This document provides an overview of geographic phenomena and their representation in GIS systems. It discusses the different types of geographic features such as artificial, natural, and mixed features. It also describes spatial and non-spatial data types. The two main groups of geographic phenomena are fields and objects. Vector and raster data representations are described, including the different vector feature types (points, lines, polygons) and how raster data divides space into a grid. The document concludes with guidelines on choosing between vector and raster data based on the type of analysis or features being represented.

1. Geographical Phenomenon and their representation
GIS Training –Day 2 @IOE, Pulchowk

2. Presentation Outline
Installation of Arc GIS will go in parallel with
Demo with real data for yesterday’s lecture
Geographical Phenomena
Raster and Vector Representation

3. Geographic Phenomena
Events that takes place in geographic space and time.
Choice for a digital representation depends on the type of
phenomenon.
Can be :
Artificial: Buildings
Natural: Rivers
Mixed Type: Pollution

4. Two types of data
Data Types
Spatial Data
Non-spatial or Attribute data
Spatial Data
Data related to location.
Example: Coordinate of center of a football ground
Non-spatial Data
Describes such aspects of the spatial data which is not specified
by its geometry alone.
 Example: Name of roads, schools, forests etc., Population or
census data etc.

5. Groups of Geographical Phenomenon
Two common groups of geographic phenomena:
Fields Objects
For every point in the study area,
a value can be determined.
All changes in field values are
gradual.
Example: Elevation, Temperature.
Well distinguishable discrete
entities.
Empty spaces in between the
features.
Study space grouped into
mutually exclusive bounded
parts.
Example: Buildings, Roads

6. Putting your data into GIS
 What original raw data is available?
What sort of data manipulation does the application want to
perform?

8. Vector Representations
 Represents each Geographic feature by
a set of coordinates.
An attempt to represent the object as
exactly as possible.
All positions, lengths, and dimensions
to be defined precisely.

9. Types:
Point representation
Defined as single coordinate pairs (x,y) in 2D and (x, y, z) in 3D
Line Representation
Defined by 2 end nodes and 0-n internal nodes in between
Area Representation
 Represent each polygon as a set of XY co-ordinates of the boundary

TIN Representation

10. Raster Representation
Entire space is broken into grid cells of a fixed or uniform size.
Used Commonly to represent Geographic Fields.
 Each grid cell is referenced by a row and column number.

11. Raster representation
with different Cell Size
Associating Coordinate System to a Raster
data
Example:
Use of raster data as a
thematic data, Land Use

12. In a Raster
A point is represented by a single
grid cell.
A line by a number of neighboring
cells strung out in given direction.
An area by an agglomeration of
neighboring cells.

13. Comparison
VECTOR
Complex data Structure
Easy association with attribute
data
Efficient representation of
topology
Overlay of several vector
polygon maps creates
difficulties
RASTER
Simple Data Structure
Difficult to associate attribute
data
Difficult in representing
topology.
Overlay and combination of
data is easier

14. REAL WORLD

15. However,
Both structure are inter-convertible.
Conversion from Vector to Raster
 Is simplest
Many well known algorithms exist.
Conversion from Raster to Vector
Much more complex operation.

16. Choice between Vector and Raster
Use VECTOR data structures for
Geographical objects like buildings ,Roads etc.
Network analysis :Transportation networks, Telephone
networks etc.
Use RASTER methods
Representing continuous geographical fields
When it is necessary to work with surfaces; Simulation and
modeling
Spatial analysis ,Map overlays, etc.

17. Conclusion

18. Reading Materials
GIS DATA Formats
http://maps.unomaha.edu/Peterson/gis/notes/DataFormatExchange.html
Complete Understanding of GIS (http://giscommons.org/ )
Introduction ,Input ,Map Processing ,Data Processing , Analysis and Output