Finding out where you are with map and compass

If you don’t know where you are exactly and you can see two or more unique landmarks which appear on your map such as mountain peaks or churches then you can easily find your position with only a map and a compass by using the following system which is called a ‘Resection’. I will take you through the process quickly. Take as long as you like to learn it.

1.   Magnetic bearing: Take a bearing on one of the landmarks. You do this by pointing the sights of your compass at the landmark and then aligning the dial of your compass with the north-pointing needle. This gives you a ‘Magnetic Bearing’ from you to the landmark.

2.   Back bearing: Add 180° to this bearing – half a circle – and you will have a ‘Back Bearing’ – the magnetic bearing from the landmark to your position.

3.   Magnetic variation: Map (Grid) North is slightly different to Magnetic North so all magnetic bearings are slightly different from map bearings. The reason for this oddity is partly the way maps are made and partly that the Magnetic North Pole wanders about slightly. So each year it will be a little different from the north shown on your map according to the map’s age. To correct for Magnetic Variation is very simple: on your map it will say that at the given date of printing the Magnetic Variation was X degrees Y Minutes East or West of True North and changing by X Degrees Y Minutes per year. A Minute is 1/60th of a degree.

Work out how many years since the map was printed. Multiply the yearly change by that figure. Add it to the given figure when the map was printed. This figure is your map’s current Magnetic Variation. It will be a few degrees and minutes.

To change a Magnetic Bearing to a Map/Grid Bearing all you have to do is subtract the Magnetic Variation.

4.   Plotting a back bearing: Turn your Magnetic Back Bearing into a Grid Back Bearing by subtracting the Magnetic Variation. Plot this on the map by using the edge of the compass or a degree measuring device and drawing a line at the correct angle from north running from the landmark towards your estimated position.

Do the whole thing again with a second landmark. Then again if you have time and another landmark for accuracy.

5.   Resection: The back bearings you have marked on your map will intersect exactly on your position if you have done the process accurately. So you will know where you are to within a few tens of yards. If you did three and they do not cross on a point then one is wrong. At least one.

TOP TIP!

The rhyme I use to remember the direction calculation is:

Mag to Grid get rid

Grid to Mag Add – for when you are doing it the other way around

Marching on a bearing

Once you know where you are, and you want to get to another position marked on the map but is out of sight or a long way off, then there is a way to direct your route called ‘Marching on a bearing’.

Measure the Grid Bearing from your position to your goal with a protractor or the edge of a compass. Add the Magnetic Variation to turn it into a Magnetic Bearing. See rhyme the box above.

Set the Magnetic Bearing on your compass and then line up the arrows on the compass with the North Seeking Needle. The sights or arrow on the compass will now be pointing to your target.

If you do not have a protractor handy then lay the direction arrow or edge of your compass on the map and along the line of your intended march. Set the dial of your compass so that North on it points to Grid North on the map. Then add the date-corrected magnetic variation from the map and you are ready to go.

Fixed landmark: Sighting along your correctly set compass you should aim to pick a fixed landmark where it is pointing and use that to march towards. This minimizes sideways drift while marching.

Artificial landmark: Where there is no visible feature on the landscape, such as some deserts or jungles, you should send a man ahead some hundreds of yards or as far as he is visible, keeping him on the bearing with instructions. You can then march up to him and repeat the procedure as required.

Grid reference

To tell someone else where you are on a map you need to give them a grid reference. The idea is that all maps you will use are marked off in grids which form squares usually 1,000m on a side. There are two digit numbers along the bottom and up the side of your map applying to these lines which form the squares. By finding the two numbers along the bottom which leads to the left side of the square you are in, then finding the two numbers up the side which lead to the bottom of your square you can describe it accurately as, say, square 3648. This is a four-figure grid reference.

REMEMBER:

Use this rhyme: along the corridor and up the stairs; bottom number first then side number second.

To be more accurate, you can make a six-figure grid reference by splitting the box you are in into ten equal segments. If you are just short of half way from the left to the right of the same box and just over half way up it then your position would be 364486. This gives your position to within 100m and this is enough for most purposes.

Judging distance by time marched

While in training you should accustom yourself to judging distance marched or run by the time taken. Time yourself over known distances. It should become possible to estimate your progress quite accurately when you know that on hard, flat ground you cover 4km per hour in full kit. Then you can experiment with different types of ground and so forth. Notice the difference.

You may be surprised how reliable your pace is in measuring distance covered. This is a very useful technique to master so it is worth some effort. If you have been marching through featureless terrain for six hours and then get hit you may need to know where you are quickly so you can call in a chopper to get out the wounded or you out of the shooting match.

If you know you have travelled 24km because you were doing 4km per hour you can quickly mark it off on your map and read off the grid reference to call in the casevac or air support. It is OK to do this in ‘Clear’ rather than code as the enemy will see the LZ anyway and to use code could in fact compromise the code.

Contouring

In hilly country you use up most of your energy going up and down hills. If you know that your goal is marked by a distant peak or whatever – so you can find it if you stray off track – then it is sometimes possible to save a great deal of energy by travelling around the intervening hills maintaining the same altitude but going off the direct route. This is called contouring.

Don’t mark your maps

Despite the above techniques for resection etc, we were always taught not to mark maps in case they fell into enemy hands. I always thought this was a little depressing but you can see the advantage of keeping the enemy in the dark. Consider this when I mention capture of enemy maps later. As a compromise I think it best to mark as little as possible on a map that is going out on patrol. Never mark static features such as temporary base camps.

Finding direction without a compass

Finding north using the North Star: North of the equator all the stars appear to revolve around one point, which is exactly due north and marked by a star called, oddly enough, the North Star or Polaris. This star is always directly north of where you are standing. From this exact north marker you can work out any direction you need for movement.

Finding the North Star: There is a certain group of stars shaped like a frying pan which also has a proper name. It is called the Plough. Just so you don’t think I know nothing these patterns are called Asterisms. See the illustration of the Plough below.