Earth models describe shape and size
geometrically. There are three groups of earth modells, see
2007, p. 38f]:
mathematical 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
especially for the heights
The different earth models descibe different
reference surfaces at different levels of details.