On Fri, Aug 16, 2019 at 05:11:22AM +0200, MD wrote: > We also then to measure the same small loop at the beginning of each trip: > A-B in four device orientations > B-A in four device orientations > A-C in four device orientations > C-A in four device orientations > C-B once > > So I could calculate an SD based on that.
You probably don't want to work out a set of SDs per-trip - there would be quite a lot of random fluctuation in the observed SDs from such a short loop, and an equivalent instrument properly calibrated and used with suitable care really ought to be able to achieve similar SDs. Combining information from a lot of such small test loops would be interesting though. We've been doing some surface measurements at home with our disto-x2 while planning some building work and I did some experimenting with setting different SDs and looking at how the loop error values are, and I found the following SDs seem to be achievable with care: *sd tape 0.002 metres ; 2 millimetres *sd compass clino 0.5 degrees *sd position 0.05 metres ; 5 centimetres The tape/compass/clino are just taken from Beet's disto-x2 article: http://paperless.bheeb.ch/download/DistoX2.pdf I've glossed over exactly what a "precision" of 2mm means and assumed it's just the standard deviation. It might actually mean the SD is even smaller, but that's pretty much irrelevant as the compass, clino and position errors will swamp any contribution from the tape (1m at 0.5 degrees is 8.7mm). The position error reflects how close I think I can reliably position the spot I've marked on the back of the disto relative to the "from" station plus the error in where the laser beam hits compared to the "to" station. I haven't tested if those are still reasonable underground. There should be less electromagnetic noise underground, but it's often colder and wetter underground than in our front garden, and I've found misty or dusty air can cause problems for the disto. I'd be interested to hear what people can actually achieve in-cave. > For sone Devices errors seem to be bound very much to orientation. In > the data saved by TopoDroid you can see the device orientation and so > we could set a sd based on the direction in which the shot was taken. > I have nit investigated this further so far. Also because we have this > data only for about 20% of the cave. This is already taken into account - you tell Survex the SD for the "tape" (read laser range-finder), "compass" and "clino" (read magnetic field measuring devices) and position (how close to the station you actually measure from) and it uses the direction of the leg to produce a 3D set of expected errors, which are then summed along each traverse. > TopoDroid also is able to flag an “magnetic anomaly” - i’m not totally > sure how but this als could be used in setting a per shot SD. i have > not looked much into this. I think you want to treat that as an in-cave indication that you should check for sources of magnetic interference and redo affected readings. Cheers, Olly _______________________________________________ Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
