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Techniques => Navigational Questions & Answers => Topic started by: Simon B on September 11, 2012, 03:41:18 PM

Title: More Magnetic Declination...
Post by: Simon B on September 11, 2012, 03:41:18 PM

For an explanation of what Magnetic Declination is & How to Correct for it, click this link to the Compass section: http://micronavigation.com/forum/index.php?topic=256.0

Hello All,

Just read Kenneth's thread but it's still sinking in for me at a slow rate. I am having brief moments of clarity that then get fogged over by my brain's default refusal to work with any kind of logic.

So, for now I thought I'd have a go at just making an adjustment on the compass for practice. I’m using the 2011 OS explorer OL13 for the Brecon Beacons.

So according to the legend on the map, as of 2011;
True North is 0 degrees and 51 minutes East of Grid North
Magnetic North is 1 degree and 53 minutes West of Grid North
Annual change is 9 minutes East.

I use Abergavenny to calculate from and I calculate I need to adjust to 1 degree 45 minutes West of Grid North. The online Magnetic Declination Calculator suggests it’s 1 degree 37 minutes West of Grid North.

For Magnetic North to True North I calculate 2 degrees and 36 minutes West. The online calculator is giving 2 degrees 26 minutes West.

1st Question.  Why or have I made wrong calculations?

I’m also trying to get my head around Mag’ Dec’ with Grid North and True North.  Say I decide to spend a day walking around the Sugar Loaf NW of Abergavenny.  2nd Question.  Do I take the Mag' North calculation against Grid North or True North? What determines this?


Title: Re: More Magnetic Declination...
Post by: captain paranoia on September 11, 2012, 06:59:19 PM
> True North is 0 degrees and 51 minutes East of Grid North

Unless you're using a map that has a True North grid representation, you can ignore True North; it's just for information.  You need to be able to relate Grid North (as per the Transverse Mercator Projected OSGB map) and Magnetic North, as per your compass.

> Magnetic North is 1 degree and 53 minutes West of Grid North
> Annual change is 9 minutes East.

> So according to the legend on the map, as of 2011;

As of what month in 2011?  If we're changing by 9' per year, then half way through a year, we'd expect to have changed by 4.5'.  So, if it's January 2011 as the start point, and we're now in September 2012, that's 12+8 months elapsed, so expect 9' * 20/12 change, or 15'.

That gives us a Grid Magnetic Angle (the difference between Grid and Magnetic Norths) of 1 degree (53-15)', i.e. 1 degree 38', which is much closer to the online calculator.  Try measuring that 1' difference with your compass.  Actually, try measuring the 1 degree difference with your compass...

The other thing to bear in mind is that, due to the Transverse Mercator Projection, the Grid Magnetic Angle varies slightly across the map.  It's not much, and I wouldn't like to say exactly how much it is, but there is a difference.  You could use the online calculator to explore this.  So, if you take a point within the map, if will have a slightly different GMA to the 'reference' specified in the map legend (which I think is specified for particular point on the map; a corner, or the centre, can't remember which).

> I’m also trying to get my head around Mag’ Dec’ with Grid North and True North.  Say I decide to spend a day walking around the Sugar Loaf NW of Abergavenny.  2nd Question.  Do I take the Mag' North calculation against Grid North or True North? What determines this?

As mentioned above, True North is of no practical use if you're using a map that does not present a True North representation.  OS maps are printed using Grid North, so you want to use Grid North and Mag North.  Assuming you want to be able to transfer bearing between map and compass, that is...
Title: Re: More Magnetic Declination...
Post by: Lost Soul on September 11, 2012, 09:40:32 PM
As Captain  Paranoia says forget true North work only with grid north.  Interestingly, the recently published series of The AA’s  Walker’s maps, which are nothing other than repackaging of the OS Explorer maps, only show magnetic deviation in respect of grid north.   That makes things much simpler and avoids all the confusion that can be visited on a user if too much information is given.  Or, in other words they’ve adopted the KIS approach - Keep It Simple.

Next thing to consider is the practical accuracy that you can achieve when out in the field in respect of all that high precision information you are given about a wandering magnetic pole and how it affects the little spot on the planet you are standing upon.   As Captain  Paranoia has indicated can you measure and set heading etc with a high degree of accuracy using the standard baseplate compass?   No you can’t.   With a good level of care you’d be lucky if you can get something to within half a degree.   What does this accuracy or lack of it mean in practice?

To understand that we need to go to the 1 in 60 rule.   This states that for every degree you are out in heading then for every 60 metres you travel you will be 1 metre out of position.   Example: heading you’re supposed to be on 300°; heading you’re actually following is 299°.  So after having travelled 60 metres in an absolutely straight line you will be 1 metre to the left of where you want to be.  Does this make any difference to you?  Not really, particularly if we stop every couple of hundred metres or so,  as we are supposed to, to reassess our position and adjust accordingly.

The nine minutes annual change deviation you mention.  If you could set your heading with a very high degree of precision on your compass but get deviation wrong by nine minutes and you accurately trot off an absolute straight line then after having travelled 60 metres you will be exactly 150 mm (or 2 lengths –ish of your baseplate compass) to the left or right of where you thought you wanted to be.

Given all the inherent inaccuracies in equipment etc we use then for all practical purposes just set deviation as 2°.  Which way do we make the corrections?  Remember it this way.  East is least and west is best.  So if you measure your heading off of the map as 067° and deviation is 2°west then add 2° to the map derived heading.  Thereby trotting of on a heading of 069°.  And don’t forget if you have taken a compass bearing off of a feature in the landscape then subtract 2° from if before you transfer it to the map.
Title: Re: More Magnetic Declination...
Post by: Hugh Westacott on September 12, 2012, 08:47:20 AM
Completely agree with the two replies.

It's worth remembering that few navigators can walk on a bearing, using a baseplate compass, to an accuracy better than two degrees, so it's worth familiarizing yourself with the useful technique of 'aiming off'. This involves building in a deliberate navigational error.

Consider the following situation. You are walking off-path towards a footbridge over a substantial stream a couple of miles away. You are in dead ground so cannot see the bridge. Your compass and navigational skills are not sufficiently accurate to be sure of reaching the exact location of the bridge. If you don't hit it on the nose you won't know whether it is located to your left or right. What to do?

Answer: build in a deliberate navigational error that will ensure that you will arrive at the stream on one side of the bridge. Then all you have to do is to turn towards the location of the bridge and follow the stream until you reach it. I find that five degrees is right for me but navigators should experiment to find the error that works best for them. In the above illustration I would add five degrees to the bearing that, in theory, would take me to the location of the bridge then, when I reached the stream, I would turn left and walk to the bridge. (If you can't find the bridge it probably means that it has been swept away in a flood!)

Aiming off is a useful technique that can be used in a number of situations.

Title: Re: More Magnetic Declination...
Post by: Simon B on September 12, 2012, 03:48:43 PM
Captain Paranoia, Lost Soul and Hugh..

Thank you . Your replies have really helped out. It starts to become obvious once you know...  ;)   

I just didn't even register the month on the map for the calculation. Silly mistake but now I'll know (it was July). That said I also think I was telling myself that I had to make very accurate calculation/adjustments. After reading and re-reading various other explanations on different webpages you tend to, well, go round in circles. It's good to hear the practical realities of what is actually possible with navigating across the land, with compass in hand and of using other techniques (aiming off) to compensate for this. An Art and not just Science.


Title: Re: More Magnetic Declination...
Post by: captain paranoia on September 12, 2012, 06:02:12 PM
> Which way do we make the corrections?  Remember it this way.  East is least and west is best.

As I said on the earlier thread, I don't like mnemonics, because I can never remember what they mean: 'East is least and west is best'?  Nope: not a thing...

I prefer to think about where grid and mag north are relative to each other, and adjust the compass accordingly.  So, if mag north is west of grid, and I want to convert a magnetic bearing to a grid bearing, I know that I have to move the compass bezel/capsule N pointer a little to the East (from left to right, or clockwise, depending on how you think about it).


Whatever you finds works for you is right, of course...

The other thing to bear in mind is that the adjustment we make when converting between grid and magnetic values is not the Magnetic Declination, it is the Grid Magnetic Angle.  The GMA is the combination of the Magnetic Declination (the difference between Magnetic North and True North) at a particular point on the Earth, and the Grid Convergence Angle (the difference between Grid North and True North) of the Transverse Mercator Projection at that point on the map.

http://www.ordnancesurvey.co.uk/oswebsite/support/knowledgebase/grid-north-magnetic-north-and-true-north.html (http://www.ordnancesurvey.co.uk/oswebsite/support/knowledgebase/grid-north-magnetic-north-and-true-north.html)

Just trying to make sure we use the correct terminology...
Title: Re: More Magnetic Declination...
Post by: Simon B on September 13, 2012, 12:16:42 PM
I don't mind using the odd mnemonic but as you yourself experience, many for me just don't stick. I think I've beeen trying to get my head round GMA (cheers) by using the visuals in my head in a way that I think sounds similar to your method.

Thanks for the links, I shall have a read through and then carry on with Lyle's UNM.

I appreciate the help with getting me started.


Title: Re: More Magnetic Declination...
Post by: Skills4Survival on September 14, 2012, 10:23:37 AM
Simon, my advice. Do not use "tricks" I would say, try to understand. You need the same amount of brainpower to remember and some tricks are regionally bound.  Here two slides to help out.


Title: Re: More Magnetic Declination...
Post by: Simon B on September 17, 2012, 02:25:05 PM
Hello Ivo

Sorry been away for a few days.

Thanks for the slides. Very clear and easy to grasp. Job done!

Title: Re: More Magnetic Declination...
Post by: Skills4Survival on September 17, 2012, 10:53:49 PM
thank you sir, if you have any improvement points, clarification improvements, let me know.
Title: Re: More Magnetic Declination...
Post by: Simon B on September 20, 2012, 03:19:04 PM
Title: Re: More Magnetic Declination...
Post by: Lyle Brotherton on September 21, 2012, 08:27:12 AM
A good thread and supplement to another thread on this subject: http://micronavigation.com/forum/index.php?topic=256.0 

I would like to add an important point.

In any system you need to minimise error, because error can be compound, and navigation is a system. Unless we are employing precision sighting compasses, an error of 2-3 degrees is very easy to make when pointing your baseplate compass at your objective. When we stop, and check our bearing, sighting a new attack point, this same error can occur and compound the first error. Add to this that none of us walk in a perfectly straight line, and that the time when we rely on our compasses most is usually in poor visibility, then we are constantly compounding small errors which can become signifianct. So I personally always account for GMA if it is 1 degree or more. There is never good reason not to practise Best Practice.
Title: Re: More Magnetic Declination...
Post by: Callum on September 21, 2012, 11:27:11 AM
I would add Lyle this advice you mailed to me, because like CP, I too do not like mnemonics, because I can never remember what they mean either.

"Think of the azimuth like a clock, with declination west before midday and east after midday—your compass needs always to point to midday, so if it is west you add to take it to 12:00hrs and if it is east, after midday, you subtract to take it back to 12:00hrs."
Title: Re: More Magnetic Declination...
Post by: Lost Soul on September 21, 2012, 03:53:10 PM
2 follow up comments. 

Lyle talks about errors and best practice in always adding / subtracting GMA.  As Lyle says errors of 2 to 3 degrees in setting / taking bearings are common.   And given that GMA in the UK is around 2 degrees then there is always the temptation to dismiss as being within the general margin of error.  Don’t!!  Get in the habit of applying it all the time so that it becomes second nature because if you don't then when you are in an area where Deviation / Variation / GMA is significant you will as sure as hell forget to add it and end up lost.

2nd point use of mnemonics and other weird and wonderful memory joggers.  None of the methods should be dismissed.  Use what works best for you and don't generally dismiss something because it leaves you cold.  For me mnemonics work.   They are coping strategies and we all have our own.  Best suited to our needs and the way our minds work.

Now some of the other strategies I have seen published leave me cold and are unfathomable but it does not make them wrong, far from it.  For me they require far too much brain power.  When on the hill wet, tired and cold the brain doesn’t function so well.  So trying to mentally grapple with a long winded memory test to perform a simple calculation is not going to work for me.  It will get corrupted or dumped.  Like wise for those who do not like mnemonics.

We just have to accept our minds all work differently, find what suits you best and stick with it.  Don’t be afraid to publish it as an equally valid alternative to something that doesn’t work for you.  Please remember what works for A might not work for B and vice versa.  It does not make either wrong or worthless,  just different.  As they say in France; Vive le difference!
Title: Re: More Magnetic Declination...
Post by: azimuth360 on October 07, 2012, 05:49:57 PM
I second the motion of applying understanding, not mnemonics.  The thing is, if you rely on the 'east is least' kind of stuff without real understanding, then you likely won't remember to apply the exception: am I going from map to land or land to map?  If you have the real understanding, then you don't need the mnemonic.
Title: Re: More Magnetic Declination...
Post by: captain paranoia on November 14, 2012, 12:55:55 PM
I've been thinking about the issue of True, Magnetic and Grid Norths, and the impending problem of the change of Grid Magnetic Angle from West to East, and the effect that will have on the use of the mnemonic 'Grid to Mag: Add; Mag to Grid: Get Rid'.  Here's my thinking...

True North
This is a physical property of the rotation of the Earth; it's point at which the the axis of rotation of the Earth meets the surface.  This changes a little, due to the changing shape of the Earth (core changes, earthquakes etc.) but not by much, and this variation can be ignored.

Celestial North
This is the imaginary point in the night sky that True North points to.  It changes with time (with a period of about 26000 years, or 1 degree in 72 years), as the Earth's rotation precesses.  Think of a spinning top as it slows down; the top still spins on its axis quite quickly, but the axis about which it rotates also rotates slowly; this is precession.  It has no impact on map and compass navigation, but does effect celestial navigation.  Slowly...

Magnetic North
The Earth's magnetic field is caused by the moving, molten, nickel-iron core of the Earth.  Lines of magnetic flux come out of the surface of the Earth at different points, at different strengths, and at different angles at different positions on the Earth's surface.  A compass aligns itself with the lines of magnetic flux as they occur at that point on the Earth's surface.  This gives the concept of a Magnetic North, as measured with a compass.  And, for a specific point on the Earth's surface, there is a Local Magnetic North, LMN.

Magnetic Declination
The concept of the difference between True North, TN, and Magnetic North due to the lines of flux is called Magnetic Declination (or Variation), and, at any point on the Earth, there is a specific Local Magnetic Declination, LMD:


Grid Convergence
The Earth is an oblate spheroid.  A map is a flat piece of paper, which attempts to represent the curved surface of the Earth; this it cannot do; try wrapping a ball with a piece of paper without leaving any creases...  Clever Mr Mercator came up with a mathematical way of drawing a map of the Earth's surface so that bearings and distances measured between two points are correct (a projection).  OSGB and UTM maps use this (Transverse) Mercator projection.  The downside of this projection is that the Grid North lines on the map, whilst being a fixed distance apart, don't point to True North.  Consider that, as we go North, the distance between lines of longitude gets smaller and smaller (until they meet at the pole).  Thus, in a map grid that maintains a fixed spacing between the Grid North lines, as we go North, the Grid North lines must diverge out from True North lines.  The only points at which Grid North and True North are aligned is on the line forming the centre of the projection space; in the OSGB projection, this occurs at 2 degrees W.  The concept of this difference between True North and Grid North is called Grid Convergence, and, at any point on the Grid, there is a Local Grid Convergence, LGC:


Grid Magnetic Angle
Now, if we look at a particular area of a Mercator Projection, i.e. we look at a map, there will be a difference between the Local Magnetic North and Local Grid North.  This difference is due to the Local Magnetic Declination (the relative position of the local magnetic field wrt True North), and the Local Grid Convergence due to the Mercator Projection of the map at that point (the relative position of Local Grid North wrt True North).  Note that Local Grid Convergence actually varies across the entire map, but this variation is relatively small, and can be ignored on large-scale (1:50k, 1:25k) printed maps*.  Let's define the Local Grid Magnetic Angle, GMA, as the difference between Local Magnetic North and Local Grid North:

GMA = (TN - LMD) - (TN - LGC)

Since, by cartographic convention, positive angles (0 <= angle <180) are E, and negative angles (-180 < angle < 0) are W, GMA measures how far Local Magnetic North is East of Local Grid North.  So, a positive GMA means LMN is E of LGN, and a negative GMA means LMN is W of LGN (as is currently the case in most of Britain).

So, we must be careful with our terminology, and must use only (Local) Grid Magnetic Angle when we wish to discuss converting between Local Grid North and Local Magnetic North; if we used the term Magnetic Declination, we would, strictly, be ignoring the Local Grid Convergence, which can be just as important a correction factor as the Local Magnetic Declination.  Generally, when map-reading, we aren't concerned with Magnetic Declination or True North; this has already been dealt with by the cartographers, who have given us a handy diagrammatic representation of magnetic, true and grid Norths.  The only way in which Magnetic Declination affects us is the change of Magnetic Declination with time, which causes the Local Grid Magnetic Angle to change with time.

Note that I have stressed the distinction between the general concepts, and the local value of those concepts, since it is essential to bear in mind that all these factors vary with position on the Earth, and in the particular mapping projection employed, and with time; they are not constants either geographically, cartographically, or temporally.

Now let's look at applying this.

Consider a point on an OS map, where the Local Magnetic North is (for example) 3 degrees West (or minus 3 degrees E) of Local Grid North.  By convention, the Local Grid North is 0 degrees.

GMA = -3 - 0 = -3
GMA = -3 degrees East, i.e. Local Magnetic North is 3 degrees West of Local Grid North (confirming what we stated above...)

Now, we can re-arrange
to give:


We can then replace these 'Norths' with actual measured bearings: Local Magnetic Bearing, LMB, and Local Grid Bearing, LGB:

LMB = LGB + GMA      Grid to Mag: Add
LGB = LMB - GMA      Mag to Grid: Get Rid

Note that, if we use the definition of GMA as above, and the E positive, W negative convention, and use signed arithmetic for the above operations, the 'Grid to Mag: Add, Mag to Grid: Get Rid' mnemonic works no matter what the values are, or where we are in the world.

nb. Unfortunately, you will often find GMA being defined as follows: "The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC ANGLE" (taken from the OS website below).  Note that the order in which the two terms are expressed in the 'difference' is the reverse of the above definition of GMA; when expressed arithmetically, this gives a change in sign, which makes the 'Grid to Mag: Add, Mag to Grid: Get Rid'  mnemonic completely wrong.  Sadly, they express this GMA in terms of degrees E or degrees W, rather than the more helpful, conventional +/-degrees E.

* With the rise of electronic mapping, where there is no useful North annotation in the corner of the map, some other means of finding the GMA will be required.  Ideally, the electronic mapping tool will have some means of determining GMA, probably using a look-up table of baseline and time delta values (rather like the printed map annotation).

Useful links:
http://www.ordnancesurvey.co.uk/oswebsite/support/knowledgebase/grid-north-magnetic-north-and-true-north.html (http://www.ordnancesurvey.co.uk/oswebsite/support/knowledgebase/grid-north-magnetic-north-and-true-north.html)
http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html (http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html)

You can put in any name and email address in the Geomag site...

Of course, having said all this, as I've observed above, I don't generally like mnemonics, and deal with grid to magnetic conversions in a completely different way, thinking about the relative position of local magnetic north wrt grid north...
Title: Re: More Magnetic Declination...
Post by: Skills4Survival on November 14, 2012, 06:00:13 PM
Very very nice. Agree on the trics...just know how to explain it to yourself.
Title: Re: More Magnetic Declination...
Post by: Callum on November 15, 2012, 02:46:36 PM
CP posted Useful links:

I believe there are better links, right here on this site.

Amore detailed and better explanation of GMA (Grid Magnetic Angle) with how and why to correct for it, has already been posted on the forum


The BGS (British Geological Survey) website for GMA (Grid Magnetic Angle) is unnecessarily complicated. A easier to use site that calculates the GMA you need to use to adjust your compass with is on this forums host site


Simply move your cursor over the place you need to determine the GMA and it is displayed in the bottom right hand box.
Title: Re: More Magnetic Declination...
Post by: captain paranoia on November 15, 2012, 06:41:42 PM
The OS description of declination and GMA is pretty terse, but the advantage of the OS website it's that it carries some 'authority'...  I did have to do a double-take on the bit about calculating variation, thinking that they were going to describe calculating GMA, but, no, they describe how to determine variation starting with GMA...

And I confess that I'd forgotten the GMA app on this website.  Naughty me...  Again, I posted the BGS site because it ought to be the definitive source of information, given that they provide the basic declination information to the OS.

My discussion was intended to provide 'another take' on the matter, for those of an arithmetic, or mathematical bent, and, in particular, the issue of magnetic north getting closer to true north, and the ensuing change of sign of GMA.  I wrote it a while back, and was prompted to post it after a couple of posts on OM where a chap was saying that variation = GMA.
Title: Re: More Magnetic Declination...
Post by: Hugh Westacott on November 15, 2012, 07:11:27 PM
I've been following this discussion and want to make a point that appears to have been overlooked.

Accurate navigation, relying solely on map and compass, in the lowland countryside of England and Wales is often more difficult than route-finding in fair weather in the mountains and moorlands of upland Britain (gasps of disbelief from the mountaineers!) for the following reasons:

1   Apart from the relatively small areas of open access, walkers are required to keep to public rights of way (PRoWs); if they don't, they are trespassing.

2   The PRoW network is often dense. In the three civil parishes that I check annually for the Ramblers Association there is a total of 19 miles of public paths.

3   PRoWs are not always visible on the ground and waymarking is often patchy.

4   The basic technique is to navigate using handrails and features such as field boundaries, woods, reservoirs, rivers etc. Contours, generally speaking, are of much less importance.

5   PRoWs are not always clear on the ground and a compass is often useful to determine the direction of cross-field paths (i.e. those that do not follow the field boundary) especially when the far side is in dead ground making the exit point invisible.

6   The most difficult areas in which to navigate accurately are popular areas such as Leith Hill, Box Hill, the Devil's Punchbowl, and Wendover Woods. The reason is that they have plenty of parking and well-meaning bodies such as the National Trust have created waymarked circular walks many of which are not shown on the map. There are animal tracks, firebreaks that are used as short cuts, and narrow paths made by courting couples seeking privacy.

7   I find that the most difficult national park in which to navigate is the New Forest. There are no PRoWs because it's common land, it is relatively flat so physical features do not stand out and, in it is  parts, densely wooded. The National Park Authority (NPA} has created a number of well-waymarked routes for walkers and riders but apart from these, exploring off-path is very difficult. I don't normally consult my satnav when walking in lowland countryside but I find it invaluable in the New Forest.

This long preamble leads me to my main point. I never make allowance for the Grid Magnetic Angle (GMA) in lowland countryside because it's not  necessary as distances between features are s short. In fact, I often guess from the map the angle that a cross-field PRoW deviates from a linear feature such as a field boundary. At a multi-path junctions, I confirm the route I want to follow by placing the compass on the map and quickly estimating the bearing. No need to bother with GMA!

Needless to say I always follow best navigation practice when walking in upland regions.

Title: Re: More Magnetic Declination...
Post by: RobW on November 15, 2012, 07:19:35 PM
Hi all,

Very interesting - the theoretical side of this is fascinating for the armchair and invaluable for the expert in the field but when trying to shape younger minds I've always found mnemonics useful (if a piece of 2x4 is unavailable).

'Add for Mag - Rid for Grid' was new to me but as has been mentioned has a limited shelf life.

I was taught 'Variation west- compass best, variation east - compass least'. It helped me when learning and has always been the way I teach others. Understanding the details is good and should always be the aim of a keen navigator but even now, when cold and tired, I check my understanding against the aide. Measure twice - walk once!

Title: Re: More Magnetic Declination...
Post by: Confused on December 27, 2015, 09:10:23 PM
This subject was one of the reasons I joined this site and purchased Mr Bs book.

I never truly understood variation, which I think is the key to everything.

"Because it is" is not the answer......but my poor bloody brain don't half hurt!!!

I get it in parts, but its probably time to dig deep until it clicks.

Oh Mother!!