Exploring GIS Data

I) GIS Data Models•Shapefiles•Geodatabase• CoverageII) TopologyIII) Database Management SystemIV) Major data Source for GISV) Data Quality

Data : A collection of related facts usually arranged in aparticular format and gathered for a particular purpose.(as temperature,..)

Information: Data which serves a purpose (as averageof temperature,..)

Database:A collection of related files organized for efficient retrievalof information.

Data source or dataset:Any geographic data such as a shape file, coverageGeodatabase, or raster.

Geographic data Model :

*

Geodatabase: A geographic database that is hostedinside a relational database management system thatprovides services for managing geographic data. Theseservices include validation rules, relationships, andtopological associations.

Feature: A representation of a real-world object in alayer on a map.

Field: A column in a table. Each field contains the valuesfor a single attribute.

I) Geographic Data Model:• A geographic data

model is a structure for organizing geospatial data, so that it can be easily stored and retrieved.

Geographic coordinates

Tabular attributes

•GIS stores and managesgeographic data in anumber of formats. Thetwo basic data modelsthat ArcGIS uses areVector, Raster.

Geographic Data ModelsVector and Raster Models

Concept of a Geographic Data

Vector vs. Raster Data Representation

PointPoint

LineLine

PolygonPolygon

VectorVector RasterRaster

Raster data are described by a cell grid, one value per cell

Zone of cells

Vector vs. Raster Data Representation

Representation of point, line, and area features: raster format on the left and vector format on the right.

Geographic Data Models

1.Vector Data Models

Vector Data:One way of representing geographic phenomena is with points, lines, and polygons.

This kind of representation of the world is generically called a vector data model. Vector

models are particularly useful for representing and storing discrete

features such as buildings, pipes, or parcel boundaries.

1.Vector data

Points are pairs of x,y coordinates.Lines are sets of coordinates that define a shape.Polygons are sets of coordinates defining boundariesthat enclose areas.

Vector data formats

Vector data formats for storing point, line, and polygon features. These formats include:

• Shapefiles • Coverages • Geodatabases • CAD files • Event tables • Triangulated Irregular Networks (TINs)

Personal GeodatabaseFeature data setFeature class (feature type = polygon)Feature class (feature type = arc)

Coverage (= feature class)Feature type (arc)Feature type (point)Feature type (polygon)Feature type (point)

Coverage (= feature class)Feature type (arc)Feature type (point)Locator (table)Raster ShapefileShapefile

Spatial file formats- Example- ArcCatalog view

Type of storing of Vector Data :

ArcGIS from ESRI uses three differentimplementations of the vector model to representfeature data:Shapefiles:.shpGeodatabases: .mdband Coverages: dwg; dxf;dgn

shapefileA vector data storage format for storing the location,shape, and attributes of geographic features. Ashapefile is stored in a folder and contains onefeature class.GeodatabaseA geographic database that provides services formanaging geographic data. A geodatabase is hostedinside a relational database management system. Ageodatabase contains feature datasets.CoverageA vector data storage format for storing the location,shape, and attributes of geographic features. One ofthe primary vector data storage formats for ArcInfo.Coverages are stored in an ArcInfo workspace.

Type of storing of Vector Data :

1. Vector data1.1 ShapefilesShapefiles are useful for mapmaking and some kindsof analysis. A great deal of geographic data isavailable in shapefile format.

Each shapefile stores features belonging to a singlefeature class.

Features in shapefiles•Shapefiles have two types of point features: pointsand multipoints.•They have line features that can be simple lines ormultipart polylines.•They also have area features that are simple ormultipart areas called polygons.

1.1 Shapefiles

Point shapes are simply single-point features such aswells or monuments. Here, well number 57 isselected.

Multipoint shapes are collections of points that allrepresent one feature.

A group of small islands could be represented as asingle multipoint shape. Here, multipoint feature 22 isselected.

1.1 Shapefiles

Line shapes can be simple continuous lines suchas a fault line on a map. They can also be polylinesthat branch such as a river. Line shapes can alsohave discontinuous parts.Polygon shapes can be simple areas such as asingle island. They can also be multipart areas suchas several islands that constitute a single state.Polygon shapes can overlap, but the shapefiledoes not store topological relationships betweenthem. The market areas of two stores could berepresented as overlapping polygon shapes.

1.1 Shapefiles

Storage of shapefiles

Shapefiles are stored in folders. A shapefile consistsof a set of files of vector data in the shapefile and adBASE®.dbf file containing the attributes of thefeatures. Each constituent file shares the shapefilename.

Shapefiles are stored in three to five files (withextensions .shp, .shx, .dbf, .sbx and .sbn).

1.1 Shapefiles

A shapefile contains shapes of only one geometry:point, multipoint, line, or polygon.

When you look at a folder of shapefiles in ArcCatalog,you see the shapefiles as standalone feature classes.

1.2 Geodatabases

A geodatabase stores each feature as a row in atable. The vector shape of the feature is stored in thetable’s shape field, with the feature attributes in otherfields.

Each table stores a feature class. In addition tofeatures, geodatabases can also store data tables,and references to other tables.

1.2 Geodatabases:

Some advantages of a geodatabase are thatfeatures in geodatabases can have built-inbehavior; geodatabase features are completelystored in a single database; and large geodatabasefeature classes can be stored seamlessly, not tiled.

In addition to generic features, such as points, lines,and areas, you can create custom features suchas transformers, pipes, and parcels.

Custom features can have special behavior tobetter represent real-world objects. You can usethis behavior to support sophisticated modeling ofnetworks, data entry error prevention, customrendering of features, and custom forms forinspecting or entering attributes of features.

1.2 Storage of geodatabase features

Geodatabase feature classes each contain onegeometric feature type. Related feature classes canbe organized into feature datasets. Feature datasetsare useful for organizing feature classes with ashared topology. They can also be used to organizefeature classes thematically.

For example,you might have three feature classes in a waterbodies feature dataset: points, representingponds; lines, representing rivers; and polygons,representing lakes.

1.2 Geodatabases:

When you look at a geodatabase in ArcCatalog, you seethe database tables as collections of feature datasets andfeature classes, or simply as standalone feature classes.

Geodatabase feature classes are stored with spatialindexes, so you can work efficiently with small areas ofvery large seamless databases. This eliminates the needto divide large, complex datasets into separate tiles.

you can see that a_workspace contains twocoverages: a_coverage and b_coverage. Thea_coverage contains an arc feature class and a ticfeature class. This coverage has polygon topology, soit contains a polygon feature class and a label featureclass as well. The dataset b_coverage is a linecoverage, so it just contains arc and tic featureclasses.

1.3 Coverages

In a Raster model, the world is represented as asurface that is divided into a regular grid of cells.

2. Raster data

The x,y coordinates of at least one corner of the raster are known, so it can be located in geographic space.

Raster models are useful for storing and analyzingdata that is continuous across an area. Each cellcontains a value that can represent membership in aclass or category, a measurement, or an interpretedvalue.Raster data includes images and grids. Images,such as an aerial photograph, a satellite image, ora scanned map, are often used for generating GISdata.The smaller the cell size for the raster layer, thehigher the resolution and the more detailed themap but increases the total volume of data thatmust be stored.

2.Raster data

Type of storing of Raster: Image Datasets• Supported image formats:

– ARC Digitized Raster Graphics (ADRG)– Windows bitmap images (BMP) [.bmp]– ERDAS [.lan and .gis]– ESRI Grid datasets– IMAGINE [.img]– IMPELL Bitmaps [.rlc]– Image catalogs– JPEG [.jpg]– MrSID [.sid]– National Image Transfer Format (NITF)– Sun rasterfiles [.rs, .ras and .sun]– Tag Image File Format (TIFF) [.tiff, .tif and .tff]– TIFF/LZW

Raster - Examples

Raster - Examples

Raster - Examples

Method Advantages Disadvantages Raster Simple data

structureRequires greater storage space on computer

Compatible with remotely sensed or scanned data

Depending on pixel size, graphical outputmay be less pleasing

Simple spatial analysis procedures

Projection transformations are more difficultMore difficult to represent topological relationships

Method Advantages Disadvantages Vector Requires less disk

storage space More complex data structureTopological relationships are readily maintained

Not as compatible with remotely sensed data

Graphical output more closely resembles hand-drawn maps

Software and hardware are often more expensiveSome spatial analysis procedures may be more difficultOverlaying multiple vector maps is often time consuming

Vector Data

• Discrete features• Points: wells, outcrops,

hydrants, tel. poles• Lines: roads, rivers• Areas: landuse, veg. types,

soil types• Uses points with x, y coor.• Can be topological or non-

topological data

Raster Data

• Continuous features• Uses grids and cells with

unique values• Rainfall, surface elevation,

depths, soil erosion• Simple data structure (rows &

columns)• File size depends on cell

(pixel) resolution

Two Types of Data Models Used in GIS

Triangulated Irregular Networks (TIN): vectors

a series of triangles capturing the topography .. x, y, z at nodes Each triangle has a defined slope and direction (aspect)

Advantages: variable data density depending on landscape significant points or lines can be encoded e.g. peaks, ridges, valleys

Disadvantages: more complex, needs more processing to generate triangle facets are often evident in processing

Summary: most DEM data today are stored and processed as raster GRIDS

Geographic DataModels

Attribute Data Spatial Data

Vector Data Raster Data

Point, Polyline, Polygon

Pixel: Picture Element

Type of Storing:Shape file:.shpMicro Soft Database access: .mdbCoverage: dwg, dxf, dgn

Type of Storing:.jpeg,img,tiff,gif,ecw,bmp,..

Concept of a Geographic Data

Attribute Management• Attribute Structure

– Add field– Delete field– Edit field

• Table manipulation– Sort– Freeze– Calculate– Summarize

Text The text data type is a fixed length sequence of characters/text. This is the data type you will likely use to record most of your attribute data. Make sure that you set aside enough length to allow for your attribute descriptions.

Short Integer

The most basic numeric data type is the short integer. The actual numeric value for a short integer is approximately between -32,000 and +32,000. Both short and long integers can store only real numbers. That is to say that you cannot have fractions, or numbers to the right of the decimal place. To store data with decimal values, you will need to use either a float or a double data type.

Type of field

Long Integer

The actual range for a long integer is approximately between -2 billion and +2 billion. Both short and long integers can store only real numbers. That is to say that you cannot have fractions, or numbers to the right of the decimal place. To store data with decimal values, you will need to use either a float or a double data type.

Date The date data type can store dates, times, or date and times. The default format in which the information is presented is mm/dd/yyyy hh:mm:ssand a specification of AM or PM. When you enter date fields in the table, they will be converted to this format.

Type of field

Float A float is a four-byte number and can store up to seven significant digits, both positive and negative. The double data type can also store decimals places. Leave the default precision and scale when you are adding floats.

Double A double is an eight-byte number and can store up to 15 significant digits, both positive and negative. The double data type can also store decimals places. You will be using the double data type for your Latitude and Longitude fields. Leave the default precision and scale when you are adding doubles.

Blob A BLOB, stands for binary large object. This data type is for items such as images,multimedia, or bits of code that are too large to be stored as a other data types.

Type of field

II) Topology

What is Topology

• Topology is the science and mathematics of relationship used to validate the geometry of vector entities, and for operations such as network tracing and test of polygon adjacency (Longley et al., 2001)

• Advantages and uses of topology:– Maintain correct data spatial relationship (Find

errors)– Efficient data storage (quickly process large

data sets)– Facilitate spatial analysis (Network analysis,

Adjacent area analysis, overlay analysis)

Topology

• Traditional topology: store and maintained in the vector data model– Connectivity: Arcs connect to each other at nodes– Area definition: polygon defined by a set of arcs that is

connected and enclosed– Contiguity: Arcs have direction and left and right polygons

• Topology in the object data model is a set of rules and software tools to define spatial relationships and behaviors, examples:– Polygons must not overlap within a data set– Lines must not overlap themselves within a data set– Points in one data set must be covered by lines in

another data set– …

Topology

Topological data structure [Node]

Based upon these nodes, "arcs" are defined. These arcs have endpoints, but they are also assigned a direction indicated by the arrowheads. The starting point of the vector is referred to as the "from node" and the destination the "to node."

The orientation of a given vector can be assigned in either direction,as long asthis directionis recorded and stored in the database.

The Example of Topological Relationships:

Topological data structure [Chain]

“Polygons" are defined by arcs. To define a givenpolygon, trace around its area in a clockwise directionrecording the component arcs and their orientations.If an arc has to be followed in its reverse orientationto make the tracing, it is assigned a negative sign inthe database.

The Example of Topological Relationships

Finally, for each arc, one records which polygon lies to the left and right side of its direction of orientation. If an arc is on the edge of the study area, it is bounded by the "universe."

The Example of Topological Relationships

Topological data structure [Polygon]

III) Database Management System

IV) Major data Sources for GIS

In the absence of a Data Quality Report, ask questions about undocumented data before you use it.

•What is the age of the data?•Where did it come from?•In what medium was it originally produced?•What is the a real coverage of the data?•To what map scale was the data digitized?•What projection, coordinate system, and datum were used in maps?•What was the density of observations used for its compilation?

V) Data Quality

……..

•How accurate are positional and attribute features?•Does the data seem logical and consistent?•Do cartographic representations look "clean?"•Is the data relevant to the project at hand?•In what format is the data kept?•How was the data checked?•Why was the data compiled?•What is the reliability of the provider?

V) Data Quality