On 19/12/2020 12:19, Martin Budaj wrote:
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 ...
Yes, one cave we did have to use magnetic calibration. I'm not sure how else we could have done it. Are there alternatives if you are underground. We might have been able to do a traverse all the way from the surface, however, they was one point which we may not have been able to get the theodolite survey through. You mentioned the variation of the magnetic field. One trip I had an alert on my return, on checking how the field had varied while we were underground we abandoned the data, luckily the diagrams where all there so it was not that difficult resurvey. I find this page very useful, http://geomag.bgs.ac.uk/data_service/space_weather/current_conditions.html It lets you set up an alert which I guess is good for the world and if the field is varying in the UK it is likely to be varying world wide? I have no real knowledge of this. Andrew