Naked Science Forum

Life Sciences => Marine Science => Topic started by: Hannah LS on 11/12/2018 13:37:44

Title: Is the 'sea level' different in different areas of the world?
Post by: Hannah LS on 11/12/2018 13:37:44
Muhammed asks:

Are there parts of the ocean that are continuous but have a different "sea level"? I know gravity is slightly different in different parts of the world as has been recorded on land, but is this phenomenon significant at sea?

Could you be in one place at sea that has a height at sea level of 0 and is used as a reference height and then travel a good distance away to another place where gravity is higher and thus sea level would actually be negative with respect to the original measurement point?


What do you think?
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: Colin2B on 11/12/2018 15:12:31

If you were to measure from the centre of the earth, then what you say would be true. However, it is very useful to have a sea level that doesnít vary with gravity and this is defined as the geoid. This is also extended onto land and is very useful eg if you have a canal which follows the geoid height (or is a fixed ht above or below it) then water will not flow from one end to the other.
The physical shape of the earth is called the ellipsoid and is the one used by gps. The GRS80 ellipsoid and the geoid vary by about +85 meters west of Ireland to about -106 meters  in the south of India near Ceylon.
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: Janus on 11/12/2018 16:14:51
Just one addendum to what Colin2B said.  The geoid follows the surface of equal gravitational potential, not the surface of equal gravitational force.  So the local strength of gravity will vary at different points along the geoid, or at "sea level".
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: evan_au on 11/12/2018 19:44:17
Footnote: This discussion is about "mean sea level". Superimposed on the geoid are wind waves and tides which are ignored when you take a long-term average.

This discussion also ignores things like the antarctic ice shelf, where the surface of the liquid water is pushed down below the geoid by the kilometer of ice floating above it.
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: Colin2B on 12/12/2018 13:12:02
What @Janus and @evan_au have added is the reason I love the geoid, there is so much richness in it that itís hard to know how much to tell a newcommer.
So, to whet MuhammedĎs appetite even further: where the geoid curves up or down a plumbline will not point geographically straight down towards the centre of the earth, but will lie perpendicular to the tangent to the geoid. Very good for detecting deposits of heavy minerals below the surface, along with measures of gravitational potential.
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: DrMortimer on 07/01/2019 15:59:02
I believe the gravity can't influence on water in such way
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: Colin2B on 08/01/2019 08:08:58
I believe the gravity can't influence on water in such way
In such a way as what?
Which of the answers are you suggesting canít be true?
Title: Is the sea level different in different areas of the world
Post by: BarryNor on 08/07/2019 02:25:21
Icy, definitely but this is more about ice floes on/in the sea giving the sense of extremes near one pole or the other or maybe on one of the Great Lakes during the spring thaw.
Title: Re: Is the 'sea level' different in different areas of the world?
Post by: alancalverd on 08/07/2019 20:43:13
If you started with a smooth homogeneous stationary sphere (a ball bearing) and covered it with water, you would expect the depth to be constant, but

the earth spins
the density of sea water varies with location and depth (salinity)
and with temperature
and the depth varies with atmospheric pressure
wind
and the position of the sun and moon

plus the ocean currents and tides flowing through restrictions

and the fact that the land is rising and falling all the time through tectonic and volcanic activity and erosion

all mean that "mean sea level" is pretty well meaningless on a global scale, even ignoring variations in g.

Not that it matters. Seafarers float on top of whatever nature provides, until they get to a harbor where the local tide tables tell them how deep it is. Aviators use maps referred to an arbitrary national sea level, but local air traffic information services refer their atmospheric pressure settings (from which we derive altitude) to that national datum so your terrain clearance can be read from the official map. The coolest GPS avionics actually displays your height above the ground because the satnav system has a very accurate geoid model.

Title: Re: Is the 'sea level' different in different areas of the world?
Post by: diverjohn on 26/07/2019 02:30:39
I recall an article by some man more learned than I, where he talked about the difficulty of measuring the height of Mt Everest since the significant mass of the Himalaya mountains had enough gravitational pull to bring the ocean up by quite a bit, thereby muddying the meaning of "height above sea level" in this part of the world.