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Name: Adrian
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: N/A 

How do you derive the sea level?

Dear Adrian-

The answer I found was taken from SCIENCENET... here is the URL...

What is the definition of sea level?

That's a tricky one. If you're talking about the heights given on sea charts then the height is given as that above mean high water springs - that is the average high water height for spring tides. Tides follow an approximate 2 weekly cycle depending on where the Moon is. Spring tides are generally higher than neap tides. So heights given on sea charts are given as the height above the average highest tide.

You can also get mean sea level on sea charts. This is an average of hourly measurements of the height of the tide over a period of at least a month, preferable a year but at most 19 years!

But heights given on maps (like those for the heights of mountains) aren't given in the same way. I think it depends slightly on which country you're in too, but the Ordinance Survey (who are the people that produce our maps in the UK) say they measure the height from the mean sea level in Newlyn, Cornwall. However, their mean sea level isn't the same as the mean sea level that the sea charts use.

Here the mean sea level is the mean of the Mean High Water and the Mean Low Water (is it getting confusing?!) Mean High Water is midway between mean high water springs and mean high water neaps. Mean Low Water is midway between mean low water springs and mean low water neaps.

So essentially there are lots of definitions that involve the tide that depend on whether you're a sea person or land person!

Wendell Bechtold, meteorologist
Forecaster, National weather Service
Weather Forecast Office, St. Louis, MO

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