How is altitude above sea level measured as tides are constantly moving?

Question

Did they pick a certain sea level or some physical feature near the sea line to measure from?

Answer 1

A quick google for 'mean sea level newlyn' turned up the link below.

To quote:

2. What is meant by "altitude above sea level" and how is the zero reference determined.
Is it a constant with respect to space and time?


Heights above sea level, like mountains or whatever, have traditionally been defined (and still are in most cases) in terms of a measurement of 'mean sea level' at one or more locations.

So, for example, here in the U.K. we define heights above sea level in terms of 'Ordnance Datum Newlyn' (ODN) which is the mean level of the sea at Newlyn in Cornwall in S.W.England in the period May 1915 to April 1921


In many countries, like the US or India with 2 coastlines there are often two or more datums

Answer 2

Your observation that tides affect sea level is correct, so the normal way to measure altitude above sea level is Above Mean Sea Level - the average.

Most oceans and seas have a tidal range of less than 2 meters, so if you are measuring a mountain that is 1,000 meters high (AMSL) the difference of the tide is not very significant. However, if you consider that the Bay of Fundy has a tidal range in excess of 20 meters, it is important that a specific datum is used, and by convention the datum for sea level is the mean sea level.

The earth is not an exact sphere, it is 40 km wider at the equator than at the poles. The accepted way to reference an elevation is therefore in reference to the mean sea level.

AMSL is the key here!

Answer 3

All of the above are wrong.

I worked for many years in aviation, so i know a thing or two about altitude!

Average sea level around the world is measured from Newlyn in Cornwall, UK - taken from the average of the high and low tide in Newlyn bay!

I don't know WHY it's Newlyn, maybe someone else could answer that one.

Answer 4

Up until the past 20 years or so, they used air pressure measurements and formulas to measure altitude. They took an average over a long period -- as long as year sometimes.

Now GPS signals can be used to determine position and altitude to high accuracy, within a few minutes.

The baseline for altitude is the "mean" (average) sea level, also called the "geoid".

Answer 5

Randy P is right. I just try to to do the calculation. 1.The factor by which length contracts, mass increases and time dilates = (135*10^3)/139.6 = 967 Velocity, v of the pion is given by 967 = 1/sq rt[1 - (v/c)^2] or v/c = sq rt[ 1 - (1/967)^2] = 0.9995 or v = 0.9995c Life time in earth's frame = (35*10^-9)*967 s Distance travelled by pion in Earth's frame = 35*967*0.9995*3*10^-1 m = 10.1 km. So te altitude where decay occurred = 120 -10 = 110 km

Answer 6

It's measured from mean sea level, ie the difference between high and low tides.

Answer 7

It is at the time Ebb Tide occurs.

ebb tide (ěb) Pronunciation Key
The period between high tide and low tide during which water flows away from the shore. Also called falling tide. Compare flood tide. See more at tide.


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