GIS Data Models

GIS Data Models

The real world can only be depicted in a GIS through the use of models that define phenomena in a manner that computer systems can interpret, as well perform meaningful analysis

GIS data models

ModelAbstraction of real-world/process/phenomenon

GIS data modelSet of constructs/rules for describing or representing the real world within a computer

Two data modelsVectorRaster

Data Models

VectorA coordinate based data structure commonly used to represent geographic features. Homogenous units are points, lines, and polygons.

• Mostly used for representing discrete geographical features such as building, wells, roads etc.,

RasterA cellular based data structure composed of rows and columns for storing images. Homogenous units are called cells or pixels.

• Mainly Used for representing continuous surface e.g. soil, elevation, rainfall etc.,

Vector Data Model

Real World > Data Modeling

Source: Bernhardsen, Tor. (1999). 2nd Ed. Geographic Information Systems: An Introduction. p 38.

Real World > Modeling Process

Source: Bernhardsen, Tor. (1999). 2nd Ed. Geographic Information Systems: An Introduction. p 39. Fig 3.2.

Vector Data Model

PointsPoints are associated with single set of coordinates (X, Y)

LinesLines are a connected sequence of coordinate pairs

PolygonsAreas are a sequence of interconnected lines whose 1st & last coordinate points are the same

Scale

Scale plays a major role in how you represent a spatial entity

i.e whether to represent an entity as a point, line or a polygon

Scale = Map Distance / Real Distance

E.g. 1: 1,000: large scale1:1,000,000: small scale

Vector Data Formats

Shapefile (ArcView/ArcGIS; ESRI)ArcInfo CoverageDigital Line Graph (DLG)Others: HPGL, PostScript/ASCII, CAD/.dxf

Your will encounter mostly Arcview shape files or ArcInfo CoverageCAD/.dxf

Conversion tools are available in most GIS platformsHowever, topology should be built to make it useful for further analysis