Measuring Sea Level: Tides, Elevation & Distance from Earth's Center

[Philosophaie]

Is there a median sea level that the logs of sea level data is based? I know the moon causes the tides which upsets the equilibrium on a constant basis. I need this sea level to be based upon the distance from the center of the Earth so with the elevation of a particular city on the Planet will be known.

 

[mgb_phys]

Each country has it's own mean sea level.
There is also a median sea level for the geoid that is used for height above sea level for things like GPS - which ironically isn't actually the level of the sea anywhere. Since the Earth isn't a sphere the distance from the centre to sea level also depends on position.

http://www.esri.com/news/arcuser/0703/geoid1of3.html

 

[Philosophaie]

The model is a ellipsoid Earth model, one part of the sphere is shorter then the other (poles shorter). Do these sites base on a changing sea level or is it quite constant?

http://geonames.usgs.gov/pls/gnispublic/f?p=116:1:1874957264251958::NO:1:P1_SHOW_ADV,P1_SHOW_FIPS55:,

http://stuff.mit.edu/geo?location=new+york

 

[ray b]

sea level data is based on a dated datum
we used 1927 USGS datum well into the 2000's
there was a ''new'' datum based on 1989 avg sea levels
but as far as I know that was seldom used

each city and county and state can use what ever it wants
as long as they tell you what it is based on
city of miami uses a datum .03 off the countys USGS datum
others may add on a 100 ft to avoid negative numbers
I would guess GPS data is based on a USGS datum

 

[mgb_phys]

I would guess GPS data is based on a USGS datum

In just raw lat/lon mode GPS wil give your the altitiude based on the WGS84 ellipsoid and a global mean sea level.
Largely by coincidence this is quite close to MSL for most of the continental USA but can be upto 30m off in other parts of the world.
If you put the GPS into a local map system (OSGB, Swiss etc) then it will use the MSL defined by that country.

As pointed out above this can vary for both historical and physical reasons. There was a famous example of a bridge between Austria and Switzerland where the precisely surveyed approach roads missed each other by 1m in height. Neither country having an abundance of sea level of their own - Austria used Germany's based on the north sea and the Swiss use the italian level based in the Mediterranean.

 

[Philosophaie]

Would it be way off base to say that the Polar Radius would be the lowest sea level point on Earth. If this is the case would world sea level also be at that same level neglecting tidal and wave forces. This would be an temporary approximation till USGS datum levels can be calculated.

http://en.wikipedia.org/wiki/Earth

Polar Radius = 6,356.8 km

Equitorial Radius = 6,378.1 km

 

[mgb_phys]

If by lowest you mean closest to the centre of the Earth then yes.

 

[Philosophaie]

At this lowest point, Polar Radius, does this mean that the world's level is about the same sea level as Polar levels all around the world except for the tides?

 

[mgb_phys]

No
The Earth is an oblate spheroid (pear shape) the polar radius is about 20km less than at the equator.
We pick an elipsoid that is a good average fit to this shape, locally sea level can vary by -106m to +85m from this average surface just due to bits of the crust that bend in vs bits that bend out.
Locally sea level can also vary by a few metres due to the different density of rock below the sea bed giving different local gravity.

 

[Philosophaie]

There are currents in the oceans such as the North Atlantic Current which flows thru convection. The warmer Atlantic current flows above the colder Artic current below. There may be similar currents in other places around the globe. Is there any currents flowing towards or against the artic to explain why the sea levels are not the same. Water is a viscous material. It will propagate evenly unless otherwise impeded. My question is: are the sea levels equal or are they impeded in some way?(neglecting the moon's tidal flow)

 

[Philosophaie]

Geocentric Latitude = ATAN((1-f)^2*TAN(Lat))

where f=1/298.25 is flattening
Lat - Geographic Latitude

Earth's Radius = Sqrt(((a^2*Cos(GLat))^2+(b^2*Sin(GLat))^2)
/(((a*Cos(GLat))^2+(b*Sin(GLat))^2))

where a - Earth's Equatorial Radius
b - Earth's Polar Radius
GLat - Geocentric Latitude