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.
