Loop Closures

[Andrew Atkinson]

On 11/11/14 20:02, Olly Betts wrote:
> I do recall Phil Underwood saying the accuracy of the electronic compass
> and clino module in the SEP was theoretically lower than that of
> sighting instruments, though having used one a fair bit I've found the
> results to be better overall (presumably due to a combination of factors
> - it's easy to not actually achieve the theoretical accuracy of a
> sighting instrument, the position error is likely to be lower as you
> don't need to get your eye on the station, there's no chance of reading
> the scale the wrong way from the nearest marked number, etc).
> 
>> > Guess I could do some trials and determine a value.
> I'd definitely be interested to hear results of any trials people have
> done comparing the newer instruments with the traditional ones.


Although I have not done any systematic investigation: Using the
different flavours of distoX I now check for errors if I have a loop
closure of more than 0.5m, whatever the loop length. With sighted
instruments they where far bigger, tended to be happy if we got less
than 2%. I did change methodology, but that was because the DistoXs
where giving such consistent results. Pockettopo default is to take 3
readings for each leg and then checks for consistence, not sure if it is
a percentage or an absolute, but this avoids the disto under/over shoot
problem.
The only directly comparable data I have is a linear section of
Charterhouse, that was surveyed the previous year by traditional
instruments then by DistoX with pda. Most of the centreline is difficult
to tell appart you can even see when the same stations where used. The
main difference is that the traditional survey had a number of blunders,
which when corrected (yes I assumed the DistoX was correct for this, but
one of them was a river going up hill.....) the centrelines where very
close. The main difference where the diagrams, traditional missed most
of the detail straightening sections, and generally missing out lots of
detail the DistoX team captured. (I do not think this was just because
of different people drawing, it was the ease of capturing the detail
that made the difference, I have seen it in my own diagrams)

My feeling is that the accuracy of both types of instruments is better
than the methodology that we use to do the survey and that due to being
able to take reading more easily the DistoX leads to better results, for
the same speed of survey, without considering the massive improvement to
diagrams that has been possible by real-time (nearly) plotting in the
cave and then drawing round it, plus the ability to measure to anything
with a single click of a button. This has really helped with the
production of cross sections, that I now do far more regularly, and Andy
Farrant has commented that they are more useful.

Andrew