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

[Wookey]

+++ Michael Lake [04-10-07 09:02 +1000]:
> 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'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.

There is nothing wrong with the current method of specifying corrections on
a per-survey basis as they are done now, but equally the ability to have more
central control of this info is also very useful, especially for large and
long-term datasets.

e.g if no calibs are specified then survex (optionally) using things like
the USGS world magnetic model to put in automatic corrections would make
your data better. Winkarst already provides this option. 

Being able to say 'put in these corrections for this dataset byt date and
position' globally would also make it much easier to get the correct numbers
put in, and kept consistent on a large dataset. CUCC's austria dataset would
certainly benefit from this (where there is generally no calibration info in
the dataset because we've laways been going to 'put it in later' (ha ha)).
This last got done in about 1990 IIRC.

No-one is arguing that the ability to specify explicit
calibrations/declinations should be removed or overruled - we're looking at
ways to more conveniently specify that data (i.e. in one place, not hundreds
of time in loads of files), and/or to use models to pick better defaults,
when no calibs are provided.

Wookey
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