Features Of A Compass
Features of a Compass
Most of us have all seen or held a compass, but what are the features on a compass and what do they mean?
Why Do We Need A Compass
A compass can help you plot your route, find where you are on a map. This can be useful, but can also be life saving if you get lost or bad weather closes in a you need to get off the mountain the quickest and safest way possible. The main uses are
- 'Setting' the map and compass so it matches what you are seeing on the ground, and that you are walking in the right direction.
- Taking bearings and making sure that you follow those bearings.
- Using a bearing to identify features, and using that bearing and those features along with a map.
- Pinpointing your location using visible features around you, by using triangulation from two or more visible features (the more the better)
Compasses come in many different designs, but basically all do the same thing. The most common ones for walking and hiking look like the one in the picture. You will need to make use of some or all off the features on a compass to complete different tasks. Before you can start to use a compass for navigating you need to know what the features on a compass are.
- Base plate
- Compass housing or compass wheel. A mark every 2º and the 'cardinal points' N,E,S and W.
- Compass needle. Red is NORTH, white is SOUTH.
- Orienteering or compass lines
- Orienteering arrow. Fixed to NORTH within the compass housing.
- Index line. This is an extension of the direction of travel arrow.(7)
- Direction of travel arrow.
- Scale rules. Can help judge distance on the map.
The main working part of a compass is the magnetic 'needle' that floats on a central pivot. The red end always points to the Earth's magnetic north pole and the outer ring is marked with the cardinal points of the compass (N-S-E-W) and every 2 degrees. These markings are used to get bearings (the direction from where you are, to where you want to go). If you rotate the ring to line-up the red north of the needle to the red arrow on the base-plate, a bearing can be be taken from the compass ring.
Where is north?
There is, however, a slight complication; magnetic north is not the same as map grid north because magnetic north (where the compass needle points) changes in different areas of the world, and also changes over time. To get a completely accurate reading you have to adjust the bearing to take account of the difference between map grid north and magnetic north. The degree of deviation is marked on printed Ordnance Survey maps but as a rough guide, you should adjust by 2 degrees by turning the compass housing anticlockwise. For longer trips, the difference can have a significant impact on navigation.
This can be discounted in the UK, but can make a significant difference in certain parts of the world.
Keep a lookout for the next few blogs where we will begin to explain the basics of navigation.