[Fwd: zero datum for measuring cave depth]

[stuart]

Hi
as you say what you can defend.....
I am into cave diving so the first Datum is based on Sea level (above & 
below) (HD)
Altitude of any cave, dry entrance, needs its above "sea level" as it has 
effects to decompression sickness...
the second datum is the  entrance... so a walk in cave the ceiling is "plus" 
/ surface entrance vertical "minus"

For us water level is the second zero point, above plus or minus below, 
however the third Datum is best around a set cave feature, around -5 
metres....
changes in water level & atmospheric pressure will change the Datum depth on 
a gauge.... So all survey data collected on the day is corrected for the 
change...
(most water levels changes are seasonal, drought, and flood effected )....
Water level being zero is not consider to be a Datum... Most caves will have 
a point worked out via the surface datum to above the water line as a second 
Datum and measure the water level changes (generally speaking this fails 
badly and is ineffective)

So really the standard datum for us is AHD (Australian Height Datum) & GDA94 
(Geocentric Datum of Australia)...
provide all cave datums relate to this there is no issue....
Fair to assume provide all cave surveys relate to their National Datum there 
should be no issue of ware it is...

Cheers Stu
----- Original Message ----- 
From: "Andy Waddington" <surveys at pennine.demon.co.uk>
To: <Rolan.Eberhard at dpipwe.tas.gov.au>
Cc: <cave-surveying at survex.com>; "Anthony Day" <anthony.day at ebnett.no>
Sent: Wednesday, February 17, 2010 10:46 PM
Subject: Re: [Fwd: zero datum for measuring cave depth]


> 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
>
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