The other day put a couple of survey legs into the Mousehole & getting the magnetic variation from the BGS site resulted in a negative correction - is this a first for a cave survey in the UK?
see http://britgeopeople.blogspot.co.uk/2014/01/somethings-happening-to-magnetic-north.html

I suspect it means the earth moved for you?

Does this mean that currently it is so close to zero that we don't need to worry too much about correction, as per Dep's prediction 7 years ago:
http://ukcaving.com/board/index.php?topic=6036.msg83067#msg83067

Rob said:

Does this mean that currently it is so close to zero that we don't need to worry too much about correction, as per Dep's prediction 7 years ago:
http://ukcaving.com/board/index.php?topic=6036.msg83067#msg83067

Click to expand...

It depends where you are. Have a look at http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html which will give you the amount of Variation depending on location.

paul said:

It depends where you are. Have a look at http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html which will give you the amount of Variation depending on location.

Click to expand...

Paul, thanks for the link.

So if i get a Main Field Declination of -1.985 degrees east, that means i need to correct in survex using:

Given the great work being done by Graham etal on creating 3d catchment cave surveys;which are superb!  The following point has most probably been considered but may be worthy of repeating.

I recently tried to overlay several old  cave surveys of obscure sites on to an OS Map;some of the surveys dating from the 1920's. The results appeared to be rather bizzare, until I calculated the difference between true and magnetic north at the time. This was approx 16 degrees;a considerable difference.

That's all