[Therion] North True Grid Magnetic Astronomical?

[Bruce Mutton]

Eric wrote

>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.

Eric
I guess we are lucky in NZ, the declination varies by about a degree a
decade, and we've only been producing maps for about four decades. The
overall accuracy today of most of our surveys is probably no better than
this (although we would kid ourselves otherwise).
Bruce