[Therion] North arrow and magnetic declination

[Benedikt Hallinger]

It is map compile time and it shows the declination for the day of map 
compilation:

As i understood, when compiling the map all directions are converted to 
geographic north, because the station positions are georeferenced to the 
coorindates. There, the date of the survey is taken into account to get 
the corrected coordinates based on the magnetic model and the 
location/survey date.

Aligning the map to geographic north and putting out the magnetic north 
is just an info "today the declination is this-and-that degrees".
Aligning the map to magnethic north makes that info crucial to be able 
to calculate geographic north.


Am 2020-05-27 22:28, schrieb Bruce Mutton:
> 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-bounces at speleo.sk> On Behalf Of Bill Gee
> Sent: Thursday, 28 May 2020 01:51
> To: Therion Mail List <therion at 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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