ERMs
Topographic Surface is the interface between the solid and liquid/gas portions of the Earth. It corresponds to the surface of the land and the floor of the ocean.
In the SRM, there is no Earth; there are only models of the Earth.
- Earth Reference Model (ERM) is a specification of the math-ematical shape of the Earth, usually in terms of a com-bination of ellipsoidal and equipotential (geoidal) surfaces. It excludes the topographic surface, and therefore generally corresponds with mean sea level.