Mean Altitude on Minmus

[Clipperride]

Not sure if this is the right place for this query as it's more about the KSP universe than a gameplay question.

On a celestial body that has no water, an altitude of 000m would be the mean altitude of the planet/moon.  But, the plains of Minmus are (as far as I know) the lowest points on the satilite and they are surrounded by mountains that rise up around them.  Why then are the flats at 0m?  Am I missing something or is this just a design quirk?  It's also odd how, after many many hours of game play, it's only just occurred to me!

[OhioBob]

While I agree that in real life a body's datum is placed close to its mean surface elevation, it is customary in KSP to place the datum at the point of lowest surface elevation.  I'm not sure why that decision was made, but we're pretty much stuck with it.  Placing the datum at the mean surface of course means that much of the surface will be at negative elevation.  This results in problems the way that KSP is currently coded.  While I've heard rumors of certain problems, the only issue that I've personally observed is with the radar altimeter in IVA mode.  The radar altimeter is suppose to display height above terrain.  It works correctly when the terrain elevation is positive, but when negative the altimeter displays the height above the datum rather than the terrain.  This makes the altimeter worthless when landing at a site that lies at negative elevation.  The same problem exists with the "height above terrain" readout in KER, and I'm guessing MechJeb as well, though I haven't tried it.  I presume these mods simply get the altitude from KSP.  It seems that in order to use negative elevations effectively, KSP needs to change.
 

Edited January 26, 2018 by OhioBob

[Spricigo]

 

There is any real advantage in following RL standard other than "follows an standard"? Genuine question, I can barely imagine why is like that in RL.

[Clipperride]

Thanks @OhioBob that makes perfect sense and something I hadn't considered.

@Spricigo As long as the system used is consistent, I can't see it making any difference as opposed to using the RL alternative (other than giving players something to think about when they notice it!). After all, a convention is just that and the way KSP deals with altitude could have just as easily been adopted by NASA etc when mapping other worlds

[Corona688]

I suspect they did that because negative altitudes on the mun would really freak out and confuse people trying to land.  You always hit something by 0m on Kerbin.

[Hodari]

18 hours ago, Spricigo said:

 

There is any real advantage in following RL standard other than "follows an standard"? Genuine question, I can barely imagine why is like that in RL.

My guess is that it would be very hard to be certain that you've found the absolute lowest possible place on the surface.  There would always be the chance that you missed one spot somewhere...or even that the surface might change somehow and a new spot might become the lowest point. Then your height measurements for every other point on the planet would no longer be correct.  Whereas the mean elevation will probably be far more constant and missing one or two extreme points wouldn't change the value that much, so you wouldn't have to go back and readjust all of your other numbers.

[OhioBob]

19 hours ago, Spricigo said:

There is any real advantage in following RL standard other than "follows an standard"? Genuine question, I can barely imagine why is like that in RL.

Other than what @Hodari wrote, I'm not sure what advantage there is to it.  However, it does make sense to me to use a reference sphere or ellipsoid that most closely approximates the mean surface of the celestial body.

I played around with the idea of setting the datum for bodies in KSP at the mean (or perhaps median) surface, but that's when I discovered the radar altimeter problem.  My rationale was this...  the atmospheric pressure that we define for a body is the pressure at the datum, or "sea level".  This makes sense for an ocean world where perhaps half or more of the planet's surface lies at sea level.  However, for bodies without oceans, I thought it made sense that the atmospheric pressure we see in the Tracking Station, etc., be the median surface pressure, with some places higher (lower elevations) and some places less (higher elevations).  But the way KSP does it, the atmospheric pressure that we see is actually the maximum pressure at the body's lowest surface elevation.  We look at Duna and think, ah, the surface pressure is 1/15th of an atmosphere.  But no, nearly the entire planet has a surface pressure less than that.  The way KSP does it, I find the atmospheric information deceptive and far less useful than if the datum were placed at the mean surface.