A time-efficient routine for the computation of terrain correction

In geoid computation, terrain correction (TC) removes topographic gravitational attraction but is computationally intensive. This research proposes two hybrid routines. The first approach calculates Individual Zonal Radius (IZR) for grid cells with slopes >45 degrees. For grid cells with IZR larger than twice the standard deviation of IZR, a hybrid approach with inner zonal radius equal to Minimum Zonal Radius (MZR) is used; for other cells inner zone radius will be twice the standard deviation of IZR. The second routine uses hybrid approach with an inner zone radius equal to MZR for cells with slopes >35 degrees and a spectral method otherwise.