[Fwd: zero datum for measuring cave depth]

[Andy Waddington]

Sometime before sending, Roland Eberhard typed
 (and on Tuesday 2010-02-16 Anthony Day forwarded):

> ... a discussion about what is an appropriate starting point
> for measuring depth, depending on the configuration of the entrance. In
> the case of a vertical pit-type entrance, the practice here has been to
> survey downwards from the lowest side of the entrance hole.

That has been our practice for vertical, open-to-the-sky entrances, too.
When quoting depth, we then use the highest such entrance. When quoting
vertical range, the highest point included would be the highest surveyed
point enclosed in the cave.

> In the case of
> a horizontal walk-in entrance, there appears to be no common agreement
> on what should be taken as the zero datum, which could be the lowest or
> highest point of the entrance opening.

Personally, I would reckon that depth would be from floor level at the
lowest point of the entrance, whilst vertcal range would include the
highest point which is vertically below some rock, although for an
entrance under a curving overhang, that might not be reasonable, as
it could be a long way above the top of the actual passage. Again,
for vertical range, you'd be looking to the highest point that could
reasonably be considered "inside" the cave.

> I've been searching the web for 
> any standards

There are standards for quality of both centre line and drawing,
and sets of standard symbols used by different organisations, but
cave depth is a bit competitive, so organisations use whatever they
think they can reasonably defend, whilst getting the biggest number
for the competitive statistic, whatever that is (depth, vertical
range, length, volume, pitch length, aven height, etc. etc.). The
trouble with attempting to establish a standard is that it would
be difficult and/or political to be seen to be defending (or failing
to defend...) the practice of one's local cavers if that might cause
a change of competitive sequence.

Andy