[Therion] North arrow and magnetic declination

[Philippe Vernant]

Hi,

This seems strange. The magnetic north change over time so why bother to choose a date and plot the magnetic north if you can correct it thanks to therion and have the map with the geographic north? The only interesting point I see would be to draw the magnetic north at the time when the cavers will go caving so they can use their compass without headache ;-)

Phil 




> On 27 May 2020, at 15:51, Bill Gee <bgee at campercaver.net> wrote:
> 
> Hello everyone -
>  
> I was looking at some of the sample code on the wiki for alternate north arrows.  At least two of them display both geographic north and magnetic north.  Those are northarrow4 and northarrow4a, both by Dirk Peinelt.
>  
> My question is this:  What date is used when calculating the offset angle for the magnetic north arrow?
>  
> This is especially relevant for caves that have been surveyed over a period of years.  The declination changes from year to year, and sometimes more often than that.  There are at least four possibilities:
>  
> 1) The date of the first survey.
> 2) The date of the most recent survey.
> 3) A date about half-way between the first and last surveys.  This assumes that the declination change is somewhat linear over time.
> 4) The date the map is compiled.
>  
> Does anyone know which date is used?
>  
> For me this is mostly academic.  I am just curious!  I have never used a north arrow that shows both geographic and magnetic north.  Most of the maps I make are for caves in Missouri.  The magnetic declination is less than 1 degree.  It is almost irrelevant here.
>  
> Thanks!
> 
> -- 
> Bill Gee
>  
>  
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