Using A Compass II
- Published: Sunday, 28 December 2014 11:26
- Written by Lance Hartley
- 1 - 2 crew
- 1:25000 topographic map
- Bega Map
- Magnetic compass
The Compass and Map
The major applications and uses of the orienteering compass (type used in our brigade) are detailed in the following paragraphs.
Taking a Grid Bearing – The procedure for calculating a grid bearing from a map, shown below is as follows:
- Place the long edge of the compass plate along the desired bearing (along line AB in diagram below) making sure that the directional arrow on the compass plate points in the direction you wish to travel.
- Turn the compass housing so that the meridian lines are parallel with the eastings on the map, and the north mark is pointed towards the top of the map.
- Read the grid bearing on the housing where the index line intersects it. Note: The bearing that has been calculated is a Grid Bearing. This bearing must be converted to a magnetic bearing if it is intended to be set on the compass. The procedure for converting bearings is dealt with here.
Setting the Compass on a Magnetic Bearing – To travel along a magnetic bearing use the compass as follows:
- Set the magnetic bearing on the compass by rotating the compass housing until the required bearing is in line with the index line on the compass plate.
- Holding the compass flat in the palm of the hand, turn around until the red end of the compass needle points to the north mark on the compass housing and is parallel to the meridian lines.
- The direction arrow now points along the required magnetic bearing.
Taking a Magnetic Bearing – The procedure for taking a magnetic bearing to an object is detailed below:
- Hold the compass in the position shown below with the direction arrow pointing to the object.
- Rotate the compass housing until the red arrow of the meridian lines is directly beneath the red (north) end of the compass needle.
- Read the magnetic bearing on the housing where the index line intersects it.
If the firefighter is travelling on a given bearing and wishes to return to the original start point simply turn around to face roughly in the direction from which they have come and reverse the compass so that the direction arrow points towards the firefighter. Orient the compass by turning the whole body until the red end of the compass needle points to the north point on the housing and travel in the direction in which the back of the compass plate faces.