Sea Level

[martinwoolwich]

I think my brain has gone soft.

My son just said to me

"If we have tides that means the level of the sea moves up and down. If this is true what do they mean when they say something is XXXXft above sea level?"



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[Wiggo]

Errm, chart datum (CD), MHWS, MLWS?

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[Deleted User YDKXO]

Clever son. So many feet above Ordinance Datum at Newlyn in Cornwall which is supposedly the average of sea levels at Newlyn but I rather suspect it is an absolute height somewhere

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[hlb]

Chart datum....Lowest possible tide. Dont think there to a few feet when measuring mountains though!!

<hr width=100% size=1> <font color=blue>Specializing in marine sanitation since 1997.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[BarryH]

erm. don't mean to be oicky, but CD is LAT. Not MLWS

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[markc]

You can always get your own back with <A target="_blank" HREF=http://earth-info.nima.mil/GandG/datums/vertdatum.html> this </A>


M

<hr width=100% size=1>http://www.geocities.com/jinto100/Jintopics.html

 

[PhilF]

can,t be bothered to read all that, isn't it lowest astronomical tide

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[markc]

dunno....I couldn't be bothered either! But it does seem that everyone has different ref points. Eg, bridges on charts are shown as height above MHWS



<hr width=100% size=1>http://www.geocities.com/jinto100/Jintopics.html

 

[PhilF]

thats so you don't bump your head, makes sense

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[whisper]

Good grief Haydn's right!! It is based on the VD (vertical datum) - of in our case a point at Newlyn and that is where the MSL (mean sea level)is decided. MSL is the level based on the AVERAGE height of tide at a given point and according to many years of records - in excess of 19yrs worth even.
Thank your son for asking this question - I'm glad I now know the answer.

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[Wiggo]

You can be as oicky as you like, Barry. I wasn't implying that they were the same thing - as others have pointed out, on charts, things are measured from all sorts of different datums - depth from CD (which is usuallyLAT, bridge heights from MHWS, for obvious reasons etc etc. The answer to the question 'what is Sea Level?' depends on what is being measured from sea level, surely.

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[mainshiptom]

Love it !

I will show it to my son !

Tom

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[alant]

Correct.
Heights, other than drying heights - those green bits - (which are measured above CD (LAT), are measured above MHWS on a chart - includes bridges, lighthouses etc.

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[whisper]

Yes and No. The height of Snowdon for example is not 3056(or whatever) feet above MHWS. It is that height above "Sea Level" at Newlyn. "Sea Level" in this instance being the average level of the sea at that particular place, which has been calculated using many years of data.
God, I'm boring myself now./forums/images/icons/blush.gif
Out !!

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