Compass work
What is a compass for?
A map is a representation of the landscape on a sheet of paper. We get information from the map and need to convert it into information that we can use on the ground. We may see a path on the map that we want to follow but this does not tell us in which direction the path goes in real life. In most cases we can tell from other features, the path may follow a stream up hill, for example. For when we don’t have those features, we have the compass. The compass, is a tool for converting directional information between the map and the ground.
The compass, as a conversion tool, works by virtue of having some features that relate to the map and other features that relate to The Earth. On the map there are grid lines, on the compass these are represented by the orienting lines.
On The Earth there is a magnetic field, on the compass there is a magnetic needle.
Taking a bearing from the map.
Lets assume we want to work out the bearing between two points so we can walk in a direct line between them. (From point A to point B).
1) Make an estimate
Have a guess at the direction in which you will be walking. By making an estimate you should avoid the embarrassment of being 180 degrees out. It also means that when you get to turn the dial in stage 3 your have less to turn it so it is less likely that you will have your compass slipping on the map. Set your compass to this estimate now.
2) Position the compass
Place the long edge of the compass along the path that you need to follow with the big (direction of travel) arrow pointing in the direction you will be walking i.e. the back of the compass should be at point A and the front at point B.
3) Turn the dial
Turn the dial so the orientating arrow is pointing up the map parallel with the grid lines. You should end up with a figure roughly what you estimated in step one. If it is vastly different you are probably 90, 180 or 270 degrees out. (Ignore the needle, the map has no magnetic field)
4) Add the magnetic variation if you need to.
5) Align the needle
Take the compass off the map and line up the magnetic needle and the orienting arrow by holding the compass flat and turning yourself round on the spot. It is very important to do this right as this is where you will unconciously influence the process.
6) Follow the direction of travel arrow
The ‘direction of travel’ arrow will point in the direction you need to walk in.
Reversing polarity
In rare occasions (usually in storage or transit) the compass needle can reverse polarity. This happened once on a navigation course because the participants gloves contained strong magnets to keep them together when not in use (instead of a couple of catches). This can be fixed by stroking the needle with one side of the magnet. If it does not fix it then use the other side of the magnet.