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.,
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
11.
12.
13.
14. Vector Data Formats
Shapefile (ArcView/ArcGIS; ESRI)
ArcInfo Coverage
Digital Line Graph (DLG)
Others: HPGL, PostScript/ASCII,
CAD/.dxf
15.
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
Editor's Notes
There are two significant model categories at present:
Graphic Models: Vector and Raster
Database Models/Structures: simple lists, sequential files, indexed files, hierarchal files, network files, relational database, etc.
Today we’ll touch upon real-world entities translated into database objects, but our main focus will be on the models used in representing GIS data graphically (Vector vs Raster)
Source: Bernhardsen, Tor. (1999). 2nd Ed. Geographic Information Systems: An Introduction. p 38. Fig 3.1. (simplified model of real world)
The process of interpreting reality by using both a real world and a data model is called data modeling.
Source: Bernhardsen, Tor. (1999). 2nd Ed. Geographic Information Systems: An Introduction. p 39. Fig 3.2. (Modeling process)
The process of interpreting reality by using both a real world and a data model is called data modeling.
