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Grid Nth to Mag Nth angle...

Question asked by NZGeoCAD on Jan 13, 2016

I'm trying to dynamically calculate the Grid North to Magnetic North angle for maps to allow navigation with a compass.


Using the Production Mapping's 'Topo North Arrow' gives 80% of what I need but doesn't give the actual angle between Grid North and Magnetic North (i.e. the value that someone navigating by compass would use). Furthermore, it appears that the coordinate system the arrow uses isn't updated dynamically- A test map with a Topo North Arrow (set to 'Update Dynamically') in UTM59S, saved, then changed to a map with UTM 60 data and positioned in the UTM60S zone still shows UTM 59S.....of course I could untick the 'update dynamically' box and change it to the correct zone OR insert a new Topo North Arrow but there is no way (?) to do this programatically/in arcpy.mapping so it wouldn't be dynamic.


The tool 'Magnetic Calculator' within the production Mapping toolbox can be used to calculate various components of the various North Arrows. While the Declination value is about right, the Magnetic Variation is quite different (60 degrees vs about a 20-24 Grid to Mag angle)...furthermore that tool requires the data to be in Lambert Conformal Conic Projected coordinate system to calculate the GRID_VARIATION field.....yet there is no coordinate system that appears to be named that exactly and instead there are regional versions....I guess that's why there are errors if the origin of the cone is offset?


Why does that tool not just accept a UTM coordinate system to calculate the Grid-Mag angle?


Does anyone have any tips?