The Compass The Basics What is a compass? A compass is a magnetised needle that points to magnetic north. You can use a compass to measure a bearing. A bearing is a measure of a direction and is the angle between the direction you want to measure and north. A bearing is a number between 0o and 360o, where 0o and 360o represent North, 90o for East, 180o for south and 270o is west. Parts of a compass Most people use a Silva type compass. This is a compass needle mounted in a liquid filled dial and this is attached to a base plate. This is the type I will explain here The Needle has 2 ends, red and white. The red pointer points north and the white one points south. The housing that holds the needle has 360 markings around the edge, some have a N, E, S, and W marked on and most have numbers at intervals around the edge (the 'dial'). These numbers are used to read the bearing. At the base of the dial there are parallel lines and an arrow that move with the dial The needle housing turns on the base plate. The base plate has an arrow pointing along the length of the plate, it might also have a ruler along some edges. The base plate might have other features as well Under the dial there is generally a (white) mark in line with the arrow on the base plate. What can you do with a compass? Primarily the compass needle will point to North. This is its function. Anything else you can do is just being clever. The ruler can measure stuff, for example distances on a map (2 cm on a 1:25000 scale map represents 500m, or 1km on a 1:50000 scale map). Using the base plate combined with the movable dial you can calculate a bearing between 2 points, either from a map or features you can see in reality Using bearings you can work out where you are if you are lost Measuring stuff There is a ruler on the compass. In a map reading sense, you can use this ruler to measure distances. Measure the distance you want to know and use the scale at the base of the map to work out what that represents. Measuring a bearing A bearing is the angle between north and the line between 2 points. The angle is measured from North in a clockwise direction. It can be a number between 0 and 360. A bearing can be calculated from features on the map or from features in reality. Calculating a bearing From the map One misconception is that you need to Set the map to calculate the bearing, you don't need to. To calculate the bearing: - Put the edge of the compass base plate in line with your intended direction, so the base plate edge touches both points. The base plate arrow should be pointing towards the second point - the point you will be travelling to. - turn the compass dial so that the parallel lines marked on the bottom of it run parallel to the maps northings (the grid lines running vertically on the map) and the arrow on the base plate is pointing to where you are going - The bearing is indicated by the mark under the dial that is in line with the base plate arrow. You can use this bearing to find things in reality. If you are stood at the first point and have a bearing to the second point you can use your compass to find out where it is Calculating a bearing from the reality This is the opposite of reading the bearing using the map. If you are standing at a point and what to find out what direction (relative to north) another point is you can use this method: - Stand with your back to the first point and look at the second point. - Hold the compass level in the palm of your hand with the base plate pointing at the second point - Keep the base plate held still and turn the dial so that the needle housing arrow points north - the red part of the needle and the arrow are lined up - The bearing is indicated by the mark under the dial that is in line with the base plate arrow. You can use this bearing to find things on the map, for example if you can identify a point in the distance you can find a bearing to it and transfer this to the map, do this with a few points and where the bearings cross this is where you are (see later) Now to complicate things a bit Magnetic Variation There are 3 North's! In Reality there is only one - Look at the north pole and you are looking north. The other 2 arn't quite north but they are the ones we are interested in. Magnetic north: This is where compasses point to and is quite near the north pole - it is in Greenland. Grid North: This is the where the northings point on a map. These are parallel lines but if they were all to point north they would be at all sorts of angles relative to each other. They are drawn as they are for clarity. A bit more complicated We are interested in Magnetic North and Grid North and these arn't aligned either, magnetic north is a couple of degrees away from Grid north, and it gets worse because magnetic north wanders about and changes. The difference between these is the Magnetic Variation. To make things right you have to alter the compass bearing you have just calculated to account for this magnetic variation. Look on the map and it will say somewhere what the magnetic variation is and when it was that and how much it is changing each year. Use this to work out what the magnetic variation for today is (A few degrees). Next modify your compass bearing. If you are going from reading the map to looking in reality, add this magnetic variation, if you are going from reality to the map, take this variation away. Magnetic to Grid - Get rid, Grid to Mag' - Add Confused yet? How to find out where you are on the map If you are sat on a hill in reasonable visibility, you can see a lot of landscape. You might be able to see a few roads, some hills, perhaps a church and some walls - in fact if you are lucky you might see everything there is a map symbol for around you. You can use these points to identify where you are. Look around you and try to identify a notable point, such as a church. Take a bearing from you to this church, then add the magnetic variation. Now locate this church on the map and then put the long edge of the compass next to it so that the arrow points towards the point. Rotate the whole compass so that the parallel lines on its dial line up with the maps Northings and your location is along the line of the edge of the compass. Now find another point that you can identify and repeat the above - where the 2 lines cross is where you are. If you can repeat this a third time to reduce errors. Walking with a compass OK you can locate yourself in clear visibility, you can work out a bearing from the map and from reality and adjust these to account for magnetic variation. So how does this help you on an expedition? If you are 'confused' as to your current location you can find out where you are using landmarks. Look at the landmarks and get a bearing to them, draw lines on the map from bearing to these landmarks and where the lines cross is where you should be. If you know where you are on the map you can use your compass to indicate which way to go, for example get to the path junction and turn 157o (Turn the compass dial so that it indicates the bearing required, line up the dial's base arrow with the pointer and the base plate arrow shows the way). This will show you which way to go, and until you get some experience with walking, and I guess if you are reading this then you haven't, the compass will show which footpath to take. There are more advanced techniques that will help you walk in a straight line cross country and for a certain distance but I don't intend to write about these here. Now you have read the basics, get your map out at home and practise getting a bearing between map points. Now go out walking, stop at a point where you know where you are and practice taking bearings to points, locate these points on the map and check your bearing Now walk a bit further and take some bearings from reality, and uses these to pin point exactly where you are. have fun