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Aubrey O'Callaghan wrote:

> A google search of NAP and datum brings up
> NORMAAL AMSTERDAMS PEIL, this translates as Amsterdam Ordnance Datum.
> This water level was recognized over three centuries ago as the referral
> point for all constructions in the Netherlands and several adjoining
> countries.
>
[snip]

>
> Probably for our purposes we can consider NAP and MSL to be equivalent
> -but I would stand corrected by someone more knowledgeable.
>
> By the way I think that the plural of datum is data... George ?

A MSL established 300 years ago cannot be trusted to represent MSL now,
though it might be close enough for specific places (maybe including the
  coast of the Netherlands). The Atlantic coast of Nova Scotia has
dipped about 1.5 metres in that time, while Norway has risen perhaps 3
metres out of the sea.

The grammatical plural of "datum" is indeed "data" but, by convention,
"datums" is used for more than one survey datum. Likewise, in scientific
terminology a single item of data is termed a "data point", never a
"datum". Of course, far too many scientists think that "data" is a
collective noun and so write that their "data is", when it should be
their "data are".


Trevor Kenchington


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
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