Academically and on paper, navigation is basic high school geometry. It's all about distance and direction. Let's look at direction first.
What is direction?
Put simply, direction is simply a system of reference that describes which way something lies relative to some point - often yourself. Basically, how to subdivide a circle relative to a particular point. The typical directions we are familiar with range from 0-360° (degrees). Why 360? Well, it's convenient. 360 can be conveniently divided into multiples of both 2 and 3 - which are fractions humans can easily deal with compared to, say, 5ths and 7ths. North (0/360°), East (90°), South (180°), and West (270°) are the Cardinal points. These conveniently lie at right angles, so if we are facing north and hold our right arm at right angles to the way we are facing, our arm will point East. We now have a relatively precise way of describing the bearing of objects relative to another object. Directions can be further subdivided. For those familiar with nautical and wind directions fractions of directions such as North-East (45°) and North-North-East (22.5°) are an intuitive means of describing where an object lies relative to an azimuth or defined direction - in most cases, 'North'.
As an aside... there is more than one method of quantifying direction. There are the more familiar 'degrees' described above which range from 0-360. NATO military compasses use 'mils' which range from 0-6400. The 'Mils' system is based on Radians. There are 2π Radians in a circle. So technically there are 6283 mils in a circle, but this gets modified to 6400 for convenience except for the Swedish armed forces which round it to a more exact, but slightly more annoying 6300 and former Warsaw Pact countries which round down to an anticlockwise 6000 mils - so beware of buying old Soviet-era compasses. Surveyors and other militaries may use gradians. I have a soft spot for Gradians. They are the metric form of Degrees - there are 400 Gradians in a circle. The mils system has the useful property that 1 mil subtends 1 meter at 1 kilometer. Useful when calling in direct (machine guns) or indirect (mortar, artillery) fire. Hey, some fellow hikers can be annoying. You just never know when this information can be useful… For civilian uses, degrees are the norm.
Count your compass as one of your best friends. Some people claim to have some sort of inbuilt direction finder. From observation, the error of these internal compasses can be anywhere from 0 to 180 degrees either side of the intended direction. This is not to say that folks don't innately know the relative direction, but this is invariably due to observation and awareness of the surroundings. So what is a compass? We are pretty fortunate living on Earth. This planet has a molten core which is inherently magnetic and generates a magnetic field around the planet.. This means that any sliver of metal that is magnetic will align along the direction lines, towards the magnetic 'north'. To any prospective interplanetary astronauts out there, leave your compass at home if travelling to Venus. That planet has no magnetic field. The field of Mars is a bit iffy too.
Aside from that, a compass should always be carried.
There are a few common types of compass. First are baseplate compasses such as those manufactured by Suunto and Silva. They are light, come in a range of prices, and relatively easy to use by scouts and competitive orienteers alike. They generally include some form of protractor, so are an all-in-one navigation tool. I use a cheap lower range baseplate as a backup - it is totally adequate. Indeed I also use them underwater to >50m depth (but if you are going to do this, make sure you get one with a black or red and white needle - you can't distinguish black/red needles underwater!). If I had more money, I would get a more expensive nicely balanced model such as the Suunto M3 Global or the Silva Ranger or 54 models with a prism for taking bearings. To those folks at Suunto and Silva who want a rigorous comparison with free gifts included, you know how to contact me...
A second type in common use is the prismatic or lensatic compass - often based on a military model. These are heavier, but better for taking bearings. There are cheaper forms of the military prismatic model which tend to be less robust with a foldable lubber line on a magnifying glass, but I haven't seen many without considerable sideways movement on the sighting mechanism, which introduces error. While I personally use a prismatic with Suunto backup, a good baseplate-type by itself will suffice.
Anatomy of a compass
The baseplate is a cool multitool. Aside from its obvious job of holding the needle mount, it serves a number of roles. The rulers in cm and inches can be used to measure distances between points to be read off the map scale. Scale marks calibrated in 1:25000, 1:50000, and 1:63360 scales can be used to directly measure shorter distances, and provide a convenient scale for measuring grid references. The direction of travel arrow indicates the direction to move when your compass is correctly aligned. There is also a magnifying glass and a stencil hole to mark features on the map. Protect the baseplate. I keep it in a cut-off sock with the lanyard poking out of the toe end.
The compass needle is luminous. Make sure you know which end points North! Generally it is red, with white as South. Some models however have red and black. The rotating bezel is marked in degrees, and used to either measure or set bearings. The index line or triangle is where you read the direction. The base bars indicate where the compass needle should lie when the compass is correctly aligned North-South, and the orienting lines help align the compass on the map grid lines. There is also a correction scale to correct for magnetic declination. I never use it as I prefer to do the correction in my head, and my Nav Data Sheet already has the correction. It keeps things simpler.
Sighting or prismatic compasses have a magnifying prism through which you can align the feature and read the bearing at the same time. Mirror compasses work on a similar principle, replacing a mirror and reverse-text degree markings for the prism. I strongly favor sighting compasses. They are more precise than a simple baseplate for taking bearings, and it is easier to check that the needle is floating freely when taking the bearing. That being said, good technique is key.
Taking a bearing to a feature
The procedure is simple. Getting it right takes a little thought and practice. First, identify the feature you want to take a bearing to. Place yourself in a steady position, say kneeling or against a convenient tree. For a baseplate compass, position your head over the centre of the compass needle to avoid parallax (sideways) error, and rotate yourself with the compass until the direction of travel points at the feature. You will need to look along the line of the direction of travel to the feature a few times to make sure it is lined up correctly. Make sure the needle is floating freely! If the compass is not level, the needle can stick on the base. Make sure also you at least 1/2 m away from metallic objects such as rifle barrels for you hunters. Normal power lines can exert an effect to about 10m, and high tension power lines and transformers can exert an effect to around 55m. Beware of wire fences, particularly electric ones. Rotate the bezel until the base bars are perfectly aligned with the compass needle. When you are happy that everything is aligned correctly, you can now read the compass (ie. magnetic) bearing from the index line.
For prismatic compasses, the same principles apply however reading the bearing is more precise. Individual brands may vary, so check the instruction manual, But basically you have a sighting line and a peep sight. Once your target, the sight line, and the peep sight are aligned, read the bearing through the prism. A word of caution... Many compass cards show both the bearing and the back bearing. Make sure you are reading the correct one!
Hang on, what's a back bearing?
In a nutshell, if a bearing is the direction to an object then the back bearing is the direction from the object to you. Calculating it in degrees is simple. If the bearing is less than 180°, then add 180 to the bearing to get the back bearing. For example if the bearing is 45°, the back bearing is 180+45=225°. If the bearing is greater than 180°, then subtract the bearing from 360. For example if the bearing is 275° then the back bearing is 360-275=85°. If you are working in mils, then substitute 360° for 6400 mils, and 180° for 3200 mils.
Transferring a compass bearing to a map
So we have a bearing to a feature from where ever we are (we may or may not know where we are!). How do we draw this on a map? This is a core skill in taking a resection. First, we have to convert the magnetic bearing to a grid bearing. If the declination is to the west, then we subtract the declination from the magnetic. Intuitively, if the declination is to the left, then we start measuring from the left of north grid. The opposite applies if the declination is to the east.
To make life easy some form of Protractor or Romer is necessary to measure and plot angles. Baseplate compasses contain their own Romer. Custom-made Romers which measure angles and have gradations to measure distances in map grid squares are an invaluable addition to your navigation arsenal. If you can't get hold of one, then a simple $2 plastic school protractor and short ruler will do the job just as well.
It's easiest to look at an example. We have set off in kind of a northerly direction in Vall d'Incles in Andorra, as one does. We take a bearing to Alt de Jucla. The Magnetic bearing is 31°. Now, we convert to Grid. A brief study of the Declination arrows in the legend indicates two things: We need to do a quick course in Catalan; and the Andorran cartographers have not made it easy. Geez... give them a Principality, and they decide to do their own thing with maps too. Most other maps explicitly provide the Grid-Magnetic angle.
The deviations (Grid and Magnetic) on this particular map are separately referred to True North. The True-Grid deviation is 0° 31' 12" W. True-Magnetic is 0° 25' 29" E as of 2011. In 2016 this will add 3x(0° 7' 25"E), so the total magnetic deviation is 0° 25' 29" E of True North. Fortunately the cartographers provide us with a wee diagram to show us that if we add the two values we get the Grid to Magnetic variation: 0° 31' 12" + 0° 25' 29" = 0° 56' 41" W. Let's make it easy and round it to 1°W ie. we subtract 1 degree. Given our compass is in 2 degree increments, it's academic whether we need to worry too much, but we'll go through the motions: 31°- 1° = 30°. We calculate the back bearing: 180+30=210°. Sketch a line along this bearing, and hopefully we should be somewhere along this line. Do this for 2 other points and we have a resection. In this particular case, a casual glance at the carpark sign next to us might be a bit of a clue as to our position.
If you are using a baseplate compass, you can use it in place of a protractor. Set the bezel to the grid bearing (31°), align the orienting lines or base bars parallel to the Eastings (the up and down lines), and move the compass up or down until the edge of the baseplate crosses the spot height (Alt de Jucla). Surprise, surprise - the line crosses our carpark. Note the map doesn't actually have to be oriented North-South, but it's a good habit to get into.
Setting a bearing from a map
Let's reverse the procedure. From our position in the carpark, we decide to head to Spot Height 2517, approximately South. Ultimately we want to walk the ridge line to Spot Height 2568. Using the protractor or the compass baseplate and the map, the bearing from the carpark is Grid 187°. To convert Grid to Magnetic, we add our 1° from above. So we set our compass to 188° and off we go. Easy, huh?
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