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In a recent image by Pavel Gabzdyl, the highest (+10.7 km) and lowest (-9 km) points on the Moon are marked.

enter image description here

Detailed info about these extremal points can be found at the following links:

Highest Point on the Moon!
Great Wall!

My question is how are elevations on the Moon exactly defined? A 1973 paper on lunar altimetry made by Apollo 15 and 16 missions defined elevation with respect to a 1738km sphere about the center-of-mass.

Is the same principle used today? What is the accepted radius of the sphere? How is measured (not defined) center-of-mass of the Moon, and how is that all related to equipotential surface (in analogy with sea level)?

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I will try to summarize current state of comments in an answer (which I am not going to accept in hope someone will post a better one).

It seems that elevation at Moon is defined relative to a sphere (since Moon, unlike Earth, has negligible flattening) with radius 1737.4 km.

Its center is Moon’s center of mass what makes sense since the reference system defines not only elevation but also two other coordinates (selenographic latitude and longitude) and axis of rotation passes through center of mass (rather than center of figure for example).

I still don't understand relation between center of mass and equipotential surfaces but this is probably irelevant, since even at Earth we no longer define elevation relative to mean sea level (which is a textbook equipotential surface) but relative to ellipsoid (with center of mass as its center).

It seems that the most authoritative source is this pdf document:

Lunar Constants and Models Document

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