On Sat, Dec 19, 2020 at 1:07 PM Andrew Atkinson <andrew@wotcc.org.uk> wrote:
I would hesitate to use magnetic legs for this purpose (there is much uncertainty with the declination as the models are not particularly precise, there is a daily variation of declination (up to 0.2 degrees!) and there might be local magnetic anomalies).
In this case the cave comes out in a valley with little gps signal, the position and orientation was set from the 2 gps positions we could get, the a traverse to the cave. So I have very little idea how that ties in with the declination model. I have been assuming that the direction it gave where OS grid as that is the format of the positions, I have no way to check this. I have set the declination to 0 to stop date and position correction. I think this it right, but advice welcome.
Ah, that was a reaction to some previous remarks in the discussion:
In places where we haven't been able to get GPS we have used a distox to set up the leg
and
I did consider using an approach similar to Andrew’s, ie to calibrate the Theodolite against a magnetic leg ...
Martin