Change in the Earth's magnetic position.

[Terry]

Recently I read that the Earth's magnetic position had moved, thus making all the data on OS maps obsolete. I have been trying to understand 'magnetic declination'. Where I live, just North of Dartmoor, the 'Grid Magnetic Angle Calculator' informs me that magnetic North is now 0 degrees 25' west. As I understand it there are 60' to a degree, thus making this change fairly insignificant, is that assumption correct?

[Lost Soul]

Hi Terry,

Welcome to the Forum.  Its bit slow these days but still a great resource. If you do a search on past threads and post you will find information on the subject. 

To answer your question.  Any OS map is obsolete on the day it was sent to the printers let alone the day it is published.  Changes are continuous.  New roads and all that. 

In reality the very small change in magnetic field is not significant in the short term.  Which means if you are using a current map then all is well.  There is no way you are going to be measuring 0.25 of a degree on a hand held compass.  However, the changes do build up as the years roll by and information given on a ageing map is going to become increasingly inaccurate.  Both in terms of magnetic data and topographical data.  Bottom line always use a current map.

Lost Soul

[Brynglas]

Good to see that there's still a bit of traffic on this forum.

Magnetic Declaration is constantly changes, albeit at a calculable rate. If your journeying over relatively short distances the declaration doesn't create such an error. Declaration obviously becomes a much more significant factored over longer distances.

I always try to calculates the factored in, but with changes of less than a degree this isn't so practicable on a day to day basis.

Some interesting info here:

https://www.ngdc.noaa.gov/geomag/declination.shtml