I am using this way to the above problem

The normal gravity at the earth surface was added to free air anomaly at the surface from EGM2008. The results are improved when compared with observed control point values by adding the RTM effects to above mentioned data of normal gravity and EGM2008 free air anomaly.

May please correct if there is something missing

You might just explore the Satellite DATA from GRACE which has already

mapped the surface of the Earth in mili-gal variations.

I usually use spherical harmonic method with EGM2008 coefficients up to degree and order 2190 to generate almost real data on the earth surface, i have checked these data with gravity observations and they had RMS around mGal in a not rocky region.

Ismael.

i think there are no missing,because i use from same approach to predict Deflection Of vertical from geopotential coefficient and RTM modeling,

before this we selet an optimal coefficient model that evaluated in the region of intrest