Cave-Surveying Digest, Vol 12, Issue 1

[Paul De Bie]

Hi, 
 
I think it is important to keep legs way under 30 m, unless the cave passage does not change over
that leg (e.g. a perfectly cylindrical shaft or tube of 100 m could be measured in 1 shot).
Measuring the cave is one thing, but sketching it, taking note of passage dimensions etc. is another
thing, and I consider this equally important.
Each time the passage dimensions change, each time the cave scenery changes, one should IMO make a
survey station and measure that leg. It is not only important for the drawing of the sketch, but
also for a correct visual representation (with cave passages, 3D etc) of the cave on a computer...
 
I agree that the nominal accuracy of laser tape is better, however at several occasions we have seen
that, when taking long shots (over 20 m) there is a higher risk of errors than when using a tape.
The longer the distance, the bigger the possibility to "miss the target". 
And many laser tapes need some time (sometimes over 1 second) to actually measure the distance. Even
when you are on target when pressing the button, you might easily "tremble off" the target while the
laser is measuring.  The use of a tripod is much recommended in such cases!
 
Once we tried measuring a 260 m pit with a Disto laser (of course in legs of say max. 50 m). It was
totally impossible to take a steady 50 m shot while hanging on a rope. We hade to revert back to the
good old 50 m tape.

Paul De Bie
http://www.scavalon.be
http://scavalon.blogspot.com
http://pollekepik.blogspot.com
 

> -----Original Message-----
> From: cave-surveying-bounces at survex.com 
> [mailto:cave-surveying-bounces at survex.com] On Behalf Of Pat Kambesis
> Sent: Tuesday, July 15, 2008 4:24 PM
> To: cave-surveying at survex.com
> Subject: Re: Cave-Surveying Digest, Vol 12, Issue 1
> 
> I think the reasoning behind going for longer shots instead 
> of shorter ones is that with fewer survey stations, it is 
> statistically possible to make fewer errors.  However the 
> longer the shot, the more difficult it is to site accurately. 
>  Digital instruments can greatly minimize user error since 
> the reader doesn't have to assume contorted positions to read 
> the shot and  are a lot easier to learn to use accurately 
> than suuntos.
> 
> Regardless of which types of instruments are used (digital vs 
> analog), the biggest limitation on the survey  is the speed 
> (or rather efficiency) of the sketcher.  Even if one could 
> shoot accurate 100 meter long legs, the sketcher still needs 
> time to accurately record data and make the sketches 
> (sketching to scale in plan, profile, cross sections).  
> "Eyeballing" a very long shot in terms of making the sketch 
> i.e. no distance reference, is very difficult with shots that 
> are much longer than 30 meters - one can't "eye" estimate 
> where to place passage/wall details with much accuracy with 
> distances greater than 15 meters (my personal experience).
> 
> pk
> 
> 
> 
> 
> 
> 
> 
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