This map of the world shows magnetic declination in different parts of the Earth’s surface. Note that declination has very little correlation with longitude. The green line shows the agonic line. If on this line a compass will point towards true north. On the isogonic lines declination may be more than 20° in the northern hemisphere and even greater values as we travel south. Easterly declinations are in red. Westerly declination are in blue and given as a negative number.
So what effect does magnetic declination have on navigation? Suppose I am in an area where the declination is 2° west. I’m facing a direction the compass tells me is north, 0°. I notice something of interest ahead of me and try to locate it on my map. Rather than being on the north-south line the point of interest will actually be at a bearing of 358° from my position on the map. 0° is the same as 360° so 2° west gives 358°. In another part of the world I might face towards magnetic north but in fact be facing at a bearing of 13° east, a significant difference.
Magnetic declination will probably be marked in the margin of your map. Some maps have a declination in each corner of the map. Use the value closest to your position on the map. If you are midway or in the centre average the relevant values. Note that the declination diagram is not drawn to scale. Don’t try to measure it with your protractor, use the values given in the text.
To make our life more interesting magnetic north moves over time. The magnetic declination information will include an annual rate of change so you can calculate how much the declination has changed since the map was printed.
An old map I have of London tells me the magnetic declination for June 1989 was 6°W and that this was expected to change by 9’E every year. In 2001 it would therefore be expected to be 4.2°W. In 2016 this predicts it will have shifted by 243’ from what it was in 1989. There are 60’ in a degree so 243’ is 4° 3’ and predicts magnetic declination in London would be 1° 57’ west by 2016. This website gives the magnetic declination in London in 2016 as actually being 2° 10’ west.
In practice we would treat both 1° 57’ and 2° 10’ as 2°. The difference does illustrate that not only does magnetic declination change over time, but the rate of change may also vary. If using old maps it is important to get up to date information.
Once you have an up to date magnetic declination, what do you do with it? This is where a lot of people get confused. When do you add it, when do you subtract it? Some maps will give you this information, relevant for the area covered in the map. Where present, follow these instructions.
When a map lacks this information there are lots of rhymes and aide memoires that have been created to teach you what to do. Some of these, however, are only “true” in certain parts of the world.
My recommendation is that you use the acronyms “MUGS” and “GUMA”. These stand for “Magnetic Unto Grid: Subtract” and “Grid Unto Magnetic: Add”.
A related rhyme is “Magnetic to Grid, get rid” and “Grid to Mag, Add”. Another acronym pair is “MUCA” and “CUMS”. The “M” stands for map and the “C” for compass in this case, but when stressed you might confuse these with “magnetic” and “chart” so I find MUGS and GUMA safer.
Yet another system is “LARS” = “Left: Add/ Right: Subtract”. This refers to if the magnetic north line on the declination diagram is to the left or right of true/ grid north. I find this less useful since it is not clear if you are applying this to grid or magnetic bearings.
What MUGS means is that if you have a magnetic bearing, taken with your compass you must subtract the magnetic declination before plotting the angle on your map. In our example above the magnetic bearing of 0°/360° has the declination of 2° subtracted from it to give the actual bearing of 358°. When converting a bearing on the “grid” to a magnetic bearing you add the declination (GUMA).
I suspect that the MUGS/ GUMA acronyms are probably British in origin, since they tend to favour a westward declination. To make MUGS/ GUMA global in application we need to learn one more thing: “West is Best, East is Least”.
“West is Best, East is Least” tells us to treat a west declination as positive and an easterly one as negative. As you should know, subtracting a negative number adds the value of the number to the total. Adding a negative number subtracts the value.
Hence, from the above examples:
0°/360°(magnetic) -2°W = 360°-2° = 358° grid (MUGS)
0°/360°(magnetic) -13°E = 360°- (-13°) = 0°+ 13°= 13° grid (MUGS)
(It is possibly more logical to treat a westward declination as negative, giving us the rather nice acronyms of “MUGA” and “GUMS”. MUGS and GUMA are very well established, however.)
The method above will be familiar to many compass users. Rather than aligning the needle with the “N” arrow on the face it is possible to compensate for magnetic declination by holding the needle pointing at the declination value on the dial. Hence if the local declination is “10°W” you hold the compass so the needle points to the “350°” mark on the bezel rather than “0°”. This is useful when walking to a bearing, although you are better walking towards a landmark rather than walking staring at your compass all the time. When sighting with a compass the values you will get will still need conversion.
Remember that metal objects on your person or in your surroundings may affect a compass reading. Overhead power cables may influence the needle from as far as 55 metres away!
“West is Best, East is Least”