Loop Closures

[Olly Betts]

[Replying to the list too as I think this is of general interest]

On Tue, Nov 11, 2014 at 03:35:55AM -0500, Peter Smart wrote:
> If I use the standard deviation approach are there commonly used
> values (othere than those inclued in survex for BCRA
> grades).  The issue is combining earlier man to man sunto and tape
> survey with more recent station to station and disto x data.
> Obviouslly the instrumental tolerance is less than the standard
> deviation.

As well as specifying SDs for the compass, clino and tape, you can also
specify them for the position error, so that would be the natural way to
handle the less well defined station positions in the older data.

I'm not sure how the accuracy of disto-x readings compares with sighting
instruments and a tape.

The distance readings from a disto are clearly more precise, but it's
not actually accurate to the nearest mm even in ideal conditions.  I
don't have a disto-x, but I do have an older model of disto, and from
having retried readings during actual underground surveys, it seems to
be at least be practically repeatable to measure between the same two
stations to within a cm or two.

It's also easy to let the laser beam "flick off" the intended target
and end up measuring to a different point to the target station
(though of course a tape can overread if you let it sag or get snagged).
Overall I suspect a disto is a little better than a tape, especially if
you're careful, but not dramatically better.

I don't know how accurate the compass and clino readings from a
disto-x are expected to be.

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.

Cheers,
    Olly