High-resolution spherical harmonic representation of the Earth's topographic gravitational potential based on a three-layer decomposition of the topography with variable density values.

Main features:
- Three-layer decomposition of the topography using information of the new 1'x1' Earth2014 topography model
- Rigorous separate modeling of rock, water, and ice masses with layer-specific density values: Rock: 2670 kg m-3, Water: 1030 kg m-3 (Ocean), 1000 kg m-3 (Inland), Ice: 917 kg m-3
- Ellipsoidal arrangement of the topography using the GRS80 ellipsoid + geoid undulations as height reference surface
- Additional compilation of a consistent rock-equivalent version REQ_TOPO_2015 using condensed DTM-heights

Processing:
- Forward modelling in the space domain using tesseroid mass bodies
- Transformation of global gridded values to the frequency domain by applying harmonic analysis up to degree and order 2190

Model versions:
- Spherical harmonic coefficients of the RWI model are provided by two versions (GM = 3.986004415e+14 m3 s-2, a = 6378136.3 m):
RWI_TOPO_2015 (topographic potential)
REQ_TOPO_2015 (topogr. potential of rock-equivalent heights)

• To allow the evaluation of the RWI model by synthesis software that by default subtracts the coefficients of a normal gravity field, two additional versions are available:
  RWI_TOPO_2015_plusGRS80 (RWI_TOPO_2015 + GRS80)
  REQ_TOPO_2015_plusGRS80 (REQ_TOPO_2015 + GRS80)

where the following zonal harmonic coefficients of the GRS80 normal gravity field are added to the coefficients of the RWI model:
C( 0,0) = 0.100000014676351e+01
C( 2,0) = -0.484167032228604e-03
C( 4,0) = 0.790304535833168e-06
C( 6,0) = -0.168725253450154e-08
C( 8,0) = 0.346053594536695e-11
C(10,0) = -0.265006548323563e-14
C(12,0) = -0.410788602320538e-16
C(14,0) = 0.447176931400485e-18
C(16,0) = -0.346362561442980e-20

Note that these coefficients are already rescaled to the above specified parameters GM and a of the RWI model.

Details about the used Earth2014 topography model can be found in Hirt and Rexer (2015, https://doi.org/10.1016/j.jag.2015.03.001).