What height is sea level?

Question

Say a town or city is 1000 metres above sea level, is that at low or high tide or do they take an average?

Answer 1

Its the mean sea level. That is the average height through all tides and seasons.in a place in Cornwall called Newlyn, projected throughout the British coastline.

There is a good article here;

http://www.pol.ac.uk/psmsl/puscience/#2

Answer 2

Yes it is the average level between High and Low water.
The only thing missing is the concept of a stilling chamber, to eliminate waves motions from the measuring system.
A stilling chamber is a vertical tube with a relatively small hole in the side, towards the bottom.
Full calculations also include the movement of the land, for example Newlyn is still sinking after the ice age. So you have a datum point which is not truly fixed.
It is sinking because all of the uk was tilted in a see saw style, by the weight of ice on the north. Conwall has many sunken river valleys (Ria's) for this reason.

Answer 3

In the UK and Ireland, mean sea level - Newlyn for GB, Malin head for the Irish Republic and Belfast for N Ireland. Mean sea level is found by averaging hundreds of readings at one point.
mean sea level varies around the coast.

In North America, they use a national vertical datum that is related to a point in Quebec province.

Shipping navigation charts use extreme low tide as local datum

GPS systems use an imaginary geoid - result from a hand held GPS can be 15-20m different from the "real" height above sea level.

For a city 1000 m up it does not much matter, but for flood studies in coastal cities, accurate local information is needed.

Answer 4

The simplest explanation is that sea level is the level which the sea would be at, if the land weren't there. The shape of the earth is not quite spherical, because of the rotation about its axis, and its shape is described as an 'oblate spheroid'; bulging about 20km at the equator. In addition, the presence of the continents and the additional gravity because of their mass causes the exact shape to vary somewhat; which is described as the 'geoid'. Further variations occur because of variations in the density of the earths crust, and the actual theoretical 'sea level' will vary slightly. Geophysicists use these slight variations when conducting 'gravity surveys'.

Answer 5

It is same as it always has been since "sea level" has been established by geological survey. With all the ice that has melted at the arctic, you would think it would have risen some. But it has not. Geological survey has not had to change their sea level point of reference yet.
Sea level is adjusted to the average.

Answer 6

in about 20 years it will be about 3feet above your head. have a nice day.

Answer 7

it would be at H tide