Using A Compass
- Published: Sunday, 28 December 2014 11:26
- Written by Lance Hartley
- 1 - 2 crew
- 1:25000 topographic map
- Bega Map
- Magnetic compass
North by Northwest
- True North: (also known as Geographic North or Map North - marked as TN on a topographic map - is the geographic north pole where all longitude lines meet. Unfortunately for the firefighter, true north is not at the same point on the earth as the magnetic north Pole which is where your compass points.
- Magnetic North: The earth's magnetic pole doesn't correspond to the Geographic North Pole and because the earth's core is molten, the magnetic field is always shifting slightly. The red end of your compass needle is magnetised and wherever you are, the earth's magnetic field causes the needle to rotate until it lies in the same direction as the earth's magnetic field. This is magnetic north (marked as MN on a topographic map).
- The compass is used primarily to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic north or true north) to some point (either a point on a map or a point in the real world). Bearings are used to accurately travel to a destination or to locate your position.
- If you are working from your map, it is called a map bearing or grid bearing and the angle you are measuring is the angle measured clockwise from true north on your map to this other point on the map.
- If you are taking a bearing off a real point on the landscape with a compass, you are using your compass to measure the angle clockwise from magnetic north to this point on the landscape. This is called a magnetic bearing. Remember that the bearing is measured clockwise
Grid and Magnetic North:
Topographic maps contain a variation diagram depicting the relationship between Grid and Magnetic North and notes:
- The angle between them
- The year when this was correct
- The value and the direction of change over time (Note: the change is often in tenths of degrees for every 3 years)
Examples using 14o as easterly variation
Converting a bearing from Grid to Magnetic
Grid bearing 89o – 14o = 75o Magnetic Bearing
Converting a bearing from Magnetic to Grid
Magnetic Bearing 50o + 14o = 64o Grid Bearing
|The diagram gives an example of how to convert a grid bearing, one taken from the map, to a magnetic bearing taking into account the same easterly magnetic variation. The grid bearing of 89 degrees needs to have 14 degrees of easterly variation subtracted from it as the diagram indicates.||The diagram above gives an example of how to convert a magnetic bearing, one taken by a compass, to a grid bearing taking into account a 14 degree easterly magnetic variation. The magnetic bearing of 50 degrees needs to have 14 degrees of easterly variation added to it to convert it to a grid bearing as the diagram indicates|
|Remember: GrandMa Sux - Grid to Magnetic - subtract||Remember: My Great Aunt - Magnetic to Grid - add|