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"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 |
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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 |
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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. |
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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 |
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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 |
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