Here is a gravity map of the Moon’s southern area made from data collected by the GRAIL (Gravity Recovery and Interior Laboratory) by S. Goossens and others.
Because the moon is not a smooth homogeneous sphere of equal altitude, gravity varies from area to area, just as it does here on Earth. The GRAIL spacecraft was able to measure gravitational differences from what would occur of the Moon was like a cue ball. The differences are expressed by color, in this case purple is the low end of the range, yellow is average and red is at the high end.
Get an more in-depth explanation at NASA Goddared Space Fight Center’s photostream at Flickr (along with other pictures).by