[Therion] North True Grid Magnetic Astronomical?

[Bruce Mutton]

Martin
Hmm.  So to further clarify the discussion appended below;

I appreciate the declination varies with time, but for most potentially
active projects in NZ the difference is less than 2 degrees, and if we are
dealing with magnetic bearings (historic maps) we accept that any degree of
accuracy will be fairly nominal.


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
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.

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.

Thanks again
Bruce

-----Original Message-----
From: therion-bounces at speleo.sk [mailto:therion-bounces at speleo.sk] On Behalf
Of Martin Budaj
Sent: Monday, 28 July 2008 11:39 p.m.
To: List for Therion users
Subject: Re: [Therion] North True Grid Magnetic Astronomical?

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
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