| It is often interesting to combine
satellite images with other, continuous, data sets. For example, such
additional data can concern altitude, soil, demography, etc. However,
one can only combine such an external data set with a satellite image
if the data are spatially defined. This allows them to be converted
into a common geographical reference system together with the image.
The data are then unambiguously established with three co-ordinates:
an x- and a y-value in the same plane, and a z-value as the third
dimension. On this basis one can visualise the change of z as a
function of x and y in a 3D representation.
The z-values can thus be elevation data, in which case one speaks
of a Digital Elevation Model (often abbreviated as DEM). In other
cases one speaks of a Digital Terrain Model (DTM).
After a satellite image has been combined with a DEM, one gets
a representation like that shown here (representation, in casu Hangzhou