[Fwd: zero datum for measuring cave depth]

[Andy Waddington]

Sometime before sending, stuart typed (and on Wednesday 2010-02-17 sent):
> Fair to assume provide all cave surveys relate to their National Datum there 
> should be no issue of where it is...

But the National Datum is a datum for zero altitude - nothing to do with
the point from which depth is measured, which is what Rolan was asking
about. A starting point for measuring the depth is something you can
create without needing to know the altitude of the cave, although knowing
the altitude of that depth-datum is useful for relating depth potential
to the altitude of possible resurgences without having to do a surface
survey for the purpose (or doing a shorter surface survey from some
identifiable benchmark).

Andy