Introduction To The Compass
The compass is an essential tool for the hill-walker. It is a precision instrument that can bring about the success or failure of an expedition. However, to the beginner it can also be a very bewildering piece of equipment. So we will start by identifying the key elements of the compass.
1. Base Plate; 2. Bearing Bezel; 3. Compass Needle – North End Red: 4. Orienting Lines; 5. Orienting Lines; 6. Index Line; 7. Magnifying Lens; 8. Direction of Travel Arrow; 9 Romer Scale; 10. Luminescent Direction of Travel Indicator
When choosing a compass, it should be easy to handle yet, compact and light weight. The compass needle capsule should be liquid filled. It should have a romer scale engraved into the base plate for use on 1:25,000 & 50,000 scale maps. The bearing bezel should be marked off with 360°.
Remember, this is a precision instrument and should be treated as such. Take great care not to drop the compass, attach around the neck with a lanyard. Always store away from other compasses and avoid electrical equipment if at all possible.
A Tale of Three North’s
There can be much confusion when it comes to referring to ‘North’, because there are, in fact, three norths! True North, Grid North and Magnetic North.
True North is the geographical north where the ‘north pole’ is, Grid North is the north to which the grid lines on your map points i.e. the top of the map, and Magnetic North is the north to which your compass points. As of 2019 this lies near Baffin Island NE Canada and is moving at approximately 30km a year.
It is very important to know what the difference is between the Magnetic and the Grid North because this is the difference between your map and your compass.
According to the Ordnance Survey and the British Geological Association this is called the Grid Magnetic Angle (GMA) and it is this angle that needs to be applied when converting between Magnetic and Grid bearings. Sometimes this is incorrectly referred to as the Magnetic Variation.
The legend of the OS Landranger and Explorer maps gives details of the variation between the Magnetic and Grid north’s along with the date and rate of change in degree minutes per year. For example, my Explorer OL33 says the GMA is 2° 27′ west of grid north for 2011. Annual change is about 9′ east. Therefore, in 2019 the GMA has reduced by 1° 12’. So the current difference between Magnetic North and Grid North is taken as 1° west (difficult to measure to 15′).
So how does that translate to the map and compass. Remember the phrase “From grid to mag add & from mag to grid get rid”. So, you have taken a bearing on the map (grid) and want to use it on a compass (mag) we add the GMA (1 degree).
Setting A Map
A lot of people put a great deal of importance on setting the map. What does ‘setting’ the map mean and is it really required?
The main purpose for setting the map is to orientate it to north. This is achieved by setting the magnetic variation (GMA) on the dial, then aligned the side of the compass with the north-south gridlines on the map. Then turn the map and compass until the compass needle falls inside the orienting arrow. The map is now oriented to north.
The one advantage of doing this, is that with the map set you can then look for symbols and features on the map and then you should be able to see that in the landscape. However, some navigators, including myself, prefer to orientate the map to the direction of travel, so that the top of the map is the direction that I am travelling.
Taking a compass bearing from the map and following it on the ground.
Place the long edge of the compass along a line on the map joining the start point with the finishing point. Make sure the ‘direction of travel arrow’ is point towards the end point.
Holding compass in place, rotate the housing so that the orienting lines are parallel to the north-south grid lines on the map and the orienting arrow is pointing toward the top of the map (grid-north). The bearing shown on the ‘index line’ is the angle between grid-north and your direction of travel.
However, the needle of the compass does not point to grid-north but magnetic-north. Therefore, we must offset the angle by the GMA using the rhyme “add for mag get rid for grid” we have to add the GMA or magnetic variation to the bearing, which where I live is 1°.
Now with the compass held in front of you, with the ‘direction of travel arrow’ pointing away from you. Turn your body so that the red of the needle lays over the orienting arrow on the compass housing. Once this is done the compass is pointing in the direction of travel that you should take.
Tip: In reasonably good visibility, select a feature on the line of the bearing and that will become the point towards which you should walk. However, be careful that you do not select a sheep or a cow! (poor visibility bearings will be dealt with later).
Taking a compass bearing from the landscape and transferring it to the map.
This is a very useful skill to learn as it will help you to fix your position (see next section).
Firstly, point the ‘direction of travel arrow’ at the feature and turn the housing so that the red needle lays over the orienting arrow. This will give you the angle between the feature and magnetic-north. Remembering the rhyme “rid for grid”, you need to subtract the GMA or magnetic variation so that you have the angle between the feature and grid-north.
Place the compass on the map and align the orienting lines with the north-south grid lines. Keeping this alignment, move the compass up or down to intercept the feature on the map. You must make sure the ‘direction of travel arrow’ is pointing towards the feature. Your position is now somewhere along the line formed by the long edge of the compass. If you have a pencil, draw a line along the long edge.
Exercise: Go outside and practice both of these techniques until you are happy with them. Do not forget the GMA and whether you need to add or subtract.
Triangulation or Resection
This is a technique used to fix you position in the landscape, assuming reasonable visibility. You will need at least two and ideally three landmarks or features that are approximately 120° apart.
Using the technique you learnt for taking a compass bearing from the landscape and transferring it to the map. Do this for all three features, so that you end up with three lines on the map. These should cross over in what we call a ‘cocked hat’ and we now have an 85% probability that our position is inside this ‘cocked hat’.
Exercise: Go outside and practice this technique until you are comfortable with it.
In Map Reading & Navigation – Part 4, we will be looking at Naismith’s Rule, Route Cards, Route Planning and Staying on Course plus Transits and Back Bearings.