[Therion] North True Grid Magnetic Astronomical?

[Eric Madelaine]

Martin Budaj wrote:
>
>> The grid north is usually not the same as the true (astronomical) north, but
>> for the purposes of cave maps and Therion I would assume it is usually
>> considered to be so.  (Not strictly correct, but I imagine the error would
>> not be detectable on any printed cave map in most co-ordinate systems)
>>     
>
> The difference is up to 3 degrees for UTM projection or up to 10
> degrees for Czech Krovak projection.
>
>   

I would add to Martin explanation that while the meridian convergence is 
fixed (for a given location), the declination varies significantly with 
time. So if you are managing a cave survey project for say, 10 or 20 
years... then the difference in declination will be significant (can be 
several degrees)

Eric.