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
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@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
_______________________________________________ Therion mailing list Therion@speleo.sk <mailto:Therion@speleo.sk> https://mailman.speleo.sk/listinfo/therion <https://mailman.speleo.sk/listinfo/therion>
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@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
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@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 _______________________________________________ Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
Thanks, Bruce and Benedikt. That answers my question and satisfies my curiosity. I am well aware of how Therion processes declination when going through the survey data. I take the trouble to make sure every set of survey data has a date associated with it so that the declinations can be properly calculated. As for putting declination on the map ... I have never done that and do not plan to start. I think it is not useful to have it on a map. In 20 years (or whenever) it will be wrong. It is better to just leave it off. One of my cave mapping friends always takes the trouble to make his north arrow indicate both geographic and magnetic north. He and I have not discussed this feature of his maps, so I do not know what his reasoning is. He is not a Therion user. -- Bill Gee On Wednesday, May 27, 2020 3:41:06 PM CDT Benedikt Hallinger wrote:
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@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 _______________________________________________ Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
27. 5. 2020 v 23:39, Bill Gee <bgee@campercaver.net>:
I am well aware of how Therion processes declination when going through the survey data. I take the trouble to make sure every set of survey data has a date associated with it so that the declinations can be properly calculated.
As for putting declination on the map ... I have never done that and do not plan to start. I think it is not useful to have it on a map. In 20 years (or whenever) it will be wrong. It is better to just leave it off.
Declination up to 1 degree could be not a problem. But there are regions where declination is more than 10 degree. And I know a false theory about development of two huge parallel caves instead of only one system because the author of that theory just omitted inclination of surveys of upper and lower part of that cave from upper and lover entrances (they were not connected). Softwares for cave data offer the automatic calculation of declination (and grid convergence), so there is not reason not to use it. Therion calculates declination according each day from the model. Maybe there should be the date when a map was generated under North sign in such case. Martin
On Thu, May 28, 2020 at 10:20:50AM +0200, Martin Sluka via Therion wrote:
27. 5. 2020 v 23:39, Bill Gee <bgee@campercaver.net>: I am well aware of how Therion processes declination when going through the survey data. I take the trouble to make sure every set of survey data has a date associated with it so that the declinations can be properly calculated.
As for putting declination on the map ... I have never done that and do not plan to start. I think it is not useful to have it on a map. In 20 years (or whenever) it will be wrong. It is better to just leave it off.
Declination up to 1 degree could be not a problem. But there are regions where declination is more than 10 degree.
It's more than 25° in southern NZ: https://www.gns.cri.nz/var/ezwebin_site/storage/images/home/our-science/land...
And I know a false theory about development of two huge parallel caves instead of only one system because the author of that theory just omitted inclination of surveys of upper and lower part of that cave from upper and lover entrances (they were not connected).
I suspect it took CUCC longer to connect Eislufthöhle to Kaninchenhöhle because we weren't correcting for declination back then (the declination in the area used to be essentially zero and the models to calculate it weren't easily available so we left it to sort out later, but gradually it increased to the point where it actually made a difference).
Softwares for cave data offer the automatic calculation of declination (and grid convergence), so there is not reason not to use it. Therion calculates declination according each day from the model.
I don't think anyone has suggested in this thread that one shouldn't correct for declination.
Maybe there should be the date when a map was generated under North sign in such case.
If you're going to put a Magnetic North arrow on, then I agree it really ought to be clear what date it's for. Bill's original message was saying that therion has options for North arrows which show both geographic and magnetic north, but *DON'T* say what date the shown magnetic north is for: https://mailman.speleo.sk/pipermail/therion/2020-May/008254.html That seems like a bug to me. Cheers, Olly
participants (6)
-
Benedikt Hallinger -
Bill Gee -
Bruce Mutton -
Martin Sluka -
Olly Betts -
Philippe Vernant