Barthelmes, F. (2009): Definition of functionals of the geopotential and their calculation from spherical harmonic models: theory and formulas used by the calculation service of the International Centre for Global Earth Models (ICGEM), http://icgem.gfz-potsdam.de, (Scientific Technical Report STR ; 09/02), Potsdam : Deutsches GeoForschungsZentrum GFZ, 32 S.: Ill., graph. Darst. p.https://doi.org/10.2312/GFZ.b103-09026