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Contributed By: Kit WilsonA compass is an essential piece of kit when in the wild. Though most people depend upon electronic GPS’s these days, a compass serves as a trusty back up whilst infield.
Those that can effectively use a compass to navigate have an advantage whilst infield – possessing the ability to travel accurately with minimal equipment.
Parts of the Compass:
Needle – A magnetised strip of metal that rotates to show North (red) and South (Black)
Base Plate – generally clear so you can see a map underneath. Has a straight edge for plotting, as well as a magnifying glass, scale conversions and ruler.
Rotating Dial – rotates to display marked bearings along its outer edge (from 0-360°).
Orientating Arrow – rotates to display North relative to the dials markings.
Direction of Travel Arrow – position to display the direction a compass should be held when sighting or plotting a bearing.
Index Line – the point at which you will take your bearing form the marked dial.
Orientating Lines – North/South lines that rotate with the dial. Designed to be aligned with a maps Longitudinal lines.
How to Read a Bearing
In general terms, most people will use the standard NORTH, SOUTH, EAST and WEST when describing a direction. Though when navigating in-field these units do not allow for the specificity that is need, hence, we use ‘degrees’ to outline a bearing between two points. When looking at a compasses rotational-axis these bearing begin at ‘North’ with a value of 0-Degrees and extend around the dial to 360-Degrees (also located at North…).
NORTH - 0°
EAST - 90°
SOUTH - 180°
WEST - 270°
Therefore, degrees allow for much more accurate bearings to be derived, an example being a heading of 202° which would be approximately SSW.
Adjusting for Declination
When you take a heading on a compass it will display the ‘Magnetic Bearing’, though in reality the ‘True Bearing’ may be a few degrees different. This variation is called ‘declination’ and is due to the earth’s non-uniform magnetic field, which places the ‘Magnetic poles’ offset from the ‘True Poles’. Where you are positioned on the globe will determine the extent to which you must account for declination. Luckily, most compasses will allow for adjustment according to the ‘declination value’ – which can be found on the areas Topographic Map.
Setting a Bearing
Setting a bearing on your compass will allow you to maintain a particular heading whilst travelling overland, mitigating the chance of missing your target coordinate or getting lost. You can set a bearing on a topographic map or by sighting a prominent land-feature – let’s look at both:
On a topographic map:
1. Place your compass on a map. Align the straight edge of the compass with your position and intended destination, ensuring the ‘direction of travel arrow’ is pointed towards the waypoint.
2. Now rotate the compasses dial until the ‘orientating lines’ are lined up with the north/south on your map. Make sure that the ‘North Marker’ on the compasses rotating dial is pointed to the maps north.
3. By looking at the ‘Index Line’ you can obtain the heading you will need to maintain. This heading will be displayed upon the dial and will range from 1 to 360 degrees.
4. You can now pack the map away. By holding the compass out with the ‘direction of travel arrow’ facing away, you can rotate until the North-end of the magnetic needle (red) is within the ‘orientating arrow’. Once this happens you have gotten “red in the shed’ and you are facing your bearing.
5. If you can keep this needle within the ‘orientating arrow’ whilst travelling, you will maintain your heading.
Sighting a landmark of feature:
1. Locate a prominent landmark or feature you want to head for.
2. Hold your compass out so that the ‘Direction of Travel Arrow’ point towards your chosen feature. Ensure you are exact when position the compass, as even a few degrees of difference can culminate in missing the destination over long distances.
3. Once positioned, rotate the compasses ‘dial’ until the ‘orientating arrow’ lines up with the magnetic north needle (red).
4. Now you have captured the bearing towards the landmark. You can view the ‘bearing value’ by looking at the ‘index line’ in relation to the compasses ‘dial’.
5. If you can keep this needle within the ‘orientating arrow’ whilst travelling, you will maintain your heading.
NOTE: Compasses with ‘Sighting Mirrors’ are far more accurate when sighting landmarks to gain a bearing. One of the best is the SUUNTO MC-2 GLOBAL MIRROR COMPASS.
Triangulating Your Position with a Compass:
If for some reason you don’t know your exact position, you can use your compass to provide an accurate approximation of your current location. To do this you will need to use ‘triangulation’, let run you through the process:
1. Firstly, you will need to identify 2-3 prominent landmarks or features that you can sight and locate on your map.
2. Secondly, by using the visual sighting technique previous mention, obtain the exact bearing towards your first landmark.
3. Now place the compass on your map. Align the ‘orientating lines’ with north/south on the map and position the straight-edge along the feature you sighted. Using a pencil, trace a line along the compasses straight-edge – your location is somewhere along this line.
4. Complete the process of visually sighting and plotting on the map for each of your features. The more feature you include the more precise your estimated position will become – but ensure that the landmarks are at least 60° apart.
5. Once completed, you should have a triangle (or intersecting lines) where the interior represents your approximate location.
