What is "Sea Level"?

Julian Solos

What is "Sea Level" as used in describing the height of a mountain?

nautica

Sea level is just what it says. It has arbritrairily been defined as zero, which is the level of the sea.

Nautica

Julian Solos

Is it local or the average of the sea levels at various locations on the Earth.

If it's local, the sea level of which sea was used as a reference point for the following description?

"It is well known that the HIGHEST mountain on earth is Mt. Everest on the Nepal-Tibet border in the Himalayas. It stands 8,848 m (29,028 ft) above sea level." [US Geological Survey, Pasadena.]

selfAdjoint

The "sea level" used is the level of the world ocean (you can see that all the oceans and many of the seas are physically linked, right?). Water seeks its own level so the sea level, at least relative to the underlying geoid (shape of the Earth's surface) is well defined. It is first defined by geodesy, using pendulums and stellar obsrvations to pin down the geoid (I suppose nowadays they use GPS). Then from this base value the benchmarks of altitude above or below sea level are established by surveying.

Monique

Um, so how about tides??

LURCH

It's averaged out for tides.

But the quote in question only speaks of Mt Everest being the "highest" mountain, and then gives its altitude above sea level. Of course, this does not take into account that the foot of Everest is also well above sea level. So although Everest's peak is the highest above sea level, it is not the tallest mountain if you measure from the foot of the mountain to the peak.

Everest is about 29,000ft from base to peak, while Mauna Kea, in Hawaai, stands nmore than 33,000ft from the seafloor. And although Chimbarazo, in the Andes, is only about 20,000ft above sea level, its peak is farthest from the Earth's center (because it's closer to the equator).

Monique

Ok, so you are standing on top of Everest, how do you know how far above sealevel it is? How do you know how far the sea is above sealevel, anyway? [6)]

For instance the Mediteranian sea, it is in connection with the Atlantic ocean through a small opening: the Street of Gibraltar.. does it mean that the Mediteranian sea is always at sealevel? (minimal effect of tides?)

But then I observed that Lake Erie in the US seemed to even have tides..

nautica

Sea Level is set to zero. It does not matter what tides do, the sea level measure does not change. I am sure if you wanted to take the time to do the calculations you could find the height of the tides due to the moon and the sun at any given time and find the exact hight of the sea. Or you could just use a gps and find it out.

Nautica

selfAdjoint

Lake ERIE tides? That's a new one on me. AFAIK the much larger Lake Michigan doesn't have tides, and I've been around it and on it a bit. The great lakes are not at sea level, because water flows down from them (over Niagra for instance) to the world ocean. The Saint Laurence Seaway which leads from an arm of the Atlantic to the lakes has locks in it to raise the ships to the higher level.

You know the height of Everest above sea level because it has been surveyed. You don't have to climb it to survey it. You just have to know its distance and its angular height above a known benchmark and then trigonometry gives you the answer. The British did a heroic scale surveying job in the Himalayas in the 19th century. Of course now with radar ranging and such it's much easier, and more accurate.

Nereid

SelfAdjoint mentioned the geoid, and the modern use of GPS in its determination.

It won't come as a surprise to any reader, I'm sure, that determining the geoid is far from straight-forward, if you want cm precision.

This page - from the GRACE website - gives an indication of what's now being done with such an accurate geoid. The whole site is really fun to explore.
http://www.csr.utexas.edu/grace/grav..._sciences.html

[Edit: posted wrong link! corrected]

Monique

Yeah, so I went to the beach today (I did).. say I wanted to measure how much the water was above set sealevel, or below, because of tides.. how would I do that? I mean.. what is the reference??

Actually the water must've been really high last night, since the sand was compressed all the way up to the dunes..

Jeebus

I have researched a bit and it is said that sea-levels can be measured by depth gauges at the end of the pier to satellite altimetry. Even the satellite, Topex-Poseidon can measure sea-levels. Now, if you wanted to meausure the sea-levels by tides I think the reference would be, I guess, tide gauges.

Monique

Well, I really don't get it [6)] how can you measure that the water along the coast of Holland is of the same level, as the water along the coast of Australia.. halfway around the world?

Is there somewhere on the world a beacon which says: this is sealevel? Like there is the Greenwhich timezone, which says: this is t0?

LURCH

That's what I mean by "averaged"; find however high the water gets at high tide, and however low it gets at low tide, determine the average between these two, and that is Mean Sea Level.

As for tides in the Great Lakes, yes we have them, but they're only about 2 inches different. This channge twice daily is almost impossible to detect without intruments and long-term records, as the change in water level daily (due to meteoralogical effects) is much greater. A few years ago here on Lake Huron, we had a seiche, and that was pretty freaky! The tide just suddenly went out many yards. A lot of people who live on the shoreline awoke to find their property doubled or tripled in size. Personally, I would have been terrified and gotten my familly out of the house and up to high ground. An out-surge like that is usually a precurser to a tidal wave. Fortunately, there was no large wave, levels just returned to normal over the course of the day.

Monique

Well, that makes sense :)

Bystander

http://www.gmat.unsw.edu.au/final_ye...n/monitr_t.htm

russ_watters

No, sea level is a local thing and you build your 3d model based on local sea levels.

To find your local sea level, take some random pier in Holland, make marks on the pilings every 15 minutes for a couple of years, and take the average. That's mean sea level for that pier in holland. Then do the same thing in Australia and in a couple of hundred other places and you can build a model.

If your difficulty isn't with figuring it out but relating it to sea level somewhere else, its just a definition. Its arbitrary. No, measuring sea level in this in Holland an Australia does not actually tell you how far the two points are from the center of the earth or really locate them in 3d space.

And as said before, building the model used to be a difficult thing, but with GPS and radar, now you can relate all those points quite precisely and build a real 3d model from it. However, even before you could build a real 3d model of the world, you could still define sea level in Holland and Australia to be the same thing. It is a rather arbitrary (though logical) choices for a datum.