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This question may be off topic, in which case I apologise.

I have a problem where water level is important and I have two water levels defined, Trinity High Water and Mean High Water springs (MHWS). I understand what MHWS is but I don't understand what Trinity high Water is and how it relates to MHWS.

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yes, this is very close to being off-topic, or perhaps I should say "under water" for this site. You could bring it above board by giving some context; how will an answer to this question will help you with your GIS? Datums and how to deal with them in maps and spatial data are on topic. –  matt wilkie Apr 26 '11 at 17:21
    
@Matt I had the same reaction (and still do) but suspected the answers might potentially be of GIS interest. Sure enough, @mwalker has turned up a fascinating (albeit arcane) tidbit about vertical datums. –  whuber Apr 26 '11 at 18:36
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3 Answers

up vote 6 down vote accepted

Searching Google with "Trinity High-Water Mark" in quotes (and adding London or Thames) returns a bunch of Google Books results from the 19th century. My favorite is this transcript of the Reports from Committees for the Great Britain House of Commons:

 5838 . Mr. Forsyth.] Is the Trinity a datum level ? -- The Trinity high-water mark is the datum.
 5839 . Where is it, as a fact? -- Twelve feet 6 inches above Ordnance datum. Trinity high-water mark is one of the heights which is commonly taken by water engineers as their datum line. The Ordnance datum is the datum taken by railway engineers, Trinity high-water mark being 12 feet 6 inches higher than the Ordnance datum, the latter being supposed to be the mean-tide level.

Apparently they had a map at the time, too, which might be on the record somewhere?

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As a corollary, at that time, the Ordnance Survey used mean high water at Liverpool's Victoria dock, but in the early 20th century they moved it to Newlyn harbour in Cornwall. I can't remember off-hand what the difference is, but it is something to investigate if you're taking spot-heights from the 19th century county series or town series maps. –  MerseyViking Apr 26 '11 at 18:10
    
And a corollary to the corollary, the OS helpfully has this page that describes in much detail the differences between datums: benchmarks.ordnancesurvey.co.uk/pls/htmldb/…:: –  MerseyViking Apr 26 '11 at 18:18
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I suspect it is the high water mark recognised by Trinity House (UK Lighthouse authority, plus other hydrological functions I expect).

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Text in the 2012 Port of London Tide Tables states the following:

Trinity High Water (T.H.W.) is deemed, by the Port of London Act, 1968, to be a level having a value of 11.4 feet (i.e. 3.475 metres) above Ordnance Datum (Newlyn).

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