1.2 Displaying geographic data
Geographic data are organized in a geographic database. This database can be considered as a
collection of spatially referenced data that acts as a model of reality. There are two important
components of geographic data (Figure 1.1.12): the geographic position and the attributes,
entities or properties. In other words, spatial data (where is it?) and attribute data (what is it?)
are distinguished. In the example of Figure 1.1.12, you can see that we have a (very simplified)
map on one side and a table on the other.
Maps are considered spatial data, since the information they contain is directly related to
certain locations on the earth’s surface. The location of the units A, F and G are specified with
respect to their X and Y coordinates.
Tables, on the other hand, do not contain direct information on a location. They contain
descriptive information (in this case the names of the land use types and the value of the land in
monetary units). If we would only have the table, the data would not be useful, since we don’t
know where the units are located. If we would only have the map, we still don’t know anything
about the units. In a Geographic Information System (GIS) like ILWIS, the link between spatial
and attribute data is the key to get real information. Only by combining spatial and attribute
data we can get answers to questions such as: Where are the land use units with a value more
than 250?