4.9.2011
Earth models describe shape and size
geometrically. There are three groups of earth modells, see
e.g. [
Voser
2007, p. 38f]:

mathematical earth models
 physical
earth models

topographical earth and landscape models
an earth model also needs a geodetic datum to
describe the position and orientation of the earth model in
relation to the earth.
different earthmodels descibe different
coordinates,
especially for the heights
The different earth models descibe different
reference surfaces at different levels of details.