Magnetic Declination – Compass Adjustments

Did you know there are ‘three’ Norths?

Magnetic Declination – What is it?

On this beautiful planet of ours, we have the Geographic North Pole (where Santa lives), Magnetic North Pole – which is where a magnetised compass needle will point towards and for some maps, Grid North. The difference between these two points is referred to Magnetic Declination, or often, just Declination.

TOPO50 maps are created utilising Grid North. Compasses point to the Magnetic North Pole. This can be a 20-degree difference.

See a potential issue here?

To complicate matters, the Magnetic North changes, dependant both on where you are, and when you are. Mainly, Magnetic North is determined by the earth’s molten outer core. Localised magnetics rocks, kilometres beneath can affect things, but primarily, it’s the magma that determines where the compass needle points. Being molten, it shifts over time, and as a result, the Magnetic Declination is also always changing.

Luckily, on all of LINZ’s Topo50 maps in current production, the Magnetic Declination is clearly marked on the bottom of each map. NOTE: This declination varies from map to map, location to location – for example – in Auckland you are going to have around 21 deg and Wellington is going to be closer to 24. It might not ‘seem’ that much difference – but over a distance, a couple of degrees quickly adds up.

Magnetic Declination – How to compensate

Now that we know that the map we have in our hands is using a different reference for north than our compass is, how do we translate between the two?

Mostly, you will need to take the difference indicated from Geographic North (generally referred to as ‘grid’ north) and magnetic north. But do we add or subtract from our figure?

An easy way to remember is this – if you are taking a bearing ‘off’ the map – you take the number off to get declination. If you are putting a bearing you have made with your compass ‘onto’ the map – you add the figure.

  • Off = subtract
  • On = Add

For example – according to your Topo50 map (indicating a 22.5 deg declination like above) you need to be heading from your current position at a bearing of 46°. Take the 46, subtract 22.5 (taking a measurement off the map) and you have a magnetic bearing of 23.5°.

Alternatively – taking a bearing of 98° off an identifiable mountain in the difference, we add 22.5 (putting it ‘on’ a map) to give us a grid bearing of 120.5°.

 

Magnetic Declination – is there an easier way?

Why yes. There is! Instead of having to add or subtract declination each time you convert from grid to magnetic or vice versa when using the compass and putting ‘the red in the shed’, aim the compass needle to the declination required on the outer ring of the bezel.

So, in the example above, you take a grid bearing of 46° Grid, twist the bezel on your compass so the direction of travel indicator is 46°, then turn yourself until the red compass needle is pointing to 22.5°.

Even easier? Get a compass with Declination Adjustment on it. Some of the higher end compasses will have the ability to rotate the outer part of the bezel, usually with the use of a small screwdriver, to permanently ‘dial in’ an areas Declination Adjustment. This means that you simply take the bearing off the map in the usual way, put the ‘red in the shed’, and go.

Caveat: We are discussing several different ways to achieve the same thing. The potential for confusion in communication arises because not everyone does things the same way. It is vitally important then, then whenever you give a bearing out, that you follow it with either ‘grid’ – meaning you have taken it off the map, or ‘magnetic’ meaning you have taken it off the compass.

37° Grid and 37° Magnetic could lead you in two very different directions.

 

Further Reading

Declination around New Zealand 

7 comments on “Magnetic Declination – Compass Adjustments

  1. Yeah – I have already been taught two different ways of addressing the issue – both instructors of course suggesting their method was the better one.

    Personally, I prefer adjusting the declination on the compass (and just leaving it at 23.5) the taking it off the map.

    Like you say – you need to be careful when starting to mix and match your references.

    1. Uhuh. I’m certainly a person for always pointing the needle at the offset (22/23/24/whatever) and point the bearing arrow (or whatever it’s called) at my target. To me it seems so simple that I’ve never really understood why others like to do all the addition and subtraction. But that’s just me, and it’s a very religious argument in some circles. 🙂

  2. Hi Kerry.

    As a small addendum, did you realise that Grid North isn’t typically Geographic North, either? It’s an artificial variant of North that’s created by LINZ for the purpose of producing the map series. If the North-South lines on NZ’s topo maps all pointed towards Geographic North, they’d all be slightly narrower at the northern end than the southern. (Curved globe represented on a flat surface blah blah.) If you laid out all topo50 maps in order, edge to edge, then looked at the representation of the country from above, either the country would appear extremely skewed or the map grid would — I forget which it is and can’t find the literature just now—I’m sure there’s at least one scientific paper floating around which shows it off graphically, possibly buried in the LINZ website. I’ll post a link if I find it.

    In effect, however, this artificial NZGD2000 grid is how LINZ was able to ensure, by design, that all Topo50 maps have a magnetic offset of 22.5 degrees (subject to natural changes over the years). In the earlier 260 series, magnetic offset from the grid varied depending on the map, but in the Topo50 series it’s consistent throughout.

    So really there are at least three North Poles (plus the others for all the other map grids in the world). There are more in some scientists’ eyes, depending on which definition is being used for North at any given time. 🙂

      1. Yep. I guess one of the potential catches is that grid north isn’t automatically True North, and (I think) is often very different. Most people I know use nothing other than Grid North and Magnetic North nearly all the time, because they’re using compasses to compare the real world via Magnetic North with a map, but it might be possible to be caught out if you were measuring direction from the Sun’s position or the North/South Celestial Pole or similar, and needed to be especially accurate, just because of True North not being Grid North so the map will probably be rotated, at least a little.

        I should add that I’m not an expert on this stuff aside from having tried to read into it here and there. I might be incorrect here or there, so you might find someone actually qualified who can explain it more reliably (or correctly!) in the future.

    1. Hi again. Small correction–In relation to the 22.5° comment, I think that screenshot you have (which seems to be from Te Ara) comes from the older 260 series, where the offsets are all different. I checked a Topo50 map which places it at 23.5° (as of 2011), so I think I said that 22.5° is the constant offset for all modern Topo50 maps when I should have said 23.5°. Cheers.

      1. Oops, another correction/retraction. I was twittering with a friend from LINZ who pointed out that this assertion is completely untrue. I’m not sure where I picked it up from (I thought I’d heard it in a presentation), but the grid is definitely not skewed such that all maps have the same magnetic offset from True North.

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