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?