In this video you will learn the basics of how to use a compass. You will learn to orient a map using a compass, take a bearing, understand the difference between True North & Magnetic North and adjust for declination.
The north end is usually painted red (occasionally black), stamped with the initial N, formed in the shape of an arrow, or a combination of the above. When left to itself it will eventually come to rest with one end pointing to magnetic north. To bring the needle to rest more quickly, it is suspended in a sealed compartment filled with air or liquid.
- AIR FILLED compasses work just fine, the drawback is that you must wait quite some time for the needle to come to rest so that you can take a bearing. It also requires that it be held stationary, so does not work well when hand held.
- LIQUID FILLED compasses are the most effective in breaking the swing of the needle quickly. The majority of compasses on the market are liquid filled with a mixture of water and alcohol.
ORIENTING ARROW AND LINES
The orienting arrow is the large arrow in the center of the compass pointing to North on the azimuth dial. Its double lines capped with a peak resemble a dog house, and it is frequently referred to as such. Orienting lines run parallel to the orienting arrow and help you to set the course. These, and other markings, are sometimes fluorescent, which provides better viewing at lower light levels such as early morning or evening, and in bad weather.
AZIMUTH RING (Bezel)
This is the dial surrounding the compass arrow that indicates degrees, from 0 to 360, in a full circle. Graduations, or degree markings, are listed by either 1, 2, 5 or 15 , with the more complete graduations providing greater accuracy. The 15 graduations are not recommended. To adjust for declination, the difference between magnetic north and true north, compasses feature a stationary, rotating, or adjustable azimuth dial.
- STATIONARY azimuth dials require that you turn the compass itself in order to align the arrow (magnetic north) and the “dog house” on the compass. You must manually correct your reading each time to reflect the declination.
- ROTATING azimuth dials allow you to adjust your compass setting for declination by aligning the arrow (magnetic north) and the “dog house”, and then adjusting the dial by the degree difference of declination. This only needs to be done each time you start off on a new bearing.
- ADJUSTABLE, set screw, azimuth dials operate like the rotating ones, but the set screw allows you to rotate the orienting arrow to compensate for declination and then lock it into place. This only needs to be done once, at the start of your trip. This is a nice feature if you are going to be using it frequently in the same area.
The base plate is almost always clear, in orienteering compasses, so that you can see through it to the map for better alignment. The direction of travel arrow runs down the center of the base plate from the azimuth ring to the front, and is often marked “read bearing here”.
Magnifying lenses are frequently inset into the base plate and are very helpful when looking at details on the map since the markings are quite small. Map scales are marked on the edges of the base like a ruler.
Those with more than one scale will generally show 1:24,000 and 1:365,000 since these are the two most common.
Rounded corners are more comfortable in the hand, and some feel they slide more smoothly across a map and its folds. Levels are not necessary but improve accuracy, especially when hand held. A lanyard, watchband, zipper pull, or tripod mount allows you to fasten the compass onto the outside of your body or pack, keeping the compass handy. They should not be made of metal.
These help you to more accurately take a line of sight heading to your chosen landmark. There are both mirror and lensatic sighting devices.
- MIRROR devices have a vertical line marked in the center of a hinged lid,with a notched sight at the top.
First line up your chosen landmark in the notched sight at the top of the mirrored lid.
Then align the center of the needle pivot with the vertical line in the mirror.
You can now accurately read your bearing in the mirror.
This device gives you approximately 2 degrees of error instead of the approximately 5 degrees of error that you get with a simple compass.
- LENSATIC devices have a vertical sight in the lid and a separate lens that is both used to align your landmark and read your bearing.
As with the mirror device you sight your bearing, line up the vertical sight with the notch on the lens, and read the bearing.
Because these are more accurate than the mirror devices, they are used by the military and often called marching compasses.
Similar to a telescope or binocular in appearance these provide no magnification. The compass and sight are inside. This allows you to sight alandmark and take your bearing all at once.
These are used to measure the angle of incline. This angle, combined with a known distance, can be used to calculate height or, with a known height, distance. It is also used for determining slope and, in snow conditions, predict avalanche hazard.
The cardinal points of the compass are the points that have names. The four cardinal points of the compass are North, East, South and West. Some directions can be given as combinations of these cardinal points. For example, North east is a direction half way between North and East. Further refinements can be added, for example North north east is a direction half way between North and North east.
The cardinal and major points of the compass.
The names of these directions can be abbreviated. For example, North would become N, North East would become NE and North North East would become NNE. Note that if a direction is given as a name (rather than as a bearing), then it should only be taken approximately. If someone says head towards the knoll just North of the main peak, they don’t mean head towards any knoll you find on a bearing of ° from the main peak.
Similarly, if you intend to give precise directions, use bearings rather than compass points. For example, use Travel for 20km on a heading of 45°M (magnetic) is better than Travel North East for 20km.
The circle of the compass is divided into 360 divisions, called Degrees. The symbol for bearing degrees is the same used for temperature degrees – °. Unless otherwise specified, when these notes show a bearing in degrees, they mean a magnetic bearing. To indicate which bearing system I’m using, I usually add M, G or T, meaning Magnetic, Grid or True, respectively.
The cardinal point North becomes 0°. East becomes 90°. South becomes 180°. West becomes 270°. Note that North is also 360°, but by convention we only use 0°.
There are three different types of North:
The most important to us as hikers is Magnetic North. This is the North that the compass needle points to. At present, Magnetic North is located under Canada somewhere. The second most important North is Grid North. This is the North which the grid lines on our map are aligned with, and is defined by the Australian Mapping Grid (well, in Australia it is!). The third North is True North, which is the direction towards the North Pole of the Earth.
In Australia, these three Norths are never the same. There is always a small deviation between Grid and True North, depending on how the map was drawn. There is usually a large deviation between Grid or True and Magnetic North.
On a map with a Grid/Magnetic deviation of 13°, and a Grid/True deviation of 1°, these bearings would be the same direction: 13°T, 14°G, 1°M. Note that a deviation of 1° equates to about 20m over 1km. If you make the mistake of using a True bearing to find your path with the compass, you could be out by as much as 250m over a 1km stretch!
Taking A Bearing To Something
Finding the Magnetic bearing to something is easy, regardless of the compass you use. We usually use the Silva style compass, so I’ll describe here how that works.
1. First, locate the object or feature that you want to take a bearing to.
2. Now point the Direction Of Travel arrow at that object or feature.
3. Finally, hold the base of the compass steady and turn the dial so that the red arrow on the base of the dial is underneath the read half of the compass needle.
4. Read off the Magnetic bearing.
Following A Bearing
Following a bearing is a little tricker.
1. First you need to convert whatever type of bearing you are given to a Magnetic bearing.
2. Then, turn the dial of the compass so that the Magnetic bearing you want is lined up with the Direction Of Travel arrow.
3. Now move the whole compass around so that the red half of the compass needle is above the red arrow on the base of the compass dial.
The Direction Of Travel arrow is now pointing in the direction you need to go.
Tricks of the trade
Here are some useful tricks to remember when you are navigating or planning a route.
Remember: Magnetic Deviation West, Compass Best. Magnetic Deviation East, Compass Least.
This means that if the Grid/Magnetic deviation is to the West, you always ADD that number to the Grid bearing to get the Magnetic bearing – that is, the Compass bearing will always be a better number than the Grid bearing.
If the Grid/Magnetic deviation is to the East, you always SUBTRACT that number from the Grid bearing to get the Magnetic bearing – that is, the Compass bearing will always be a lesser number than the Grid bearing.
To convert from Magnetic to Grid, go the other way. Instead of adding, you subtract, and vice versa.
If you are heading towards a linear feature such as a road, fence or river, and you want to find a point like a cattle grid, gate or bridge, use a technique called bearing off. To do this, you aim to miss the target. Sounds strange? Well, here’s an explanation.
If you head directly towards a target, you will probably miss. Say you want to find a gate located on a road. You set your bearing to head directly for the gate. On the way, you move around a few obstacles, which only compounds the error in your compass bearing. So you arrive at the road, but you don’t know whether the gate is to your left or right.
So aim, say, 4° to one side of the target. By aiming to one side of the target, you will always know what direction to travel to find the target. But make sure you aim off far enough to allow for the error in your compass bearing. One way to allow for error is to aim off by 4°, then round off to the next furthest mark on the compass dial.
For example, if the gate I am travelling to is supposed to be at 105°M, I would aim North a little, so subtract 4° to get 101°, but round to the next furthest marking, which is 100° (my compass has 2° markings). If I’d rounded to 102°, the error in my bearing could have taken me to the other side of the feature, and I’d get geographically embarrassed.
Another example. The feature I want to get to is supposed to be at 260°M. I would aim North a little, so I’d turn the dial to 264°. Since 264° is already on one of the markings on the dial, I’ll leave it there.
Remember that bearing off or aiming off only works if the feature that you are looking for is located on or near a linear feature such as a road, fence or river. Also, it doesn’t matter whether you aim North of your target or south of your target, as long as you aim to miss.
Following a Bearing
Never make the mistake of following your compass. Use it as a navigation tool, not a walking guide. If you keep following your compass, you may end up far off track because you will keep walking around trees and boulders, looking for creek crossings and finding gates in fences.
A better way to navigate is to use your compass to find a landmark or feature that is clearly visible, that also happens to be on (or close to) your bearing. For example, use boulders, huts, bridges, nearby mountains. Once you have selected the object, put the compass away and walk towards the object. When you reach it, repeat the process.
Whatever you use, make sure the object you pick is between yourself and the place you are heading. This way, when you arrive at that object, you know that you are heading in the right direction to get to your destination. Then you can repeat the process to get closer to your target.
Another hint – don’t pick a “landmark” which is ambiguous. For example, from a long way off, you may see one tree that looks a lot taller than the other ones. When you get up close, all the trees are tall, and it’s hard to pick which one was the tallest one. Similarly, from a long way off, that rock may seem to have a lot more moss on in that the others around it. However, when you get up close, all the rocks have moss on them, in different shades of green, and you can’t quite pick the rock you were aiming for.
Every now and then, take a back bearing to make sure you are in the right direction from where you came. To do this, simply turn the whole compass (leave the dial alone) until the white or black half of the compass needle is above the red arrow on the base of the dial. If you look along the Direction Of Travel arrow, you should see the place or feature that you came from. If not, move yourself so that you can!
Written by Peter Shave