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 <[email protected]> On Behalf Of Bill Gee
Sent: Thursday, 28 May 2020 01:51
To: Therion Mail List <[email protected]>
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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