Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
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
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?

share|improve this answer
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:…:: – MerseyViking Apr 26 '11 at 18:18

I suspect it is the high water mark recognised by Trinity House (UK Lighthouse authority, plus other hydrological functions I expect).

share|improve this answer

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).

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.