Declination/Magnetic Variation

Richard Phillips said:

Am I right in thinking that if magnetic N is 2deg W of grid N, then this
would be Easterly declination since grid N is Easterly relative to magnetic
N?

I don't think so... see
http://en.wikipedia.org/wiki/Magnetic_declination

Bottom line: remember from grid to mag, add, from mag to grid, rid.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Declination is the deg' east or west of grid north. In other words if you
have 2degW of declination, your compass needle is pointing 2degW of grid
north.

"Richard Phillips" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Hello all,

Been trying to figure this out this morning once and for all, really I
just want to check my understanding of terminology is correct.

Am I right in thinking that if magnetic N is 2deg W of grid N, then this
would be Easterly declination since grid N is Easterly relative to
magnetic N?

In other words, is declination describing grid N relative to magnetic N
(as I think is the case) or vice versa?

I think this is the case by the way, because when my compass is pointing
to magnetic N and the map is pointing to grid N (2 deg E) then the
declination scale on my compass indicates that this is Easterly
declination.

Cheers,
Rich.

Peter Clinch said:

Bottom line: remember from grid to mag, add, from mag to grid, rid.

Nice mnemonic! I've never really felt the need to fiddle around with the
couple of degrees difference between mag North and grid North. I doubt
whether you can take a bearing with a Silva type compass accurate to
within 2 degrees anyway - to do any better you'd need a prismatic
compass. As long as I'm carrying enough food and have shelter, I'm not
particularly bothered with compass navigation. I've ended up in
interesting places on the occasions when I've got lost! The last time
this happened I was on the complex ridge system in the middle of Arran.
The mist was coming and going and I hadn't a clue where I was on the map
- even if I could be bothered to get it out and struggle with it in the
wind! I just shrugged my shoulders and flogged on knowing I would get to
the sea eventually - which is what happened. Mind you, it would have
been a longer walk to the sea from the Cairngorms!

Jim Ford

Peter/den,

I see it now, thanks.

I was ok in so far as using the compass was concerned, but my understanding
of e.g. "10 deg W declination" was the wrong way around.

So declination refers to magnetic N relative to grid N then, I thought it
was the other way around.

I think what actually confused me was my compass (Silva Ranger 3 I think it
is). It has red markings on the bottom of the dial part for declination, to
the left it's marked "W declination" and to the right it's "E declination".
I think the idea is for W declination I turn the dial towards "W
declination" (sounds obvious), but if I turn it 20 deg, the scale I am using
on the dial is the "E declination" side; I think this is where the confusion
was coming from!

All ok now I think.

Cheers,
Rich.

"den" <[email hidden]> wrote in message news:[email hidden]...

Quoted message said:

Declination is the deg' east or west of grid north. In other words if you
have 2degW of declination, your compass needle is pointing 2degW of grid
north.

"Richard Phillips" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Hello all,

Been trying to figure this out this morning once and for all, really I
just want to check my understanding of terminology is correct.

Am I right in thinking that if magnetic N is 2deg W of grid N, then this
would be Easterly declination since grid N is Easterly relative to
magnetic N?

In other words, is declination describing grid N relative to magnetic N
(as I think is the case) or vice versa?

I think this is the case by the way, because when my compass is pointing
to magnetic N and the map is pointing to grid N (2 deg E) then the
declination scale on my compass indicates that this is Easterly
declination.

Cheers,
Rich.

I've never been a big fan of mnemonics personally, I *usually* find if I can
understand the principles of something then a mnemonic is not needed!

I doubt I'll ever use this is reality, but I feel happier saying that when
(I think) I understand it all.

Better to understand it and not use it, eh? 🙂

Rich.

"Jim Ford" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Peter Clinch said:

Bottom line: remember from grid to mag, add, from mag to grid, rid.

Nice mnemonic! I've never really felt the need to fiddle around with the
couple of degrees difference between mag North and grid North. I doubt
whether you can take a bearing with a Silva type compass accurate to
within 2 degrees anyway - to do any better you'd need a prismatic compass.
As long as I'm carrying enough food and have shelter, I'm not particularly
bothered with compass navigation. I've ended up in interesting places on
the occasions when I've got lost! The last time this happened I was on the
complex ridge system in the middle of Arran. The mist was coming and going
and I hadn't a clue where I was on the map - even if I could be bothered
to get it out and struggle with it in the wind! I just shrugged my
shoulders and flogged on knowing I would get to the sea eventually - which
is what happened. Mind you, it would have been a longer walk to the sea
from the Cairngorms!

Jim Ford

By the way, that "in other words" is a nice neat way of thinking about it!

R.

"den" <[email hidden]> wrote in message news:[email hidden]...

Quoted message said:

Declination is the deg' east or west of grid north. In other words if you
have 2degW of declination, your compass needle is pointing 2degW of grid
north.

"Richard Phillips" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Hello all,

Been trying to figure this out this morning once and for all, really I
just want to check my understanding of terminology is correct.

Am I right in thinking that if magnetic N is 2deg W of grid N, then this
would be Easterly declination since grid N is Easterly relative to
magnetic N?

In other words, is declination describing grid N relative to magnetic N
(as I think is the case) or vice versa?

I think this is the case by the way, because when my compass is pointing
to magnetic N and the map is pointing to grid N (2 deg E) then the
declination scale on my compass indicates that this is Easterly
declination.

Cheers,
Rich.

Jim Ford said:

Nice mnemonic!

Taken from "Mountain Navigation", by Peter Cliff. There's a couple
of others in there, but, ummm, I can't remember them! ;-/

Quoted message said:

I've never really felt the need to fiddle around with the
couple of degrees difference between mag North and grid North.

It's changed over time, and probably also changes according to
where you are in the grid to some extent: when I started map
reading as a Scout it was about 7.5 degrees, which *is* worth
accounting for!

Quoted message said:

whether you can take a bearing with a Silva type compass accurate to
within 2 degrees anyway - to do any better you'd need a prismatic
compass.

My compass is a Silva with a sighting prism too, the 54 NL, though
to be honest I don't use the sighting feature much. Most often I
walk on very approximate bearings (like "just to the left of
North"😉 where I don't correct. If you can keep track of where you
are that's usually quite good enough, and orineteers' thumb
compasses don't allow any better than that.

Quoted message said:

As long as I'm carrying enough food and have shelter, I'm not
particularly bothered with compass navigation. I've ended up in
interesting places on the occasions when I've got lost!

Often the case, but if just wandering along until you get to some
known point takes you out onto a collapsing cornice it could Ruin
Your Whole Day... The descent from the top of Ben Nevis in clag is
a prime example that's cost peoples lives when they got it wrong.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

In message <[email hidden]>, Jim Ford
<[email hidden]> writes

Quoted message said:

Peter Clinch said:

Bottom line: remember from grid to mag, add, from mag to grid, rid.

Nice mnemonic! I've never really felt the need to fiddle around with
the couple of degrees difference between mag North and grid North.

I've never bothered in this country either, but I've just got back from
South Africa (where we climbed Table Mountain and the West Peak of the
Helderburg). Magentic declination there is 23 deg W. That gave me a bit
of a shock (and a frantic few minutes of searching my memory to try to
remember what to do to the compass).

Fortunately the weather was so clear that navigation wasn't a problem.
The temperature was though, 35 deg C 'off piste' in the Helderburg
looking for a fruit bat cave was no joke.

Regards
--
Neil Pugh

Richard Phillips said:

I've never been a big fan of mnemonics personally, I *usually* find if I can
understand the principles of something then a mnemonic is not needed!

It varies with me, and some things I use them, others not, and in
some cases some mnemonics work for me while others don't. Use
whatever works for you, "Mag to grid, rid" just hits the spot for
me. I do know the principles and can work it out, but if the
mnemonic lets me short-circuit straight to the answer then I find
that's handy, especially in a stress situation where thinking gets
a lot harder!

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

The message <[email hidden]>
from Peter Clinch <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

I've never really felt the need to fiddle around with the
couple of degrees difference between mag North and grid North.

Quoted message said:

It's changed over time, and probably also changes according to
where you are in the grid to some extent: when I started map
reading as a Scout it was about 7.5 degrees, which *is* worth
accounting for!

Just had a look at my earliest Lake District map. 10 degrees 50 minutes
W on 1st June 1951.

--
Roger Chapman

Neil Pugh said:

I've never bothered in this country either, but I've just got back from
South Africa (where we climbed Table Mountain and the West Peak of the
Helderburg). Magentic declination there is 23 deg W. That gave me a bit
of a shock (and a frantic few minutes of searching my memory to try to
remember what to do to the compass).

Yeah, it's a similar magnitude in New Zealand. I was about halfway
through my trip before I found that out. On a similar note, I've heard
people say you can't use a Northern Hemisphere compass in the Southern
Hemisphere because of the dip angle but I didn't find any problems.

"Peter Clinch" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Richard Phillips said:

I've never been a big fan of mnemonics personally, I *usually* find if I
can understand the principles of something then a mnemonic is not needed!

It varies with me, and some things I use them, others not, and in some
cases some mnemonics work for me while others don't. Use whatever works
for you, "Mag to grid, rid" just hits the spot for me. I do know the
principles and can work it out, but if the mnemonic lets me short-circuit
straight to the answer then I find that's handy, especially in a stress
situation where thinking gets a lot harder!

Yeah, same here really. I find if I understand something I can normally
figure it out again. I rarely trust myself to remember even simple
mnemonics though!
Like you say, whatever works best 🙂

Rich.

Quoted message said:

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

"Peter Clinch" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Jim Ford said:

Nice mnemonic!

Taken from "Mountain Navigation", by Peter Cliff. There's a couple of
others in there, but, ummm, I can't remember them! ;-/

Quoted message said:

I've never really felt the need to fiddle around with the couple of
degrees difference between mag North and grid North.

It's changed over time, and probably also changes according to where you
are in the grid to some extent: when I started map reading as a Scout it
was about 7.5 degrees, which *is* worth accounting for!

It tells you the magnetic variation in the margin of OS
1:25000 and 1:50000 maps, and how much it is expected
to change during the lifetime of the map.

In message <[email hidden]>, Roger
<[email hidden]> writes

Quoted message said:

Just had a look at my earliest Lake District map. 10 degrees 50 minutes
W on 1st June 1951.

My oldest maps show:

Lands End 1st Jan 1947 - 11 deg 53 min W
Saffron Walden 1st Jan 1946 - 9 deg 44 min W
Gloucester 1962 - 9 deg W

http://www.ngdc.noaa.gov/seg/geomag/declination.shtml

Has a web page for computing variation for lat/long and date. As a test
I put Lands End into it for 1947 and it came up with:

Declination = 12° 1' W changing by 0° 8' E/year

which corresponds pretty closely with the OS figure above.

Regards
--
Neil Pugh

In article <[email hidden]>, Neil Pugh
<[email hidden]> writes

Quoted message said:

In message <[email hidden]>, Roger
<[email hidden]> writes

Quoted message said:

Just had a look at my earliest Lake District map. 10 degrees 50 minutes
W on 1st June 1951.

My oldest maps show:

Lands End 1st Jan 1947 - 11 deg 53 min W
Saffron Walden 1st Jan 1946 - 9 deg 44 min W
Gloucester 1962 - 9 deg W

http://www.ngdc.noaa.gov/seg/geomag/declination.shtml

Has a web page for computing variation for lat/long and date. As a test
I put Lands End into it for 1947 and it came up with:

Declination = 12° 1' W changing by 0° 8' E/year

which corresponds pretty closely with the OS figure above.

But you should beware that the geomag figure is the difference between
true north and mag north. Usually we are interested in the difference
between grid north and mag north and that is what is given on the OS
maps.

Because the OS grid is a distortion of the surface of the earth to make
it flat the lines of grid north are not actually parallel - this is
known as the convergence of the grid. On the central meridian of the
grid (2 deg W) grid north and true north are aligned. Either side of the
central meridian the grid north the grid curves away from true north as
a function of both the distance from the central meridian and the
latitude.

The convergence varies throughout the UK from ~ -3.6 degrees (square NB)
to ~+3 degrees (square TG).

--

Dominic Sexton

This thread made me think about GPS and digital maps. How does one work
out what the current variation is with only these available?

OK the GPS has some kind of look up table for it, but I don't think the
actual value is easily available, especially if you are not actually at
the location.

The digital map (Anquet, Mapwise etc), you can't go to the bottom of the
sheet and look it up, as on a paper copy.

I am currently in Norway but preparing for a week near Braemar. I have
printed out copies of the maps I will need and will have my GPS, but I
want to set up my compass and, without a paper map copy, cant see how to
dig up the variation.

While on this topic, I found myself at the junction of two UTM (grid)
areas last weekend in Norway, but near Sweden.
On the same sheet, UTM zone 32 changes to UTM zone 33. one side is
"grid get rid" but the other is the opposite. Could be very confusing.

--
Hbunnet : (replace "nospam" with "athome" to reply)
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/

Hbunnet <[email hidden]> wrote

Quoted message said:

This thread made me think about GPS and digital maps. How does one work
out what the current variation is with only these available?

I have always had my GPS set to AUTO mag variation, and it has always
led me where I wanted to go.

None of that "From map to terrain add" and "From terrain to map
subtract" stuff.
;-)
--
Gordon Harris

In article <[email hidden]>, Hbunnet
<[email hidden]> writes

Quoted message said:

This thread made me think about GPS and digital maps. How does one work
out what the current variation is with only these available?

OK the GPS has some kind of look up table for it, but I don't think the
actual value is easily available, especially if you are not actually at
the location.

The digital map (Anquet, Mapwise etc), you can't go to the bottom of the
sheet and look it up, as on a paper copy.

I am currently in Norway but preparing for a week near Braemar. I have
printed out copies of the maps I will need and will have my GPS, but I
want to set up my compass and, without a paper map copy, cant see how to
dig up the variation.

Well it seems to me that you have two options:

1. Get some kind soul to look it up on a map!

2. Use Geomag to get the declination for the location and then
adjust for the convergence.

Braemar is almost exactly 57N & 3.4W. Putting those into the Geomag
calculator

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

gives us 4.62 degrees west.

Convergence of the grid is calculated by multiplying the longitude
difference from the central meridian (west being positive) by the sine
of the latitude. See:

<http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
emsinfo/guidetonationalgrid/page19.html>

The central meridian is 2 degrees west and Braemar is 3.4 degrees west
so the longitude difference is 1.4 degrees. Multiplying that by the sine
of the latitude gives us 1.17 degrees. Therefore grid north is 1.17
degrees west of true north.

As magnetic north is also west of true north the convergence is taken
away from the magnetic declination obtained earlier. 4.62 - 1.17 = 3.45
degrees west.

If you are going to the west or north of Braemar the convergence will
increase so I would tend to use 3 degrees. If you are going to the south
or east the convergence will be smaller so go for 4 degrees. If you are
in the Braemar area pick whichever suits you.

Quoted message said:

While on this topic, I found myself at the junction of two UTM (grid)
areas last weekend in Norway, but near Sweden.
On the same sheet, UTM zone 32 changes to UTM zone 33. one side is
"grid get rid" but the other is the opposite. Could be very confusing.

Sounds life an interesting place to be!

--

Dominic Sexton

The message <[email hidden]>
from Dominic Sexton <[email hidden]> contains these words:

Quoted message said:

Quoted message said:

Well it seems to me that you have two options:

Quoted message said:

1. Get some kind soul to look it up on a map!

Quoted message said:

2. Use Geomag to get the declination for the location and then
adjust for the convergence.

Quoted message said:

Braemar is almost exactly 57N & 3.4W. Putting those into the Geomag
calculator

Quoted message said:

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Quoted message said:

gives us 4.62 degrees west.

Quoted message said:

Convergence of the grid is calculated by multiplying the longitude
difference from the central meridian (west being positive) by the sine
of the latitude. See:

Quoted message said:

<http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesyst
emsinfo/guidetonationalgrid/page19.html>

Quoted message said:

The central meridian is 2 degrees west and Braemar is 3.4 degrees west
so the longitude difference is 1.4 degrees. Multiplying that by the sine
of the latitude gives us 1.17 degrees. Therefore grid north is 1.17
degrees west of true north.

Quoted message said:

As magnetic north is also west of true north the convergence is taken
away from the magnetic declination obtained earlier. 4.62 - 1.17 = 3.45
degrees west.

Quoted message said:

If you are going to the west or north of Braemar the convergence will
increase so I would tend to use 3 degrees. If you are going to the south
or east the convergence will be smaller so go for 4 degrees. If you are
in the Braemar area pick whichever suits you.

Thanks for that Dominic that is a useful source. I will still look at a
paper map though!
:-)

--
Hbunnet : (replace "nospam" with "athome" to reply)
_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/