[Fwd: zero datum for measuring cave depth]

[stuart]

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 were not linked, there for plausible that each cave could have 
a higher or lower vertical range, without the NHD you would have no way of 
linking the surveys other than a surface survey of all Cave Datum's (Ent 1 
to Ent 2 -100m) and if in time the caves were linked, then each survey most 
likely will contradict each other if you cant align your Datum's.... while 
each cave VR may differ the "system" VR still has to equal 120m in the above 
case....

For us we borrow a $50,000 linked DGPS.s One is set up on a trig point on a 
nearby hill, the other, all caves are given a NHD / GDA point at the 
entrance to the cm, this links all surveys...all cave survey Datum's are 
normally in caves, mostly below the water lines.... all cave datum's are 
first linked to the surface datum.... In some cases the paddocks are pined 
with survey marks in the same way.... pingers are used (magnetic field 
beacons) underwater survey marks are transposed to the surface...A "thumper" 
(a very big version of a pinger) works through rock for around 150m 
vertical.... This impart takes up for slack with Tape & compass... As divers 
not in your league... And for us water level is zero but not the Datum (just 
a piont of reference) and common to have 2 datums...

I would admit getting your NHD on your datum is a bit of a trick....and most 
wouldn't have this accuracy...
but the lack of it in any format, i can understand how old surveys, other 
group surveys, incomplete survey, wouldn't line up, and confusion over 
vertical ranges....

Cheers Stu
----- Original Message ----- 
From: "Andy Waddington" <surveys at pennine.demon.co.uk>
To: <cave-surveying at survex.com>
Sent: Thursday, February 18, 2010 10:09 AM
Subject: Re: [Fwd: zero datum for measuring cave depth]


> 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
>
>
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