[Therion] North True Grid Magnetic Astronomical?

[Martin Budaj]

On Mon, Jul 28, 2008 at 10:39 PM, Bruce Mutton
<bruce.mutton at paradise.net.nz> wrote:
> Does the North Arrow plotted by Therion always point to true (astronomical)
> north regardless of any of the other settings?  Your response to a) & b)
> below suggests it is so.  This would be different to the usual practice in

Yes, exactly.

> NZ as I understand it, where we plot an arrow to grid north with, often, a
> secondary arrow to magnetic north as it was at the time of surveying.  (Yes,
> if Therion were to do this it would need to plot an average or a range of
> magnetic norths)  To adopt a co-ordinate grid and then point to true
> (astronomical) north seems counter intuitive to me, but I accept that there
> are many different ways of doing things.

The grid north is always indicated by crosses/grid lines, so we use
the north arrow to indicate astronomical north exclusively.
I think the true north should be always displayed, because the grid
north depends on your choice of geodetic coordinate system.
As you mention, the magnetic north is ambiguous so we do not display it at all.

> Therions current behaviour is probably fine, I just need to know what it is!
> Perhaps the Therion book entry on North true/grid could be updated to
> clarify the direction the north arrow points.

OK, it will be more verbose.

Martin