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:[email protected]] 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 _______________________________________________ Therion mailing list Therion at speleo.sk http://mailman.speleo.sk/mailman/listinfo/therion
