I found an interesting document that explains some of the aspects of this
question (and probably why online gravimetric maps are rare) while giving
a very interesting background on, er, geodesy in the USA during the last
~50 years:

http://geodesy.ceegs.ohio-state.edu/50_years/Olentangy.pdf

I wonder what the equivalent Russian (I dare not say USSR) paper looks
like.

Peter

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