[Fwd: zero datum for measuring cave depth]

[Andy Waddington]

Sometime before sending, stuart typed (and on Thursday 2010-02-18 sent):
> curious...
> a National Datum is zero at Sea level all the mountain heights are 
> calculated from this....
> Your cave datum is also zero to workout the vertical range in a cave, 
> however if the NHD is not linked to your survey datum then all caves 
> surveyed may end in confusion with no piont of reference....
> Meaning if you have 3 entrances to a cave (openly linked) regardless of 
> which entrance the survey datum is in or if all 3, the vertical range has be 
> the same...
> So Ent 1 (top of hill) 200m NHD / cave datum.... vertical range -120m
> Ent 3 (efflux) 80m NHD / cave datum.... vertical range +120m
> Ent 2 (hill side) 100m NHD cave datum.... vertical range +100m / -20m

If the caves are linked, then the depth is 120m and the vertical range
is 120m, regardless of the altitude of the entrances. If entrance 1 didn't
exist (but the passage reached that height inside the cave) then the
depth would be 100m and the VR still 120m. None of those figures depends
on the altitude and all can be determined by survey inside the cave. Of
course, the figures could also be obtained by survey outside the cave,
and if you have a decent reference system with surface benchmarks that
let you do short surface surveys to points at the cave entrances, that
might get you a more reliable figure.

The original question was about where in the entrance the zero datum for
the depth figure was placed, which is not a matter of accuracy, but one
of definition, and that was what I was addressing.

I don't dispute anything you are saying - it is just not addressing
the original question.

To clarify that again - one takes the lowest closed contour of a
pit entrance, and that counts as the level at which the outside
world becomes the underground, for the purposes of calculating depth.
Rolan's question was about the setting of that level - some cave
surveys you will see appear to use a different definition, and he was
asking if there was a standard.

If you want to use more modern methods to calculate altitudes for your
entrances, like survey grade GPS, then you do indeed need to know about
your National Datum (here in the mainland UK it is mean sea level at
Newlyn), and the relationship between your mapping agency's vertical
control network and the geoid to which GPS altitudes refer. In the UK,
you get a 36 Mb dataset which provides a grid of conversion figures
between simple GPS-derived references and the "real" grid/altitude
reference. That depends on a lot of survey infrastructure, and a
detailed gravity survey over the whole country. It's all good stuff,
but slightly beyond the needs (and technology) of most cave surveyors,
and makes not an iota of difference if all you have is an entrance
and want to know where to start surveying to get a figure for depth
to compare with some other cave on a world deep cave list... Whether
you then link that start point to a "real" altitude so you can compare
passage altitudes with nearby but unlinked caves is up to the surveyor,
but doesn't affect the point that you choose in the first place.

Andy