What does gravity have to do with sea level?

[mark2142]

TL;DR Summary

I want to understand the role of gravity

https://www.physicsforums.com/threads/what-is-sea-level.13108/post-136240
As far as I understand the sea level is the level of the sea water above the geoid or surface of the earth. The level of sea is same everywhere around the world because its all connected. So to calculate the global sea level to find the elevation of a mountain we take local mean sea levels on the beaches around the world with all the tides in consideration and find the average. That's the sea level of the world.
But in a video on youtube by @minutephysics called 'What is sea level?' what its all talking about gravity?
Sorry I am unable to post the link. But thank you in advance.

 

[phinds]

Gravity varies with distance from the center of the Earth. Thus if you are at sea level you have standard gravity and if you are 3 miles above sea level up on a mountain then gravity is less for you.

 

[Arjan82]

No, that's actually not how sea level is defined. It is a somewhat abstract definition that takes a gravitational model as measured by satellites as input and then computes what the height of the water would be at a certain location if the landmass wouldn't be there but its gravity would.

In this model also water is added, I guess up to the point where all oceans would be at their actual averaged water level. Note that the difference between high and low tide in the middle of the ocean can be much lower than near a shore, https://www.researchgate.net/publication/334235446_Tidal_Forested_Wetlands_Mechanisms_Threats_and_Management_Tools/figures?lo=1:

 

[phinds]

Summary: I want to understand the role of gravity

The level of sea is same everywhere around the world because its all connected.

That is not correct. The Atlantic and Pacific oceans differ by something like 2 feet.

 

[Arjan82]

That is not correct. The Atlantic and Pacific oceans differ by something like 2 feet.

2 feet compared to what?

 

[phinds]

2 feet compared to what?

Each other. That is, the mean sea level differs between the two.

 

[Arjan82]

Each other. That is, the mean sea level differs between the two.

Sorry, I don't get that :). Sea level at both oceans is by definition zero (as I understand it anyhow) so if the sea level between the two differs, then it is not compared to sea level since that difference would be zero... so compared to what then? Center of the earth? Then I think it is way more than two feet and also different from location to location. Compared to some spheroid model of the earth? Or... compared to what?

 

[PeroK]

But in a video on youtube by @minutephysics called 'What is sea level?' what its all talking about gravity?

The video seems to do a good job of explaining the complexities and the standard definition of sea level. Yes, it's all about gravity, as it must be if you think about it.

 

[Doc Al]

Here's the video:

 

[Bandersnatch]

@mark2142 The video goes into detail about this, so you may need to be more specific about the point of your question.

But maybe rephrasing what it said will help.

It's not true that 'The level of sea is same everywhere around the world because its all connected'. There are places where strength of the gravitational field is a bit higher or lower, and the water pools (i.e. raises higher) in the places that have a bit higher gravity (because e.g. the crust is denser below that spot, or there's the mass of a continent pulling on it).
So you can have actual sea at different levels in different places, even after taking tides into account, even though it's all connected.
In yet another words, the fact of 'being connected' doesn't equalise the level to some idealised surface, but to the actual shape of the gravitational equipotential. Which can be bumpy.
After all, you don't want to use some global average and end up telling somebody on an actual beach somewhere that they're below or above sea level. (might still happen with tides, but let's ignore them)

Say, if you were to imagine a perfect sphere made of some relatively non-dense material, for example silica (or even iron) somewhere in space. You drop some water on this sphere, and it forms a perfectly even sea level everywhere - same distance from the centre of the sphere.
If you then bury a huge lump of lead at one spot - even if you did this in such a way that the ideal shape remains unchanged - the water on the surface of such a sphere would flow a bit more towards the spot with the denser lump underneath it. You'd have a spot with sea level above the global average.

The video makes a point that even in places where there is no water, the sea level is defined along the surface where water >would< flow if it could - i.e. a gravitational equipotential surface.

 

[Rive]

...to calculate the global sea level to find the elevation of a mountain...

Historically, 'sea level' goes down to specific points used as geographic references.
For example 'Metres above the Adriatic'

The whole gravity thing is a later addition.

'Global sea level' as reference of height is a difficult topic. The sea level depends on more things: temperature, salt content (=>density of water), gravity... Thus this (sea level, in global sense) is mostly for global (sea) things (models) only. For height measurements it's more common to use a reference surface defined independently.

The 'sea level' to reference surface change (of reference) can be quite confusing, especially when it hits some 'well known' heights of mountains or so.

 

[anorlunda]

Don't forget seasonal prevailing winds.

Don't forget that Earth is an oblate spheroid.

Bottom line as others already said, sea level is very complex.

 

[mark2142]

It is a somewhat abstract definition that takes a gravitational model as measured by satellites as input and then computes what the height of the water would be at a certain location if the landmass wouldn't be there but its gravity would.

Okay! That is quite interesting. You mean a gravitational model is programmed in your gps device/satallite and it will based on your location(longitude and latitude) compute gravity and calculate the height of water in that particular location. Then for elevation gps device must need the total height of us from the geoid/surface? Can you clarify a bit more?

It's not true that 'The level of sea is same everywhere around the world because its all connected'. There are places where strength of the gravitational field is a bit higher or lower, and the water pools (i.e. raises higher) in the places that have a bit higher gravity (because e.g. the crust is denser below that spot, or there's the mass of a continent pulling on it).
So you can have actual sea at different levels in different places, even after taking tides into account, even though it's all connected.

Okay. That makes sense.

Bottom line as others already said, sea level is very complex.

It is!

(Thank you for so many answers)

 

[pbuk]

Okay! That is quite interesting. You mean a gravitational model is programmed in your gps device/satallite and it will based on your location(longitude and latitude) compute gravity and calculate the height of water in that particular location.

A GPS device doesn't have to compute gravity, it simply reports your height above the reference WGS84 geoid, which includes a model of the equipotential surface.

https://en.wikipedia.org/wiki/World_Geodetic_System

Edited following #17 below:

A GPS device doesn't compute gravity, it simply reports your height above the reference WGS84 ellipsoid, which could be ±100 metres from the equipotential surface. Note that due to atmospheric factors, GPS height location can add another ±100 metres inaccuracy.

 

[mark2142]

A GPS device doesn't have to compute gravity, it simply reports your height above the reference WGS84 geoid, which includes a model of the equipotential surface.

https://en.wikipedia.org/wiki/World_Geodetic_System

(I found this. Hope these links are right. https://www.usgs.gov/faqs/what-geoid-why-do-we-use-it-and-where-does-its-shape-come
https://www.gpsworld.com/data-collection-of-wgs-84-information-or-is-it/)

Lets say this WGS84 reference system and the WGS84 geoid or imaginary equipotential ball is defined by the scientists. So is our GPS encoded with it and tells us our height or it communicates with the satallite where this model is stored?

 

[pbuk]

So is our GPS encoded with [WGS84] and tells us our height.

Yes, this, and your position, although most GPS units are also encoded with other geoids so they can display positions for plotting on local maps and charts. See for example this list from Garmin: https://support.garmin.com/en-GB/?faq=CuRXNp4JBU25QHE8cwRI26.

This is necessary, among other things, to deal with the fact that the continents are moving relative to the WGS84 geoid, some faster than others.

As someone (almost) said above, GPS is really REALLY complicated.

 

[Baluncore]

So is our GPS encoded with it and tells us our height or it communicates with the satallite where this model is stored?

The altitude correction is not relevant, nor needed, unless you leave the ground or the sea surface.

Most GPS units report only the WGS84 latitude, longitude, and the approximate height above the WGS84 ellipsoid. The difference between the WGS84 ellipsoid and sea level ranges over about ±100m. When height accuracy is needed, the Earth Gravitational Model EGM 2008 is used by the mobile receiver to compute the altitude correction. That gives the user control of the model employed.

GPS positions are accurate because multiple satellites, to the north and south, east and west, partly cancel the variations in ionospheric delay. That is not true for the vertical height, since the Earth is always in the way of the reverse sight. For that reason, GPS derived height continuously drifts, and cannot be as accurate as is the position estimate.

Accurate heights must be obtained using differential GPS, where a reference receiver is placed at a nearby permanent benchmark, defined for the local mapping grid.

 

[pbuk]

Most GPS units report only the WGS84 latitude, longitude, and the approximate height above the WGS84 ellipsoid.

I didn't know that, I thought they used the EGM 2008 geoid (which is part of the WGS84 specification), but it seems that you are right, for Garmin and Trimble at least (and so therefore almost certainly GPS software built into phones etc.)

Phone apps seem to get height at ground level closer than this though (particularly when you add in the atmospheric inaccuracy), I guess they cross-reference to map data.

 

[Baluncore]

If you think you can measure height to better than ±1 m with direct GPS, think again. There are a couple of other confounding problems with measuring sea level height.

The first is the solid Earth tide. The Earth is elastic and so immediately responds to the tidal effects of the Sun and the Moon. That introduces a vertical variation in the height of a reference benchmark or tide gauge, by about ±500 mm.

The second is near the coast, where deformation by the loading effect on the Earth's body, due to the mass of water being continuously redistributed by the delayed ocean tides, is about the same magnitude as the solid Earth tide.

https://en.wikipedia.org/wiki/Earth_tide

 

[pbuk]

If you think you can measure height to better than ±1 m with direct GPS, think again. There are a couple of other confounding problems with measuring sea level height.

Absolutely not, but my experience is that it is much better than the ±100 m I might expect with the difference between the WGS84 ellipsoid and geoid. Having said that, I only check when I'm near sea level (I spend a lot of my time at sea level!) and perhaps the difference is less there.

 

[mark2142]

As someone (almost) said above, GPS is really REALLY complicated.

Ok I guess I will leave it for later. Seems hard. Thank you guys for your valuable time.

 

[Baluncore]

Welcome to PF.

That is incorrect.

Exactly what is incorrect?

 

[phinds]

The Pacific and Atlantic oceans are separated by about two feet.

Old news. See post #4. When responding in a thread, it's a good idea to READ the thread first.

And I agree w/ @Baluncore --- WHAT is incorrect?

Also, I don't get what you mean by "the Earth is always seen in reverse".

 

[Laroxe]

In looking at sea level it might also be worth considering how the spin of the earth creates an equatorial bulge which effects the sea and the land mass adding to the gravitational effects. The redistribution of the mass of water from polar ice also causes gravitational changes, which in turn effect the spin speed.