Ultimate Navigation part 6: Grid Magnetic Angle

In part 6 of our navigation series Lyle Brotherton explains why there is more than one north!

"The navigation term grid magnetic angle (GMA) sounds daunting, but it isn’t! Here you will learn exactly what it is, and how and where to use it."


The Norths

If you stick a pencil vertically through an orange and imagine it is the Earth on its axis, the point at the top would be true north. This is where all lines of longitude originate. Running across the orange, imaginary parallel lines would be lines of latitude. This is the universal coordinate system used on maps the world over.

The British Isles cover a small section of the Earth, and lines of longitude and latitude curve, so Ordnance Survey introduced its own grid system: the National Grid. These are the blue lines on OS maps (grey on Harvey), and all the vertical ones point to grid north.

Our compasses, however, point to magnetic north, and this angular difference between the north on your compass (magnetic) and the north on your maps (grid) is called grid magnetic angle, or GMA (not to be confused with magnetic declination or variation, which is the difference between true north and magnetic north). When we transfer a bearing from our compass to our maps, or vice versa, we need to take the GMA difference into account.

All OS and Harvey maps state what the difference between magnetic north and grid north is for the area covered (see below).


Making the adjustment

  • To adjust for GMA when transferring a bearing taken with your compass (mag) to a map (grid), SUBTRACT thegrid magnetic angle from your compass bearing.
  • To adjust for it when transferring a bearing taken on your map (grid) to your compass (mag) – ADD the grid magnetic angle to your compass bearing.

A simple mnemonic to help you remember how to do this is ADD for mag, RID for grid.


In Use

If the difference is 1.5° or less, we can forget about it. So for example in Devon, where it is currently less than 1°, we need not bother; but if we are over in East Anglia, where it can be more than 3°, we must take it into account.


The adventurous among us can calculate our local GMA when out navigating:

  1. Locate exactly where you are on the map; the more accurate the fix, the more accurate your result.
  2. Take a bearing with your compass on a distant feature, ideally one that is narrow such as a radio mast and which is identifiable on your map.
  3. Make a note of this bearing.
  4. Now take a bearing on the map from your position to the feature (on the map).
  5. The difference between these two bearings on an OS or Harvey map is your local GMA.

A couple of degrees might not seem much, but if you were 2° out when you took your bearing and another 1° when transferring it, your compound error including GMA could be 5°, which over a kilometre means you will miss your target by more than 87m!

The magnetic north pole is also moving over time, so check the date of your map. You can find the up-to-dateGMA on the British Geological Survey’s website at www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html. If your map is more than 5 years old, visit this website and write on your map the GMA plus the date.

If navigating abroad you’ll need to take account of the variation between magnetic north and true north (magnetic declination or variation) if a local grid system is not used, such as in Canada and America – where the variation can be over 20°!