ASL on other planets

[Sirrobert]

Ok so I understand ASL on earth. It's the avarage sea level.

But in KSP (I have no idea if we also do it in real life), we also use ASL for other planets, even though they have no sea.

Now for some this makes abit of sense. ASL on minmus is exactly the height of the flats. But on the Mun, I don't think I've ever even seen a spot below ASL, so it's hardly the avarage height of anything.

So how does this work?

[Rakaydos]

I usually see the phrase "datum" instead of Sea level used for other bodies.

Not sure how they deturmine "datum" altitude, though.

[Lexif]

The datum is where you want it to be, that's how it works. For planets with interconnected oceans, it's kind of obvious, but even then it's not so simple in the real world. The earth's sea levels are at different distances from the earth's center as the earth is not spherical and does not have a homogenous density profile... (Plus other effects, like tides, currents, etc...) Even on earth, different countries use different datums. For example, they built a bridge over the Rhine between Switzerland and Germany, and both countries use different datums for their altimetry data. A contractor made a mistake like confusing a sign when converting between the datums, and the bridge heads were off by a meter or so because of that...an expansive mistake. If I remember it right, the germans use a datum based on mean sea level at some north sea port, while the swiss use the exit of Lac Leman at Geneva, but which is in practise linked by measurements to mean sea levels in the adriatic sea or something like that...