Remember that gravimetric measurements are an established part of geology/geophysics and have been around for a long time. I believe it was in the late 1800s that gravimetric surveys in India showed that the Himalayas seemed to exert less of a gravitational pull than expected, which led to the discovery that mountains have extremely deep "roots", that is, mountains are like icebergs, with the part you see much smaller than the part beneath them.
The point is that, even at the rough industrial level as opposed to precision labs, it's rather amazing the precision that can be achieved.
This brings me to one of my favorite hurricane faqs:
http://www.aoml.noaa.gov/hrd/tcfaq/C5c.html