[Therion] Detecting errors

[Max D]

> 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

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