Question 8: How do we represent geographical information in a computer?
Geographic information contains
locations, shapes and descriptions of the information. A geographic
information system uses a computer to
combine and analyse multiple
sets of geographic information. In order to achieve this, it is necessary to
code the geographic information into a format that the computer understands.
Representing location
Firstly, the geographic information needs to be located using some sort of
cartographic format that the computer can understand. This requires geographic
information
in a GIS to also have information about the map projection and coordinate system
in which it is stored entered. This data is usually input by the GIS operator
and stored alongside the geographic information.
Describing shape
Secondly, the computer needs to have a way of storing the shape of the geographic
information. This might be the locations of points, the shape of lines such
as roads, or the shape of polygon boundaries such as the outline of a building
(this
is known as vector data). It may also store geographic information as a grid,
with each cell in the grid containing a subset of the geographic information
(this is known as raster data). Generally, the computer relies on the complex
shapes being simplified into a number of simple, linked, tables (often referred
to as a database) containing the coordinates of the features and information
about how each shape relates to those around it.
Describing the information
Finally, the computer needs to hold descriptions of the geographic information.
This is also done using tables containing descriptions that are linked to the
tables containing the shape information.