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In the UK, heights above sea level use the "mean spring high tide" level as their datum. This I hope needs no further definition, its meaning appears obvious to the undersigned Brit. But the US terms "mean high water" and "mean higher high water" are not obvious. Could they be defined in the article? Also it would be useful to know how they relate to "mean spring high tide". Maproom (talk) 13:45, 29 September 2009 (UTC)
It appears to me that this article is about the (Chart) Soundings Datum, not the Chart Datum. I have seen the term Chart Soundings Datum inaccurately referred to as Chart Datum. However, Chart Datums, in a stricter sense, define how geographic coordinates are used on a map. The most prominent Chart Datum, in the age of GPS, is WGS84.
In the UK Admiralty Chats at least, "Chart Datum" does mean sounding datum. The datum that the geographic coordinates are referenced to is typically called the "horizontal datum". 22.214.171.124 (talk) 09:58, 3 May 2012 (UTC)