[Fwd: zero datum for measuring cave depth]

[Anthony Day]

-------- Forwarded Message --------
From: Rolan.Eberhard at dpipwe.tas.gov.au
To: csg-editor at bcra.org.uk
Subject: zero datum for measuring cave depth
Date: Mon, 15 Feb 2010 13:38:12 +1100


Hi 

I am a caver from Tasmania, Australia. The surveyed depths of our three
deepest caves are very close - within a metre or two of each other. This
has triggered 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. However, I
note that some cave maps from overseas show the zero datum as the
highest side of the entrance pit. This could mean a difference of tens
of metres in the surveyed depth of the system as a whole. 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. I've been searching the web for
any standards or discussion regarding this question, and have so far
drawn a blank. If you have any information or advice based on UK
practices, I would very much appreciate hearing from you. 

Thanks, Rolan 

Rolan Eberhard 
48 Fergusson Ave 
Tinderbox 
TAS, 7054.