*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.