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 <[email protected]> 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 > > > _______________________________________________ > Therion mailing list > [email protected] <mailto:[email protected]> > https://mailman.speleo.sk/listinfo/therion > <https://mailman.speleo.sk/listinfo/therion>
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