Bearings
Taking a Magnetic Bearing Hold the compass parallel to the ground making sure the direction of travel arrow is pointing at the object or feature you wish to take a bearing of. Holding the compass steady and still pointing at the object or feature turn the compass housing until the North indicator on the housing matches up with the red end of the Magnetic needle. The read of the bearing at the index line this is the magnetic bearing to the object or feature. To Walk on a Bearing Once you have carried out the above procedure take note of the bearing. Then without moving the graduated circle, hold the compass in the flat of your hand, turn the whole compass around keeping your eye on the compass needle as it moves. Once the north point of the compass needle is aligned with with the north point "Red on Red in the compass housing, the compass is set. You are then ready to head off in the direction, that the direction of travel arrow is pointing. Not the direction that the magnetic needle is pointing if you do that you will only be going in a north direction. |
Images produced from Ordnance Survey Get - a - map -service. Image reproduced with permission from Ordnance Survey
How to take a Grid Bearing First identify on the map where you are and where you want to go. Place the edge of the compass so it bisects your position and your destination, making sure that the direction of travel arrow is pointing towards your destination. Once you have done that, hold the compass in position on the map and turn the compass housing or bezel until the north indicator and the red half of the compass are pointing to the top of the map,ie, grid north, and the orienteering lines are parallel to the vertical grid lines. Without disturbing the compass read off the grid bearing at the index line. In this case the grid bearing is 316 degrees (see above). Its advisable at this stage to take a note of the bearing. Remember you have to convert this grid bearing into a magnetic bearing, so you must add the grid magnetic angle (GMA). Example: If the GMA was 3 degrees, and we have the grid bearing of 316 degrees you would add the 3 degrees converting the grid bearing to a magnetic bearing of 319 degrees. Note: When you take direct bearings from one point to another, it is not always possible to walk direct on that bearing to the objective. You may have to make slight detours due to obstacle in your way and of course the type of terrain you are crossing will dictate your movement. Plot the direct bearing onto the map and mark it with a pencil. Study the map, look at all the features and the terrain, check each side of your bearing on the map looking for any potential pitfalls, also check for features which aid you along the route; this is especially important in bad visibility or at night. Choose a safe route. If need be break the route down to shorter legs. you should never cover long distances on a single bearing this will only lead to mistakes and disorientation. Remember this simple rule. "Grid to Mag = Add" "Mag to Grid = Get Rid" |
Choice of Route: The choice of route is choosing the best and safest route to get to your destination and not necessarily the quickest one. Looking at the map Route A to B is the direct route to your destination going totally cross country this would probable be ok in good weather conditions and you where really competent in your navigational skills. Route C to D is the best option and will take bit longer this will depend on the path and the track that you eventually join. And also on this route you only have two small stretches cross country. |
Listed below are some Guidlines Keep the compass at least these distances away to avoid incorrect readings. Buildings, high tension cables and sub stations - 55 to 60 metres. Vehicles. pylons, metal gates and cattle grids - 18 to 20 metres. Note: These distances are only a guide, just be aware of your surroundings. Points to Remember when Taking Bearings A compass is sensitive to iron and steel and other ferrous alloys. Even small quantities near the compass can cause a false reading. Common items which may affect the reading are overhead or buried electrical cables which generate magnetic fields, also buried pipe lines, wrist watches and even the wire support in a bra (so ladies be aware). Backbearings The difference between a bearing and a back bearing is 180 degrees. Therefore, given the bearing and if it is less than 180 degrees, to find the back bearing add 180 degrees; or if the bearing is more than 180 degrees subtract 180 degrees. This is handy if you want to re-trace your steps back to your last known location. Example: Forward bearing 040 deg, the back bearing would be 220 degrees. Forward bearing 300 degrees the back bearing would be 120 degrees. Conversion Grid and True Bearings The convergence between grid and true north is 01 degrees and 41 minutes. The relationship is constant and not affected by date and time. To convert: Grid bearing to a True bearing - add 01 deg, 41 min. True bearing to a Grid bearing - subtract 01 deg, 41 min. |
