Author Topic: First negative magnetic correction for UK cave survey?  (Read 1186 times)

Offline alasdair neill

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First negative magnetic correction for UK cave survey?
« on: March 07, 2014, 09:44:21 am »
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

Offline Bottlebank

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Re: First negative magnetic correction for UK cave survey?
« Reply #1 on: March 07, 2014, 10:12:14 am »
I suspect it means the earth moved for you?
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Offline Rob

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Re: First negative magnetic correction for UK cave survey?
« Reply #2 on: March 07, 2014, 10:34:09 am »
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
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Offline paul

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Re: First negative magnetic correction for UK cave survey?
« Reply #3 on: March 07, 2014, 12:12:48 pm »
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

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.
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Offline Rob

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Re: First negative magnetic correction for UK cave survey?
« Reply #4 on: April 04, 2014, 03:07:27 pm »
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, 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:

*calibrate compass 1.985
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Offline exsumper

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Re: First negative magnetic correction for UK cave survey?
« Reply #5 on: April 04, 2014, 06:40:06 pm »
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.

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