On 5. Sep 2019, at 08:28, Olly Betts <olly@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>