Describing the Information
As well as recording where features are,
geographic information also needs to be capable of recording what features
are. This is known
as attribute information.
When using a paper map, this aspect of the geographic information is held in
the legend located at the margins of the map rather than being located within
the mapped area itself. Consequently, it is known as aspatial information because
the attribute component of geographic information does not hold any information
about location – it is purely descriptive information about what the
points, lines and polygons represent.
Describing vector data
In the same way that the information needed to draw points, lines and polygons
using the vector method can be held in one or more tables, so the attribute
information can also be held in tables. The following table holds a information
about two polygons that form the boundaries of two agricultural fields. The
crop planting history of the last two years is recorded in the table.
Note that the table above contains the name of each polygon so
that the polygon
topology tables can be linked
to this table. Geographic information systems are able to perform this linkage.
Describing raster data
Raster data does not use
tables of information linked to the grid shapes to describe the information.
Instead, each grid cell contains a single code
that
represents the geographic information held within the cell. Only one piece
of information can be contained within each grid cell with the raster method
of representation meaning only very limited description information can be
held by the raster method.