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Compass Deviation

How do you calculate the GMA on Landranger/Ordinance Survey Maps?

I am a cadet and as part of my training syllabus i have to teach other cadets lessons. Unfortunately i drew the short straw and ended up with the Map and Compass lesson: Grid Magnetic Angle at which i am hopeless.

If anyone knows an easy way to calculate the Grid Magnetic Angle for a landranger/ordinance survey map i would be grateful.

It was eplained to me by someone else that allowed me to fully understand it, but i have forgotten it completely, it involved clocks.

Please keep it simple and easy to understand as other websites either didn't explain how to do it or did so in incredibly complex ways.

GMA is also refered to as the magnetic varience or magnetic deviation.

Regards

Mike

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True Magnetic Variation Compass Readings and Deviation



What has caused the deviation of magnetic north coordinates on a compass?

Is it due to the magnetic change within the Earth's core?

Yes, in effect, the magnetic deviation is caused by a fluctuation in the strength and location of the magnetic north pole, which in turn is controlled by the earth's internal dynamo. Scientists have actually mapped the earth's magnetic field, and produced something similar to a contour map with lines pointing to the magnetic north pole from wherever you are in the world. The magnetic north pole is currently sitting about 60km off the western coast of Ellesmere island near Canada, and is moving toward Russia at a rate of about 60km per year.

Deviation can also be caused by localized variation in the magnetic field, often influenced by Iron deposits, though some anomalies are still not fully understood.

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