Compass
There are generally four types of compasses:
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Air compass or standard compass. This is an open air compass where the magnetic needle floats on a point in air. Because it's in air, the needle can take a long time to settle into position.
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Induction dampened compass. This compass uses electromagnetic movement to help dampen the motion of the needle so it settles into position faster.
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Liquid filled compass. This compass is filled with a liquid and the liquid dampens the motion of the needle so it settles into position faster.
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Electronic or Solid State. This is like what would be in a smartphone where the device contains a magnetic sensing chip.
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Commonly used liquid-filled compasses:
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Care of your compass.... Most compasses are rugid devices, but there are a few things you need to do to make sure it stays in good condition: 1) Do not hit/bump your compass. 2) Do not spin your compass by the lanyard. 3) Do not bring your compass near a stong magnet (like mag-mount antennas). 4) Do not leave a liquid filled compass in a hot car. 5) Do not store your compass near metal for extended periods of time. Prior to orienteering events, there's usually a compass check station that's setup at a known Azimuth to allow you to check your compass' accuracy.
Simple or Zipper Compass
The most basic, useful for general direction finding. Usually in 5° or larger increments. Not useful for orienteering. Least accurate.
Thumb Compasses
Useful for sport orienteering. Made to wear on a runner/hiker’s thumb for fast use on maps without wasting time. Usually no more than 2 degree increments.
Baseplate Compass
Usually in two degree increments. Especially useful with maps and plotting courses because it has orienting lines and a clear base to see the map. Good for most uses (including field navigation) except professional use.
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Mirror Compass
Usually 2 degree increments. Can often be used as a baseplate compass as well as in the field. Features a mirror and sighting marks.
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Sighting Compass
Usually 1 degree increments. Can be used professionally. Used when you usually have little to no landmarks to navigate by and rely on compass accuracy.
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Lensatic Compass
Usually 1 or 2 degree increments. Main selling point is they’re tough and used by military. Sighting wire for the front sight and a rear lens for the rear sight fold out to help shoot bearings.
Prismatic Compass
Most sophisticated but primarily used for surveying. Similar looking to Lensatic. Can be held by hand or mounted on a tripod. Compass in degrees or mils.
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The following discussion assumes the use of a mirror compass (above image, 2nd from left) such as the Suunto MC2.
​​​Orienting the compass
A compass is just a fancy protractor. It can be used on maps that use true north, or it can be used in the field using magnetic north.
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A compass can be rotated in any direction which allows you to align or orient 0° in different positions.​ Only one orientation is correct though; that is where the compass' 0° is oriented or aligned to the North. Orienting and aligning a compass to 0° is critical.
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When working on a map you would orient the compass' 0 ° with True North and you only use the orienting lines, not the orienting arrow. Unless you're aligning the map to real-world magnetic North, you don't use the magnetic needle or orienting arrow when working on a map.
When working with a map, you orient the compass by rotating the compass so North (0°) is in the same direction as the map's True North (TN or ★) and the orienting lines of the compass are perfectly parallel with lines of longitude (which are themselves aligned with True North). This will make the compass 0° match with the map 0° (True North).
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Note: If Grid North and True North are very close (less than 1°), you might be able to get away with using the grid lines for aligning the compass' orienting lines to the map.
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When do you rotate the whole compass vs. just rotating the bezel to align the compass with North?
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If you know the correct azimuth ahead of time, set the azimuth by rotating the bezel first so the azimuth is under the index line. Then, once set, you would rotate the entire compass while lying it on the map to orient the compass with North.
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If you want to determine an azimuth, then you would first align the compass edge on the map between two points (mind the direction of travel arrow!) then rotate the bezel until the compass' 0° is pointing True North and the orienting lines are parallel to the lines of longitude. Then you would read the azimuth under the index line.
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In the field, you would orient the compass 0° using the compass magnetic needle that always points to Magnetic north. You do this by "Boxing the Needle" or putting "Red in the Shed." This will make the compass 0° match with magnetic North of the Earth (Magnetic North).
In summary:
Aligning the compass to a map.
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Mind your direction of travel arrow! If that's wrong, you'll be 180° off!
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Align the course of travel with a straight edge parallel to the direction of travel arrow (usually either of the two sides of the compass will work).
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Rotate the compass bezel so that the Orienting Lines (not arrow) are aligned with the lines of longitude and "N" points North. Sometimes, the UTM grid can be used as long as you're aware of the difference between Grid North and True North.
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Aligning the compass to magnetic north in the field.
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The north pointing side of the compass needle is normally painted red.
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“Boxing the needle” or “Put Red in the Shed” or “Put the dog in the doghouse” is when, as you’re pointing the “Direction of Travel Arrow” in the desired direction, you turn the compass housing so that the Orienting Arrow completely encompasses the red part of the compass needle. See picture to the right, the red outline around the red needle is the orienting arrow.
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“Parallax” is the error created when you do not look at the compass face straight-on and you see a skewed view of the orienting arrow and magnetic needle. This prevents you from perfectly boxing the arrow. Adjust your mirror angle to minimize parallax.
Parts of a mirror compass:
A. Baseplate- The transparent bottom flat part of the compass.
B. Capsule- Part of the compass that houses the needle. Usually liquid filled to help prevent the needle from swinging back and forth too much.
C. Declination Scale- Not on all compasses. This is used to correct the compass for magnetic declination. If your compass has this, set it to 0 if you intend to manually adjust for declination. Otherwise, always set it to the current declination first, before taking a bearing.
D. Dial- Also called Bezel or Housing. The rotating part of the compass that has the Azimuth angles listed on it.
E. Direction of Travel Arrow- The arrow on the baseplate that must point in the direction of desired travel.
F. Magnetic Needle- The magnetic part of the compass that aligns with the Earth’s magnetic field. Has a North (usually red) and South (usually white) part.
G. Inclinometer (clinometer)- Part of a compass to indicate the grade or slope of terrain.
H. Index Line- Also called lubber line. The part of the baseplate that aligns with the Direction of Travel Arrow that you use to read the compass dial once it’s oriented properly. I is the back azimuth index line for the azimuth dialed-into the Index line.
J. Orienting Arrow- The large arrow (usually red) on the compass housing that points north and is aligned with the compass needle.
K. Orienting Lines- The parallel lines on the housing of the compass. Used to align the compass with the map.
L. Romer (Roamer) Scales- Scales on the side of the compass to assist in measuring distances on a map. For direct use on USGS topo maps, these scales should be 1:24,000 ratio.
M. Sight- Like your gun sites, this is the aiming part of the compass to allow you to find objects at a desired bearing.
N. Sighting Line- On a mirror compass, this is the line that bisects the sighting mirror.
O. Slope (Grade)- A scale on the side of the mirror. On a topographic map of a matching scale (1:24,000) as indicated on the scale, lining up the map contour lines with the closest matching pattern will tell you how much that terrain slopes.
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Magnetic zone balancing- Magnetic lines around the earth are not uniform. At the north and south poles the magnetic flux lines are perpendicular to the surface of the earth. The needle of a compass must be balanced to read accurately. This is called Magnetic Inclination. There are 5 different balancing zones around the Earth. Balancing is done to prevent the needle from dragging on the compass face.
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Most Compasses are made specifically for certain regions and are balanced only for that region. Unless the compass is a global compass that is made to be balanced all over the globe, you cannot use accurately a compass outside of the region it’s made for. For instance, a compass made for North America will not be accurate in South America.
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Care of your compass
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Prior to use, ensure the compass is clean, the needle swings freely, there are no cracks in the liquid filled capsule, and the bezel rotates freely. Before you shoot a bearing in the field, hold your compass level and turn around to ensure the needle swings freely!
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NEVER expose your compass to chemicals like DEET! Here's why.
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Take care of your compass! Do not abuse it or store it near other magnets or metal.
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Do not swing your compass around or hit it against something. Do not store it in extremely hot or cold environments.
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Prior to competitions, if a field calibration check is available, take advantage of it!
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Use of a compass around metal will result in inaccurate readings if using the magnetic needle. In order to obtain accurate readings, stay at least 20' from metallic objects and power lines. If you are closer, the metal will throw your compass off. (This is my experience based on some data I've taken and will be happy to share. Some sources such as USMC recommend further!)
TIPS:
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1) In the field, setting the angle of the mirror so you look directly down upon the compass face is important. That's what allows you to see "Red in the Shed."
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The first image shows perfect mirror angle and on target. Red is in the shed. Notice the magnetic needle is in the middle of the orienting arrow (the shed) and there's even space all around it.
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The second image shows what you will see if the mirror angle needs to be adjusted. Holding still, adjust the mirror angle so that you have an equal amount of space around the needle.
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The third image shows incorrect azimuth and poor mirror angle.
2) Put "Red in the Shed" to head towards your destination at the set azimuth. Once you arrive, turn around and confirm you're on course by using your compass to look back at your starting point and put the black needle in the orienting arrow ("Let Black take you back"). This is called a reverse azimuth or back bearing.
3) If you're on hilly terrain, before you start, turn around and find a tall object (like a tree) behind you using the black needle in the shed. This is because you might not be able to see a ground starting point when you arrive at your destination in order to confirm with your back bearing (#2). When you arrive at your destination, use that object to confirm you're on course using the back bearing.
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4) On a long leg, you can stop mid way and re-shoot your bearing or back bearing. That's an advantage for having a mirror compass, you can "set it and forget it."
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5) Know your compass! What are the azimuth divisions; 1°, 2°? Set your course and double check!
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6) Double check your declination setting. You can do everything right and never find your target if your declination is incorrect.
7) When using a compass, think SOS: Set, Orient, Shoot/Sight.
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Set:
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On a map, SET the compass on the map so that the Direction of Travel arrow points in the direction you are traveling and the points (start and finish) are perfectly aligned with the edges of the compass.
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In the field, Dial into the compass under the top index mark the direction you want to travel.​
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Orient:​
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On a map, holding the compass steady on the points from the "SET" step, rotate the bezel so "N" points towards the North and the orienting lines are perfectly parallel to a nearby line of longitude (depending on your location, sometimes the UTM gridlines are close enough to True North that you can use them. Check the declination diagram. Do NOT use the Range and Township lines!)​
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In the field, keeping the compass self aligned (the mirror line bisects the compass pivot point), rotate your whole body so that Red is In The Shed.
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Shoot/Site:​
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On a map, read the azimuth that is aligned with the top index mark.​
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In the field, making sure your "Orient" step is still good, look through the siting whole at the bottom of the mirror and find a distant object that falls in the groove. Note that there's a groove on the top of the mirror too if you need to site something higher. Do NOT walk the leg while looking through your compass. Sight an object on the desired azimuth and walk a straight line towards it. Close your mirror compass when walking so as not to flash the sun accidentally into other competitor's/hiker's eyes!
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