Geodetic Datum Type geodDatumType extGeodDatumType refinedGeodDatumType refinedExtGeodDatumType POSC 1.0 Geoidal The geodetic is a geode, described as a surface of constant gravity potential. An example is mean sea level. These datums are generally set by standards groups, and involve considerable measurement history. Depth The datum is set as a specified depth. Barometric The datum is a surface of constant barometric pressure. Surface The datum is a surface, whose meaning is generally determined by the application using it. For example, a datum may be the top of a geologic formation.