This is the earth's geoid - the surface of constant gravitational and rotational potential energy, and
represents the surface that water would take under that influence. It gives a good visualisation of
what GPS has to correct for.

http://icgem.gfz-potsdam.de/ICGEM/potato/Geoid-eigen-6c-Ellipsoid.jpg

There are more accurate gravitational maps from GRACE and GOCE missions, but the datasets are on a
commercial basis.