# What counts as "0 mGal" in these gravity maps?

In the following gravity map of Mars the changes in gravity are shown according to mass distribution:

Map removed

However, since no moon or planet other than the Earth with 9.80665 m/s² has a surface gravity defined by standard, I wonder what counts as 0 mGal there. We see the differences from an obviously average gravity that isn't given. The average surface gravity of Mars is 3.72076 m/s². Do you know whether the value was used on that map?

I have a same question for the following gravity map of the Moon (was 1.625 m/s² taken as average?):

Map removed

I wonder why they don't mention which gravity they use as standard. Gravity maps like these are far more informative:

Map removed

• I have a hunch that the maps show deviations form a spherically symmetric potential on a spherical surface with some reference radius, but I cant say that with surety. You may find this answer and others there helpful, as well as the discussion of Bouguer gravitational anonaly in this question. You might also find discussions on this and this page interesting.
– uhoh
Commented Apr 6, 2020 at 11:49
• @uhoh The "0 mGal" value should be mentioned in Bouguer anomaly maps.
– user35272
Commented Apr 6, 2020 at 12:08
• There is no "0 mGal" value. It's mostly differences to what you would expect from a perfectly homogeneous body. All the effects of height, oblateness, latitude, terrain shape ... have been subtracted. It's not a "gravity map" that tells you which gravity you feel at any given point. Commented Apr 6, 2020 at 14:30
– user35272
Commented Apr 6, 2020 at 15:09
• I will when I have time. Writing an answer takes a bit more time than commenting. Btw, you might want to comply with the licenses of the images you copied here. In wikipedia they are licensed cc-by so that you are required to name the author. Commented Apr 7, 2020 at 10:50