Survex Digest, Vol 4, Issue 3

[Mike McCombe]

> Michael Lake said:
>
> | The mag declination correction for a given area might not have a linear
> | relationship with the date. Sometimes it is given as so-many degrees per
> | year on a map but sometimes it's given as a table of values. I'd rather
> | have to add a correction explicitly myself than survex guess it for me
> | and not realise it. Also if I get the data wrong for a map by entering
> | it incorrectly then the mag dec would be wrong too.
> |
> | my 2c worth.
> |
> I agree. Not only that, but many people calibrate their compasses in such 
> a
> way as to include both compass error and magnetic deviation in one figure.
> Letting Survex deal with one part of this (I do not see any way in which 
> it
> can automatically deal with my compass!), will either lead to confusion or
> to some folks not doing it correctly - or at all!
>
> Graham

With daily variations in magnetic declination in the UK typically 0.2 - 0.5 
degrees (see http://www.geomag.bgs.ac.uk/observatories.html for examples) it 
seems to me to be good practice to calibrate the compass against a known 
physical baseline at the start of each day's work. Just taking the published 
long-term rate of change of declination would be a poor second-best and not 
something to be encouraged!

Mike