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Compass & Bearings


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Compass & Bearings

  1. 1. The Compass and Bearings
  2. 2. A. Parts of a compass
  3. 3. B. Setting and following a bearing on a compass <ul><li>Rotate the compass dial until the bearing comes opposite the direction of travel arrow and over the index line. </li></ul><ul><li>Hold the compass against your body with the direction of travel arrow pointing away from you and turn your entire body until the RED end of the needle points to “N” on the dial. (RED is in the SHED!) </li></ul><ul><li>Look up and select a distant object, such as a rock, tree, etc., in line with the direction of travel arrow. </li></ul><ul><li>Walk to this object, concentrating on the object, not the compass (i.e., it is okay to go around barriers, as long as you get to your selected object). </li></ul><ul><li>Repeat steps 2 through 4 until reaching the destination or turning point. If it is a turning point, set your new bearing and continue to the next turning point or destination. </li></ul>
  4. 4. C. Finding a bearing from a map <ul><li>Place the compass on the map and join together the start and destination with the long edge of the compass. </li></ul><ul><li>Check that the direction of travel arrow points the way you intend to go. </li></ul><ul><li>Rotate the compass dial until the orienting lines are parallel to the longitude lines and “N” is at the top. </li></ul><ul><li>Important: ignore the magnetic needle. </li></ul><ul><li>Read the true bearing (map bearing) at the index line. </li></ul><ul><li>Before using this bearing, it must be converted to a magnetic bearing by adjusting for declination. </li></ul>
  5. 5. <ul><li>Correcting for declination </li></ul><ul><li>The difference between map north (true north) and magnetic north is called declination </li></ul><ul><li>Declination depends on where you are on the earth! </li></ul><ul><li>Declination is constantly changing, so declinations on maps may be wrong! </li></ul><ul><li>Declination can be east or west. If the compass needle is pulled to the east of of true north, like in Alaska, it is called east declination. </li></ul>
  6. 6. 5. If your declination is east then magnetic north is greater than true north and the map bearing is greater than the magnetic bearing. You need to make the two worlds equivalent by adding or subtracting the declination. This is illustrated in the figure. <ul><li>Map bearing - Declination = Magnetic Bearing. </li></ul><ul><li>Magnetic bearing + Declination = Map Bearing. </li></ul><ul><li>This can be difficult to remember, so use the following to help you: </li></ul><ul><li>Map to compass, dump it; </li></ul><ul><li>Compass to map, add it back. </li></ul>
  7. 7. E. How do you determine declination? <ul><li>Declination is often printed on maps and charts. Caution: declination changes over time! </li></ul><ul><li>Use a declination calculator such the one at: </li></ul><ul><li>Determine declination yourself using your map and compass </li></ul><ul><ul><li>If you know your location, take a bearing to another known landmark such as a mountain, lake, etc. This will be a magnetic bearing. </li></ul></ul><ul><ul><li>On your map, determine the true bearing between the same two locations. </li></ul></ul><ul><ul><li>The difference between your map (true) bearing and the magnetic bearing is the declination. Subtract the magnetic bearing from the true bearing to find the declination. </li></ul></ul>
  8. 8. F. Triangulation: locating your position on a map <ul><li>Take bearings to two or preferably three landmarks. These will be magnetic bearings. </li></ul><ul><li>Convert your magnetic bearings to true bearings. Because you are going from “compass to map” you need to add the declination to do this. </li></ul><ul><li>Set one of the true bearings on your compass. </li></ul><ul><li>Lay the edge of the compass next to the landmark you sighted and turn the entire compass (not dial) until the orienting lines are parallel to the longitude lines on the map. </li></ul><ul><li>Draw a line along the edge of the compass (extend if necessary) </li></ul><ul><li>Repeat steps 3 through 5 for the other two bearings. </li></ul><ul><li>Your location will be where the lines cross (or inside the small triangle made by the lines). </li></ul>