Mag dec from date. was Re: Survex 1.1.1 test version uploaded

[M.J. Green]

On Oct 7 2004, Michael Lake wrote:

> M.J. Green wrote:
> > With cavern understanding dates, would it be possible to put in 
> > corrections between magnetic north and grid north based on date?  It is 
> > of course not obvious where to put in this general data, or perhaps 
> > survex is tending towards processing everything together, and then 
> > extracting the data that is required.
> 
> 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 was thinking of being able to specify the magnetic corrections at a list of dates, with linear interpolation in between the specifyed data.  I should have been clearer about what I meant.

> I'd rather 
> have to add a correction explicitly myself than survex guess it for me 
> and not realise it.

On the other hand having the correction all in one place, you can check that it is right, whereas if it is spread through your dataset, it would be much harder to check and have it consistent.

> Also if I get the data wrong for a map by entering 
> it incorrectly then the mag dec would be wrong too.
> 
I am not sure I know what you mean by mag dec, do you mean the angle of the magnetic field compared to the gravitational field.  If so surely this is not considered in a compass and clino survey.

Cheers,
Martin