Re: [Therion] North arrow and magnetic declination

[Olly Betts]

On Thu, May 28, 2020 at 10:20:50AM +0200, Martin Sluka via Therion wrote:
> > 27. 5. 2020 v 23:39, Bill Gee :
> > 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-marine-geoscience/earth-s-magnetic-field/declination-around-new-zealand/nz-magentic-declination/75157-1-eng-GB/NZ-magentic-declination.png

> 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
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Re: [Therion] North arrow and magnetic declination

[Martin Sluka via Therion]

> 27. 5. 2020 v 23:39, Bill Gee :
> 
> 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.

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Re: [Therion] North arrow and magnetic declination

[Bill Gee]

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  On Behalf Of Bill Gee
> > Sent: Thursday, 28 May 2020 01:51
> > To: Therion Mail List 
> > 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
> ___
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> 

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Re: [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  On Behalf Of Bill Gee
Sent: Thursday, 28 May 2020 01:51
To: Therion Mail List 
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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Re: [Therion] North arrow and magnetic declination

[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  On Behalf Of Bill Gee
Sent: Thursday, 28 May 2020 01:51
To: Therion Mail List 
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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Re: [Therion] North arrow and magnetic declination

[Philippe Vernant]

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  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 
> https://mailman.speleo.sk/listinfo/therion 
> 
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[Therion] North arrow and magnetic declination

[Bill Gee]

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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