gis

1. GIS Data Models

2. 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

3. GIS data models
Model
Abstraction of realworld/process/phenomenon
GIS data model
Set of constructs/rules for
describing or representing
the real world within a
computer
Two data models
Vector
Raster

4. Data Models
Vector
A 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.,
Raster
A 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.,

6. Vector Data Model

7. Real World > Data Modeling
Source: Bernhardsen, Tor. (1999). 2nd Ed. Geographic Information Systems: An Introduction. p 38.

8. Real World >
Modeling
Process
Source: Bernhardsen, Tor. (1999). 2nd Ed.
Geographic Information Systems: An
Introduction. p 39. Fig 3.2.

9. Vector Data Model
Points
Points are associated
with single set of
coordinates (X, Y)
Lines
Lines are a connected
sequence of
coordinate pairs
Polygons
Areas are a sequence
of interconnected lines
whose 1st & last
coordinate points are
the same

10. 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 scale
1:1,000,000: small scale

14. Vector Data Formats
Shapefile (ArcView/ArcGIS; ESRI)
ArcInfo Coverage
Digital Line Graph (DLG)
Others: HPGL, PostScript/ASCII,
CAD/.dxf

16. Your will encounter mostly
Arcview shape files or ArcInfo Coverage
CAD/.dxf
Conversion tools are available in most
GIS platforms
However, topology should be built to
make it useful for further analysis