Hi Bill I played with Dirk's north arrow back in 2018, making my own adjustments, and came to the conclusion that the magnetic north reported is that at the date of map compilation. Looking at my comments in my metapost I am not absolutely convinced however. Regardless, the reporting of a magnetic north direction needs to be accompanied by the date and location to avoid confusion. Dirk's north arrow, and my variation, both report the date, so we have that covered, and Therion uses the 'centre' of the map as the location.
It is somewhat relevant in New Zealand. The magnetic deviation is approximately 20 degrees and has varied 2 or 3 degrees over my lifetime, and it varies by a similar amount across our caving regions. There are surveys that span three decades. The only time I have encountered a problem with plan maps in Therion related to magnetic deviation is when a recent survey is added, and the compiling software version pre-dates that survey (geomagnetic model cuts out prior to the survey date). In this circumstance Therion sets the magnetic deviation to zero and does not report the error, or reports it as something unrelated. Bruce From: Therion <therion-boun...@speleo.sk> On Behalf Of Bill Gee Sent: Thursday, 28 May 2020 01:51 To: Therion Mail List <therion@speleo.sk> Subject: [Therion] North arrow and magnetic declination 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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