For Earth, the reference surface for "GPS" latitude, longitude, and altitude is the WGS84 ellipsoid, with equatorial and polar radii $a, b$ of 6378.1370 and 6356.7523 kilometers, respectively. Altitudes are calculated as the distance from a point to this reference surface along a line normal to the surface, not along a line from the geocenter. Math is given in this answer.
Does the Moon's selenographic coordinate system have a reference surface? Is it a sphere or an ellipsoid or something else? What are it's dimensions?
I've looked in Wikipedia, and in
- A Standardized Lunar Coordinate System for the Lunar Reconnaissance Orbiter
- Determination of Selenographic Positions
- Selenographic Control
but these are not helpful and the Moon's "selenography" has been evolving as lunar exploration and metrology has increased over time.
Question: What are the dimensions and shape of the Moon's reference surface for selenographic latitude/longitude?
below: exaggerated illustration of Earth's ellipsoid, from here.