The Compass & Some Navigational Basics
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Before jumping into how to use a compass, we should first explain some of the verbiage invovled.
The Compass
​First up is the compass itself. Understand that these articles are only in reference to lensatic compasses or other compasses that allow for accurate sighting and directional measurements. They are not for the handy compass on your wristwatch or the cheap, flat compasses from your local gas station, which only give a general idea of direction and are completely inadequate for actual navigation. If your compass doesn’t possess the same basic features as this one, it's likely not up to the task. Do your own research before relying on any given type or style of compass in the field.
For this article, we will be using a Cammenga Model 3H tritium-illuminated lensatic compass. This is the exact compass currently issued to the US military and, with the exception of much more expensive models, probably among the highest quality lensatic compasses on today's market. It is accurate, waterproof, made of solid aluminum and, short of hitting it with a hammer, virtually indestructible with a closed cover. Beware, this compass is also widely imitated by a deluge of cheap, Chinese lookalikes. Don’t be fooled. While slightly better than the gas station compasses, they are nowhere near the quality of the original, American-made model. If you are using a different style compass of equal quality, all the principles in these articles still apply, although the fine details of their execution may require slight adjustment.
With that knowledge, we can move on to some compass anatomy.
From right to left in the image we have:
Thumb Loop: This aids in holding the compass level when sighting off an object.
Lens: For reading the azimuth markings during use without having to move the compass. The lens bracket folds forward over the dial for storage, simultaneously locking the otherwise free-floating dial in place, to prevent damage in the event of a sharp impact.
Sighting Slot: Used in conjunction with the sight wire in the cover to sight off an object.
Dial: Certainly the most familiar part of the compass, this contains the north arrow and the azimuth markings in both degrees (red) and mils (black).
Index Line: The immovable, black line etched into the glass, under the bezel, that indicates the azimuth.
Bezel & Bezel Indicator Line: The bezel is grooved on the outside and rotates in three-degree increments. The white bezel indicator line is etched into the glass of the bezel and is used to align with the north arrow in a number of measurement applications. It also glows green in the dark, for use at night or in poor lighting.
Sight Wire & and Luminous Inserts: The sight wire is used for sighting off objects in conjunction with the sighting slot. The two luminous inserts, directly above and below the sight wire, are used to help orient the compass and aid in finding the sight wire in bad lighting situations.
Scale Edge: The flat edge of the compass is marked in meters and allows distances to be measured on a 1:50,000 scale topographic map. It is marked up to 6000 meters with 3000 meters at the point where the compass folds. This also provides a straight edge for drawing on a map. The scale edge can measure distances on maps of other scales, though a conversion factor would have to be used.
​​Some Navigational Nomenclature:
Azimuth: Like length, mass or volume, 'azimuth' is the term for the measurement that all compasses are actually measuring. It is a measurement of angle in a clockwise, 360 degree arc along the horizon. North is set at 0 degrees and South at 180 degrees. Technically, 0 and 360 degrees are both north. However, just as the number twelve on a clock also takes the place of zero, zero on the compass dial is the same as 360, so markings only read up to 359 degrees. Hence, you will never actually read an azimuth of "360 degrees" just like you never read a time of "zero o'clock".
Bearing: Very similar to azimuth (and the two words are often used almost interchangeably) this is the number of degrees to a destination. For instance, if you measure the azimuth to a destination that is 130 degrees, you might then say that to get there one must set a course at a bearing of 130 degrees.
Magnetic North & True North: Remember when we said that most people would be incorrect that a compass needle points due north? Well, a compass needle only actually points to what is called 'Magnetic North'. Magnetic North is the direction to the north pole of the earth's magnetic field. Here's the issue--'Magnetic North' is not located at the actual, geographic North Pole--the point on the globe that the earth actually spins around. The direction from any spot on earth to the geographic North Pole is what is called 'True North' and is what is actually being referenced by the phrase 'due north'. True North and, its counterpart, True South are what the lines of longitude on a globe run between. Magnetic North also is continuously moving, which makes this distinction all the more important. In the image below, you can see the historical location of magnetic north since 1900. So, if some little boy in northern Alaska decided to go find Santa at the north pole in the year 1900, and had simply trusted that his compass was taking him north, he actually would have been heading almost due east to Canada!
Declination: Now that we know the difference between True North and Magnetic North, we come to the solution. Declination is the measurement of the amount of error a compass will have at any given point on the earth. Declination values vary depending on location and will be marked on any topographic map of your area of interest. If you want to know the declination in your area, but don’t have a map, declination values can also be found online from a variety of sources. For more information see our article on How to read a Topographic map.