Actually, there is some elasticity. You have to factor in the changes in gravity due to the passing of the Moon, and, if near the coast, the extra loading due to high tides. People living on the less dense type of rock would be paying less than those on basalts or other dense rocks, and altitude would also change the force - the higher the less. When I did my OU geophys, we had a tremendous number of corrections to apply to readings. Mind you, they could be done by computer.

Penny