"Sea-level" on various planets?

Hi,

First off, is there another term for "sea-level", when talking about
planets? "Zero-Altitude Reference Point" or something? Second what is
this point on the planets and moons of our solar system and how is it
decided?

Thanks

Andre

In article , Tarken
wrote:
First off, is there another term for "sea-level", when talking about
planets? "Zero-Altitude Reference Point" or something? Second what is
this point on the planets and moons of our solar system and how is it
decided?

The datum points for the "geoids" of other planets are decided
by selecting some particular feature and declaring it to be "zero".
Quite arbitrary.
I do not know if the vertical, latitudinal and longitudinal
references for other planets are coincident in time and space - were I
to be choosing the points, I'd nail them all to a prominent rock
somewhere that's not obviously unstable. But I'm not everyone.
BTW, what *exactly* do you mean by "sea level"? Do you mean the
mean level of the sea, as defined by your local hydrographic survey
authority? Or do you mean the "Lowest Astronomical Tide level" for the
site. The two can, of course, differ by some 10s of feet. And when
engineers design altitude-critical equipment, with one part of the team
working to altitudes above MSL, and the other part of the team working
to aptitudes above LAT, then expensive mis-fits can occur. *VERY*
expensive.
Sea level is fixed, right? Wrong. In individual human careers,
fixed items have moved by up to 14ft w.r.t "sea level", without
earthquake, and without nearby similar equipment being affected.

"Sea Level" is as artificial as the price of gold - there's some
relationship to immutable laws of nature, but a much stronger
relationship to the chances and contingencies of history.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

Sea level is fixed, right? Wrong. In individual human careers,
fixed items have moved by up to 14ft w.r.t "sea level", without
earthquake, and without nearby similar equipment being affected.

14 feet? According to
http://www.greeningearthsociety.org/...s/2000/sea.htm, mean sea level
has gone up maybe 10 centimeters over the last century. Maybe.

In article , Dave & Janelle wrote:
14 feet? According to
http://www.greeningearthsociety.org/...s/2000/sea.htm, mean sea level
has gone up maybe 10 centimeters over the last century. Maybe.

You're missing the point. Maybe I expressed it badly. I'll try again.
In the time it takes to turn a page, "sea level" has moved by up to
14 ft. This has had consequences, expensive ones.
The important point is, of course, the quotes around ""sea level"" -
different people have different conventions about what constitutes "sea
level", and there have been major engineering projects where people from
different branches of engineering have got confused over which convention is
being used on which page of a design brief. The consequences were, in the
particular case I worked beside and watched being built, *expensive*. 14 ft
of steelwork that's not there is rather difficult to insert. Particularly
when you don't notice the mismatch until after you've put the 20,000 ton
process module on top of your steelwork and tried to run it. Oh, and the
only (mobile) crane in the world that can pick up the process module has
moved on to it's next job.

"Sea level" is a term that has an approximate physical meaning, but
once you get beyond a certain level of detail, then the exact terms of your
specification do matter. And those detailed definitions can be substantially
different.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

Aidan Karley wrote:
BTW, what *exactly* do you mean by "sea level"? Do you mean the
mean level of the sea, as defined by your local hydrographic survey
authority? Or do you mean the "Lowest Astronomical Tide level" for
the
site. The two can, of course, differ by some 10s of feet. And when
engineers design altitude-critical equipment, with one part of the
team
working to altitudes above MSL, and the other part of the team
working
to aptitudes above LAT, then expensive mis-fits can occur. *VERY*
expensive.

What I mean by sea-level is a "zero-refence point", from where all
geographical hights are measured. For example my house is at 400m, and
I am flying a plane at 2000m. The alternative would be to use height
measurements which are actually distances from the centre of the
planet,

My house would then be 6371.01m + 400m
and my flight at 6371.01 + 2000m

though I get the feelings that most people would not find this the most
convenient of "vertical" reference points.

Andre