Help with magnetic variation/declination ?

TarfHead

I'm trying to brush up on map/compass skills and have created confusion in my own head around magnetic variation (MV). Let me sketch it out ..

I have an OSI Discovery series Sheet 56, where magnetic variation is given as a 1995 value of 8d24m. Annual decrease is given as 8.5m. If I want to use this map in 2019, I should then re-calculate MV as 8d24m, minus (8,5m times 24 years).

Correct ?

If so, that works out as 8d24m minus 3d24m, or 5 degrees.

Still correct ?

To check my work, I used Google to see what the current MV is/should be,and got a figure closer to 2d.

Where have I gone wrong ? Do I even need to descend down this rabbit hole ?

cruizer101

At a guess it may not be a linear relationship so the change per year varies per year also. e.g. in 1995 decrease is 8.5m but by 200 may be 9m.
A change small enough that going from the 1995 reading and using 8.5 per year you would be grand for a few years but its now 24 years later so no valid.
Thats just me guessing mind, but as you say yourself is it just a rabbit hole, a few degrees really isn't much especially when you consider user error.

Tabnabs

Magnetic declination is not a fixed entity, it changes for reasons we still don't fully understand (https://www.mnn.com/earth-matters/climate-weather/stories/magnetic-north-shifting-by-40-miles-a-year-might-signal-pole-r)

So the information on your map is accurate for the time of publication, but is now out of date and incorrect. If looking for the correct information on the internet, then reliable sources are always your best bet. I can find nothing on the OSI to update the correction, but the American NOAA website has a useful reference tool that I would expect to be about right. https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination

So for Dublin, it's 2.66° W ± 0.38°, so just shy of three degrees on the compass in your hand.

TarfHead

cruizer101 wrote: »

At a guess it may not be a linear relationship

I have since found a 2006 version of Sheet 56. MV is given as 6d26m, with annual variation 9.6d.

Definitely not linear !

Thanks

Carawaystick

https://www.forbes.com/sites/trevornace/2019/02/05/earths-magnetic-north-pole-has-officially-moved
says its starting to move faster than before