The digital finite-element height reference surface (DFHRS) research and development project (www.dfhbf.de) (German word DHFBF = Digitale Finite Elemente Höhenbezugsfläche) is aiming at the computation of a parametrized and georeferenced height reference surface (HRS), called N=N(p|B,L,h) in the following (fig. 1). The HRS parameters p are stored in a DFHRS data-base (DFHRS_DB).
Depending on the height system type, physical heights H (fig 1) are called the orthometric heights, the normal heights (the European height system), or the spheroidal normal heights (NN-heights). The respective HRS, abbreviated with N (fig. 1), is accordingly either the geoid, the QGeoid or the NN-surface. N is always the height of the HRS above the reference ellipsoid.
The main practical target of the DFHRS project dealing with the development of integrated approaches for the computation of Geoid or QGeoid HRS N (fig. 1) is to enable the conversion of ellipsoidal GNSS heights h (determined at the earth topography) into the physical earth gravity field based standard, or “sea-level” heights, H by the direct transformation H = h – N.