Fresh, Fresher, Freshest: Building Vocabulary Through Rhythm and Rhyme
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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