What is "sea level" on other planets?

[Themohawkninja]

When we measure the heights of mountains, we say that they are "X feet/meters above sea level", which means that the summit of the mountain is X feet/meters above the mean level of the sea. Where is sea level on planets like Mars, where there is no sea to act as a baseline?

[Kerbface]

Yeah, I always wonder how they decide how tall a mountain like Olympus Mons is and all that.

[Brotoro]

They choose the datum ("sea level") elevation arbitrarily. On Mars they chose it to correspond to an atmospheric pressure below which ( elevation above which) liquid water could not exist no matter what the temperature was.

[searlefm]

the reason we use sea leave is because sea covers around 70%~ of the surface of earth its common hear,

such a term is irrelevant on most planets in the Sol system, so average altitude would be a more appreciable term on planets/astral bodies with out liquid on the surface.

In KSP we use lowest point on the planet as the reference number on the top of the HUD.

i hope this helped you

[metaphor]

What Brotoro said. Mars and other planets have a "datum" around which elevations are measured. For example, Venus's elevations are measured from a radius of 6051 km from the center of the planet.

For mountains, usually the height above the surrounding terrain is used when comparing mountains on different planets.

[Dunbaratu]

For mountains, usually the height above the surrounding terrain is used when comparing mountains on different planets.

The problem with that is that the "edge" of a mountain is often a purely arbitrary point. The land starts out flat, then slopes up and up slowly getting steeper and steeper - where along that slow rise is the "bottom" of the mountain? Where does the "slightly tilted plain" become "the mountain"? With a thing like a volcano it's a bit easier because they often do have a distinctive shape above surrounding terrain, but for uplift mountains it's harder to define the "edge" of the mountain. For example, most of Tibet is uplifted high by the same tectonic effect as the Himalaya mountains, but it's a large sloping upward toward the mountains. Whereabouts is the northern "edge" of the Himalayas? If the land is at a grade of 5% is that enough to be part of the mountain? What about when it rises to 10%? How about 15%? 50%? At what point is it steep enough to be "the" mountain and not the plain?

[Lexif]

The problem with that is that the "edge" of a mountain is often a purely arbitrary point. The land starts out flat, then slopes up and up slowly getting steeper and steeper - where along that slow rise is the "bottom" of the mountain? Where does the "slightly tilted plain" become "the mountain"? With a thing like a volcano it's a bit easier because they often do have a distinctive shape above surrounding terrain, but for uplift mountains it's harder to define the "edge" of the mountain. For example, most of Tibet is uplifted high by the same tectonic effect as the Himalaya mountains, but it's a large sloping upward toward the mountains. Whereabouts is the northern "edge" of the Himalayas? If the land is at a grade of 5% is that enough to be part of the mountain? What about when it rises to 10%? How about 15%? 50%? At what point is it steep enough to be "the" mountain and not the plain?

You can approach that problem with statistics. Take a look at the hypsographic curve of earth, for example:

It shows that earth has two domains, continents are mostly 0--1 km high, oceans mostly 5--4 km deep. So it makes sense to compare Mount Everest's height to sea level, but to compare, say, Mauna Kea's height to the average depth of the oceans. Also, for Mount Everest, you might just as well argue that it rises above the Indian plains, as it sits on the watershed between India/Nepal and Tibet. (I think?) But the Tibet plateau is just mindboggling. https://upload.wikimedia.org/wikipedia/commons/6/66/MountEverestRelief.png

The hypsographic curve of Mars shows two domains, for example, for the differen hemispheres. The other planets basicall only have one peak, making a single datum the most logical choice.

Putting the lowest point of a body to zero elevation sounds impractical to me if you didn't design the planet yourself. Because you first have to discover the lowest point.

And by the way, the "sea level" contour on earth is just a surface of equal gravitational potential. Because the earth isn't a sphere, and has an equatorial bulge, Mount Everest isn't the furthest point from the center of the earth. That's the Chimborazo in Ecuador. It's only 6268 m high, but very close to the equator.

https://en.wikipedia.org/wiki/List_of_mountains#Summits_farthest_from_the_Earth.27s_center

[Flixxbeatz]

Zero meters. Except Jool, with -98 meters.

[DerekL1963]

And by the way, the "sea level" contour on earth is just a surface of equal gravitational potential.

Depending on which coordinate standard you use... cartographers invented "standards are fun because there are so many of them" long before computer marketing 'geniuses' did. In the UK, they use a set of measurements performed back in 19teens and twenties that averaged sea height at a particular location in Cornwall.

[Themohawkninja]

Zero meters. Except Jool, with -98 meters.

I am referring to reality.

[Lexif]

Depending on which coordinate standard you use... cartographers invented "standards are fun because there are so many of them" long before computer marketing 'geniuses' did. In the UK, they use a set of measurements performed back in 19teens and twenties that averaged sea height at a particular location in Cornwall.

Those differences are "fun". I remember when they built a bridge between the swiss side of the Rhine and the german one. They accidentally subtracted the difference between the two national datums used instead of adding it (or somesuch). So they had to recast the foundation on one side as it was something like half a meter of.

But what I meant was the surface of an (overly) idealized ocean, leaving out other effects like wind, tides etc.

[pyrosheep]

wouldn't it just be the mode altitude above lowest point?

[WilliamW2010]

On 7/20/2013 at 11:14 PM, searlefm said:

the reason we use sea leave is because sea covers around 70%~ of the surface of earth its common hear,

such a term is irrelevant on most planets in the Sol system, so average altitude would be a more appreciable term on planets/astral bodies with out liquid on the surface.

In KSP we use lowest point on the planet as the reference number on the top of the HUD.

i hope this helped you

Does this apply to Moho (meaning the Mohole stretches down to 0 meters in elevation)? 

[Nuke]

On 7/20/2013 at 1:14 PM, searlefm said:

the reason we use sea leave is because sea covers around 70%~ of the surface of earth its common hear,

such a term is irrelevant on most planets in the Sol system, so average altitude would be a more appreciable term on planets/astral bodies with out liquid on the surface.

In KSP we use lowest point on the planet as the reference number on the top of the HUD.

i hope this helped you

the mode makes more sense than the average as it kind of filters out all the outliers. but then again you could always just use the lowest point if you dont want to deal with negative elevations. and thats not exactly unheard of on earth either (see death valley). besides as resolution increases you  might find lower lows and higher highs and then you either have to revise all your elevation data or put up with the offsets.

 

i just realized i walked into a zombie apocalypse.

Edited May 27 by Nuke

[kerbiloid]

The lunar sea level is the average of the lunar ebb and the lunar flow, caused in the lunar seas by the Earth tides.