-between. This data representation allows
the user to easily reconstruct or visualize the original image.
Figure 5
A vector representation of the same diagonal line would record the position of the line
by simply recording the coordinates of its starting and ending points. Each point
would be expressed as two or three numbers (depending on whether the representation
was 2D or 3D, often referred to as X,Y or X,Y,Z coordinates (figure 2). The first
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number, X, is the distance between the point and the left side of the paper; Y, the
distance between the point and the bottom of the paper; Z, the point's elevation above
or below the paper. The vector is formed by joining the measured points.
Some basic properties of raster and vector data are outlined below.
Each entity in a vector file appears as an individual data object. It is easy to
record information about an object or to compute characteristics such as its exact
length or surface area. It is much harder to derive this kind of information from a
raster file because raster files contain little (and sometimes no) geometric information.
Some applications can be handled much more easily with raster techniques
than with vector techniques. Raster works best for surface modeling and for
applications where individual features are not important. For example, a raster surface
model can be very useful for performing cut-and-fill analyses for road-building
applications, but it doesn't tell you much about the characteristics of the road itself.
Terrain elevations can be recorded in a raster format and used to construct digital
elevation models (DEMs) (figure 3). Some land-use information comes in raster
format.
Figure 6
Raster files are often larger than vector files. The raster representation of the
line in the example above required a data value for each cell on the page, whereas the
vector representation only required the positions of two points.
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The size of the cells in a raster file is an important factor. Smaller cells improve image
quality because they increase detail. As cell size increases, image definition decreases
or blurs. In the example, the position of the line's edge is defined most clearly if the
cells are very small. However, there is a trade-off: Dividing the cell size in half
increases file size by a factor of four.
Cell size in a raster file is referred to as resolution. For a given resolution value, the
raster cost does not increase with image complexity. That is, any scanner can quickly
make a raster file. It takes no more effort to scan a map of a dense urban area than to
scan a sparse rural one. On the other hand, a vector file requires careful measuring
and recording of each point, so an urban map will be much more time-consuming to
draw than a rural map. The process of making vector maps is not easily automated,
and cost increases with map complexity.
Because raster data is often more repetitive and predictable, it can be compressed
more easily than vector data. Many raster formats, such as TIFF, have compression
options that drastically reduce image sizes, depending upon image complexity and
variability.
Raster files are most often used:
For digital representations of aerial photographs, satellite images, scanned
paper maps, and other applications with very detailed images.
When costs need to be kept down.
When the map does not require analysis of individual map features.
When "backdrop" maps are required.
In contrast, vector maps are appropriate for:
Highly precise applications.
When file sizes are important.
When individual map features require analysis.
When descriptive information must be stored.
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Raster and vector maps can also be combined visually. For example, a vector street
map could be overlaid on a raster aerial photograph. The vector map would provide
discrete information about individual street segments, the raster image, a backdrop of
the surrounding environment.
Digital Map Formats- How Data Is Stored
The term file format refers to the logical structure used to store information in a GIS
file. File formats are important in part because not every GIS software package
supports all formats. If you want to use a data set, but it isn't available in a format that
your GIS supports, you will have to find a way to transform it, find another data set,
or find another GIS.
Almost every GIS has its own internal file format. These formats are designed for
optimal use inside the software and are often proprietary. They are not designed for
use outside their native systems. Most systems also support transfer file formats.
Transfer formats are designed to bring data in and out of the GIS software, so they are
usually standardized and well documented.
If your data needs are simple, your main concern will be with the internal format that
your GIS software supports. If you have complex data needs, you will want to learn
about a wider range of transfer formats, especially if you want to mix data from
different sources. Transfer formats will be required to import some data sets into your
software.
Vector Formats
Many GIS applications are based on vector technology, so vector formats are the most
common. They are also the most complex because there are many ways to store
coordinates, attributes, attribute linkages, database structures, and display
information. Some of the most common formats are briefly described below
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Common Vector File Formats
Format NameSoftware
Platform
Internal or
TransferDeveloper Comments
Arc Export ARC/INFO* Transfer
Environmental
Systems Research
Institute, Inc. (ESRI)
Transfers data
across ARC/INFO*
platforms.
ARC/INFO* Coverages ARC/INFO* Internal ESRI
AutoCAD Drawing Files
(DWG)AutoCAD* Internal Autodesk
Autodesk Data
Interchange File
(DXF)
Many Transfer Autodesk
Widely used
graphics transfer
standard.
Digital Line graphs
(DLG)Many Transfer
United States
Geological Survey
(USGS)
Used to publish
USGS digital maps.
Hewlett-Packard
Graphic Language
(HPGL)
Many Internal Hewlett-PackardUsed to control HP
plotters.
MapInfo Data TransferFiles (MIF/MID)
MapInfo* Transfer MapInfo Corp.
MapInfo Map Files MapInfo* Internal MapInfo Corp.
MicroStation Design
Files (DGN)MicroStation* Internal Bentley Systems, Inc.
Spatial Data Transfer
System (SDTS)
Many (in the
future)Transfer US Government
New US standard
for vector and raster
geographic data.
Topologically Integrated
Geographic Encoding
and Referencing
(TIGER)
Many Transfer US Census Bureau
Used to publish US
Census Bureau
maps.
Vector Product Format
(VPF)
Military
mapping
systems
BothUS Defense Mapping
Agency
Used to publish
Digital Chart of the
World.
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Raster Formats
Raster files generally are used to store image information, such as scanned paper
maps or aerial photographs. They are also used for data captured by satellite and other
airborne imaging systems. Images from these systems are often referred to as remote-
sensing data. Unlike other raster files, which express resolution in terms of cell size
and dots per inch (dpi), resolution in remotely sensed images is expressed in meters,
which indicates the size of the ground area covered by each cell.
Some common raster formats are described below
Format NameSoftware
PlatformInternal or
TransferDeveloper Comments
Arc DigitizedRaster Graphics(ADRG)
Militarymappingsystems
Both US DefenseMapping Agency
Band Interleavedby Line (BIL)
Man BothCommon remote-sensing standard.
Band Interleavedby Pixel (BIP)
Many BothCommon remote-sensing standard.
Band Sequential(BSQ)
Many BothCommon remote-sensing standard.
Digital ElevationModel for
(DEM)
Many TransferUnited StatesGeological Survey
(USGS)
USGS standard format digital
terrain models.
PC PaintbrushExchange (PCX)
PC Paintbrush Both Zsoft Widely used raster format.
Spatial DataTransfer
Standard (SDTS)
Many (in thefuture)
TransferUS FederalGovernment
New US standard for both rasterand vector geographic data;raster version still underdevelopment.
Tagged ImageFile Format(TIFF)
PageMaker Both Aldus Widely used raster format.
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An Example of Raster and Vector Integration
Figure 7: An Example of Raster and Vector Integration
Vectors & Raster Data Models - Merits & Demerits.
RASTER MODEL VECTOR MODEL
Advantages
Simple data structureEasy and efficient overlaying
Compatible with RS imageryHigh spatial variability is efficiently
representedSimple for own programmingSame grid cells for several attributes
Disadvantages
Inefficient use of computer storageErrors in perimeter, and shapeDifficult network analysisInefficient projection transformationsLoss of information when using large
cells Less accurate (although interactive) maps
Advantages
Compact data structureEfficient for network analysis
Efficient projection transformationAccurate map output.
Disadvantages
Complex data structureDifficult overlay operations
High spatial variability is inefficientlyrepresented
Not compatible with RS imagery
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Hybrid System
It is an integration of the best of Vector and Raster Models. The GIS technology is
fast moving towards Hybrid model GIS.
The Integration of Vector and Raster System Hybird System
Figure 8: The Integration of Vector and Raster System Hybird System
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Analysis of Geographic Data
ANALYSIS - What? & Why?
The heart of GIS is the analytical capabilities of the system. What distinguish the GIS
system from other information system are its spatial analysis functions. Although the
data input is, in general, the most time consuming part, it is for data analysis that GIS
is used. The analysis functions use the spatial and non-spatial attributes in the
database to answer questions about the real world. Geographic analysis facilitates the
study of real-world processes by developing and applying models. Such models
illuminate the underlying trends in geographic data and thus make new information
available. Results of geographic analysis can be communicated with the help of maps,
or both.
The organization of database into map layers is not simply for reasons of
organizational clarity, rather it is to provide rapid access to data elements required for
geographic analysis. The objective of geographic analysis is to transform data into
useful information to satisfy the requirements or objectives of decision-makers at all
levels in terms of detail. An important use of the analysis is the possibility of
predicting events in the another location or at another point in time.
ANALYSIS - How?
Before commencing geographic analysis, one needs to assess the problem and
establish an objective. The analysis requires step-by-step procedures to arrive at the
conclusions.
The range of geographical analysis procedures can be subdivided into the following
categories.
Database Query.
Overlay.
Proximity analysis.
Network analysis.
Digital Terrain Model.
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Statistical and Tabular Analysis.
Spatial Analysis
It helps us to:
Identify trends on the data.
Create new relationships from the data.
View complex relationships between data sets.
Make better decisions.
Geographic Analysis
Analysis of problems with some Geographic Aspects.
Alternatives are geographic locations or areas.
Decisions would affect locations or areas.
Geographic relationships are important in decision-making or modelling.
Some examples of its application:
Nearest Neighbour.
Network distances.
Planar distances.
Spatial Analysis - An Application
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Image 1
Where should we build a road from a point A to point B?
How do we minimise the impacts of building this road?
Relationship of Modelling to Analysis
Decision Models search through potential alternatives to arrive at a
recommendation.
Decision support models process raw data into forms that are directly relevant
to decision making.
Data characterisation models are used to develop a better understanding of a
system to help characterise a problem or potential solutions.
Difficulties of Geographic Analysis
Plenty of data.
Spatial relationships are important but difficult to measure. Inherent uncertainty due to scale.
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any data sources.
Difficult to make data sources compatible.
Difficult mathematics.
Quantity vs. Quality Questions.
Multiple objectives.
GIS can address some (but not all) of these difficulties.
Network Analysis
Network models are based on interconnecting logical components, of which the most
important are:
1. "Nodes" define start, end, and intersections
2. "Chains" are line features joining nodes
3. "Links" join together points making up a chain.
This network can be analyzed using GIS.A simple and most apparent network
analysis applications are:
Street network analysis,
Traffic flow modelling,
Telephone cable networking,
Pipelines etc.
The other obvious applications would be service centre locations based on travel
distance.
Basic forms of network analysis simply extract information from a network. More
complex analysis, process information in the network model to derive new
information. One example of this is the classic shortest-path between two points. The
vector mode is more suited to network analysis than the raster model.
A Road Network
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Image
Tabular Statistical Analysis
If in the above road network we have categorised the streets then in such a case the
statistical analysis answers questions like
What unique categories do I have for streets?
How many features do I have for each unique category?
Summarize by using any attribute?
Database Query
The selective display and retrieval of information from a database are among the
fundamental requirements of GIS. The ability to selectively retrieve information from
GIS is an important facility. Database query simply asks to see already stored
information. Basically there are two types of query most general GIS allow: viz.,
Query by attribute,
Query by geometry.
Map features can be retrieved on the basis of attributes, For example, show all the
urban areas having the population density greater than 1,000 per square kilometer,
Many GIS include a sophisticated function of RDBMS known as Standard Query
Language (SQL), to search a GIS database. The attribute database, in general, is
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stored in a table (relational database mode.) with a unique code linked to the
geometric data. This database can be searched with specific characteristics. However,
more complex queries can be made with the help of SQL.
GIS can carry out a number of geometric queries. The simplest application, for
example, is to show the attributes of displayed objects by identifying them with a
graphical cursor. There are five forms of primitive geometric query: viz.,
Query by point,
Query by rectangle,
Query by circle,
Query by line,
Query by polygon,
A more complex query still is one that uses both geometric and attributes search
criteria together. Many GIS force the separation of the two different types of query.
However, some GIS, using databases to store both geometric and attribute data, allow
true hybrid spatial queries.
Overlay Operations
The hallmark of GIS is overlay operations. Using these operations, new spatial
elements are created by the overlaying of maps.
There are basically two different types of overlay operations depending upon data
structures:
Raster overlayIt is a relatively straightforward operation and often many data sets
can be combined and displayed at once.
Vector overlayThe vector overlay, however is far more difficult and complex and
involves more processing.
Logical Operators
The concept of map logic can be applied during overlay. The logical operators are
Boolean functions. There are basically four types of Boolean Operators: viz., OR,
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Conditional operators were already used in the examples given above. The all
evaluate whether a certain condition has been met.
= eq 'equal' operator
ne 'non-equal' operator
< lt 'less than' operator
gt 'greater than' operator
>= ge 'greater than or equal' operator
Many systems now can handle both vector and raster data. The vector
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