Geographic Phenomena and their Representations

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

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

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

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.

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

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

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.

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

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.

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

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.

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

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.

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.

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

Geographic Phenomena and their Representations

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