National Geodetic Survey - http://www.ngs.noaa.gov
Geodetic transformation operating on geographic coordinate differences by bi-quadratic interpolation. Input expects longitudes to be positive east in range 0-360° (0° = Greenwich).
The latitude, longitude and ellipsoidal height offsets at a point are derived by interpolation within the gridded data. Separate grid files are given for latitude, longitude and ellipsoidal height offsets. The grid file format is given in documentation available from the information source. Bi-quadratic interpolation is used to derive the offset values. For the forward calculation the interpolated value of the offset is then added to the source CRS coordinate value to give the coordinates in the target CRS. Reversibility Iteration is required for the reverse transformation. The coordinate reference system for the coordinates of the grid nodes is the source coordinate reference system for the forward transformation. Then in forward transformations the offset is obtained through straightforward interpolation of the grid file. But for the reverse transformation the first grid interpolation entry will be the value of the point in the second coordinate reference system, the offsets are interpolated and applied with sign reversed, and the result used in further iterations of interpolation and application of offset until the difference between results from successive iterations is insignificant.