[Therion] Detecting errors

[Benedikt Hallinger]

You could also indicate the misalignment with instrument correction commands, but i dont know the exact command at the moment. I think that would not with  different orientations of the same device...

> Am 05.09.2019 um 13:10 schrieb Max D <md at foxel.org>:
> 
> 
> 
>> On 5. Sep 2019, at 08:28, Olly Betts <olly at survex.com> wrote:
>> 
>>> 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 -
> 
> Currently we set the SD per calibration - which works out to about every second trip. We Take the "Error stddev" from Topodroid Calibration Results - e.g.0.1058 in the attached Screenshot.
> 
> Thinking of it, this might be the wrong approach because it only considers the instrument.
> 
>> *sd tape 0.002 metres ; 2 millimetres
>> *sd compass clino 0.5 degrees
>> *sd position 0.05 metres ; 5 centimetres
> 
> Interesting. I will read up on the difference between tape and position
> 
>>> 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.
> 
> But I would need a extra set SD data per device orientation. 
> So I know that if the display is to the right the Disto tends to reassure an Azimuth 2 degrees to high and if the display is to the left.
> And when the display is to the left it tends to give an Azimuth 1 degree to low.
> 
> But perhaps I just should try to do more and better calibrations instead of fixing stuff afterwards.
> 
> 
>>> 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.
> 
> Good Advice!
> 
> Thanks!
> 
> --max
> 
> <Screenshot_20190720-135115_TopoDroid.jpeg>
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