[Therion] North True Grid Magnetic Astronomical?

[Martin Budaj]

On Mon, Jul 28, 2008 at 12:20 PM, Bruce Mutton
<bruce.mutton at paradise.net.nz> wrote:
> So just to clarify if I understand correctly;
>
> If we want to plot our maps with respect to magnetic north (to be compatible
> with 'old' uncorrected surveys for example) then we do not specify
> declination and we do not specify any co-ordinate system.  If we specify a
> grid to be plotted it will be aligned with magnetic north.

It would be true if there were no changes in declination. As the
magnetic declination changes over time, you should perhaps specify a
difference of declinations for old and new data to have them both
aligned.

> Also if we do specify a co-ordinate system and;
> a) adopt Grid North, the north arrow would plot parallel to the (NS) grids,

Here the grid is exactly parallel to the page margins and the north
arrow is (usally slightly) rotated.

> or,
> b) adopt True North, the north arrow would NOT (necessarily) be parallel to
> any of the grids.

Here the north arrow is parallel with left/right paper margin and the
grid is rotated.

Martin