Walt wrote
Way back when I was a navigator on a KC-135 using Grid Navigation we
referenced the DG to Grid North, not True North. Big difference between
the two, although I think I know what you're hinting at.

Right! And at PanAm, we paid real big bucks for those 'low precession'
gyros. Actually they had an anti-precession controller into which one
would set the latitude as one progressed.

Bob Moore