Maps/Navigation #3, 6/13/99: Orientation, Part 3, Using Magnetic Compasses

This is Part Three of our series on orientation—basic terrestrial navigation. In Part One I briefly described GPS receivers, their use, and limitations. Last time I introduced basic concepts of orientation and went into detail on using maps and map grid systems. If you missed either part, see "Back Issues" in our library.

Answers to Part Two Assignment

I concluded Part Two with an assignment: contact the USGS and order:

a map catalog of your state
USGS map symbols brochure
USGS map "Los Indios Canyon, NM"; #36106-F8-TF-024.

Then I asked you to use the Los Indios Canyon map and note:

What is located at 13S 326400 4046300?
What is the approximate elevation of that object?
What is the terrain like at that point?

If you answered:

An oil well is at 13S 326400 4046300.
The approximate elevation of the well site is 7849 ft.
The terrain at that point is very hilly with a moderate slope. A road climbs the hill to the well.

...you're correct. Congratulations!

What's at 13S 326400 4046300

If you participated in the assignment and plotted the location correctly you've learned several valuable things:

how to order catalogs and maps from the USGS;
how to find a map object from its UTM coordinates;
how to use contour lines to previsualize terrain;
how interesting maps can be.

If you didn't participate—oh well!

Orientation, Part 3

Using Magnetic Compasses

(Maps/Navigation #3, August 7, 1999)

In my article on GPS I mentioned GPS manufacturers' disclaimers that warn us never to depend entirely on one navigation system, but always to have a backup navigation system available. In orientation that backup system will almost always be a magnetic compass and map. Magnetic compasses are easy to use, very reliable, versatile, and cheap enough for you to own several different kinds. I routinely use a magnetic compass to take fixes on landmarks to back up my GPS readings. I've even used them to take fixes on distant objects I couldn't approach directly for a GPS fix, or which weren't even on the map I was using.

backup navigation system

Before I get into details, here are some definitions that will make it easier to communicate:

Orientation Glossary

adjustable compass: a compass whose azimuth circle can be rotated relative to the lubber's line.
azimuth circle: a circular compass scale graduated in angular units: degrees, cardinal points, or other units, usually clockwise from north or 0°.
bearing: the angular direction to a landmark.
boxing mark: a box, arrow, line, or other mark permanently fixed to point to the N or 0° index on a compass azimuth circle. The boxing mark is usually part of the capsule. To "box" a compass, align the compass needle or card with the boxing mark, north-to-north, or with N or 0° on the azimuth circle.
capsule: a sealed transparent case which houses the compass needle, the azimuth circle, and the boxing mark. The capsule may be filled with liquid to damp needle or card swinging.
card: an azimuth circle mounted on a compass needle. The card rotates relative to the lubber's line.
cardinal compass points: on a compass azimuth circle, the indications N, E, S, and W, and the intermediate points NE, SE, SW, and NW. These eight points divide the azimuth circle into coarse 45º segments.
course: angular direction of travel.
deduced reckoning ("dead" or "ded" reckoning): an approximate navigation method which integrates bearings with estimated speed over the ground to produce approximate fixes. In my use of the term I simply mean fixes made by direct observation of the environment.
fix: the position indicated by the intersection of two or more lines of bearing.
landmark: a recognizable real object in the terrain.
lubber's line: a line or mark on the compass body that points toward the direction of travel. Simple compasses may use north or 0° on the azimuth circle as a lubber's line. On sighting compasses, the sight centerline is the lubber's line.
map object: a picture or symbol used on a map to represent a landmark or other object.
needle: a magnetized pointer resting on a pivot in the capsule, free to rotate relative to the lubber's line and azimuth circle;
north: grid north: the direction to the earth's geometric north pole along a meridian; indicated on maps by the vertical grid lines;; magnetic north: the apparent direction to the earth's magnetic north pole, not usually indicated by a grid on maps. This net magnetic north comprises all the magnetic effects acting on your compass;; true north: the direction to the earth's geographic north pole, indicated in life approximately by Polaris; indicated on maps approximately by the vertical grid lines.
resolution: the precision of an indication possible on an instrument scale. A compass card with 2º markings has greater resolution than one with 5º markings.
rose: an azimuth circle marked in cardinal compass points.
sighting compass: a compass with a mirror or peep sight aligned with the lubber's line.
simple compass: a compass with the lubber's line fixed at north or 0°, and a fixed azimuth circle.

	Magnetic Compasses, In General There are two types of magnetic compass: needle- and card-type. Both are available in simple or adjustable models. In either type the north-seeking end of the needle or the north mark on the card points to net magnetic north.

Basic Compass Features	All compasses have some common features: a needle or card that points northward; an azimuth circle, either fixed beneath the moving needle or printed on the moving card. A transparent capsule which holds the needle or card and the azimuth circle; a "boxing" mark on the capsule used to align the needle or card with the azimuth circle north or 0° mark.
Advanced Features	Some compasses have extra features to make them more convenient for navigation: a compass body marked with a lubber's line; a way to rotate the boxing mark and azimuth circle relative to the lubber's line. On needle compasses it may be an adjustable capsule or boxing needle; on card models, a moving bezel with printed boxing mark. A liquid filled capsule to damp needle or card swings. It makes using the compass much easier and more certain. Declination adjustments allow precise correction for global magnetic errors. This feature is a must in a car or boat compass, a helpful addition to a hand compass. A sighting device to make selecting landmarks more precise. Compasses with a front and rear peep sight or a mirror sight allow resolving bearings to a degree or so. With care, any hand held compass can resolve bearings of ±2º or 3º. Luminous dial marks are fun, but trying to walk a compass course in the dark is too dangerous for me. I'd rather sleep at night. If you must have a luminous dial, try to find one with tritium luminescence. It costs more, but it's by far the best kind.

	Using Compasses Without Maps Compasses work best when used with maps, but they are useful even without maps. You can do three important things with a compass alone: determine the bearing to a landmark; walk a course to a landmark; determine your location relative to landmarks. Simple Compasses Simple compasses have limited features, but they do what's important—indicate magnetic direction. A simple compass has a fixed lubber's line and azimuth circle with at least a rose with the cardinal points marked; additional intermediate marks are better; an azimuth circle or card marked in degrees gives the best direction resolution, but it may be harder to read. 5° or 10° resolution is OK.
	Where is that landmark? Where do I want to go? (Determining a Magnetic Bearing; Walking a Magnetic Course)
Working to a number	There are two ways to use a compass to determine the bearing of a landmark or to walk a course: (1) working to a number, and (2) working to a point. They work equally well; the one you use depends on the type of compass you use and your own personal preferences and skill. (1) To work to a number: "box" the needle (line it up north to north with the lubber's line or azimuth circle N or 0° mark) The number on the azimuth circle or compass card that points toward the landmark is its bearing. (2) To work to a point: point the lubber's line or boxing mark toward the landmark. Use a card's bearing indication directly; subtract the bearing indicated by the north end of the needle from 360. Align the needle with the number; walk in the direction indicated by the lubber's line or N or 0° on the azimuth circle. I use whichever method lets me hold the compass comfortably.	Working to a point Card compass Needle compass
	To Determine a Landmark's Magnetic Bearing with a Simple Compass
	(1) Stand directly facing the landmark. (2) Hold your compass directly in front of your body. ...then For Needle Compasses: (3) Look down at the compass and "box" the needle (turn the compass until the needle points to N or 0° on the azimuth circle). (4) The number on the azimuth circle that points directly toward the landmark indicates its magnetic bearing. ...OR (3) Point the lubber's line toward the landmark; (4) Subtract the needle indication from 360 to obtain the landmark bearing. For Card Compasses: (3) Look down at the compass and point the lubber's line directly toward the landmark. (4)The number on the card indicated by the lubber's line is the landmark's magnetic bearing.
	Communicating Bearings and Courses to Others There is often a great difference between magnetic direction and true; remember to express that clearly and consistently when making notes or telling bearings or courses to other people. Always express bearings or courses with three digits and indicate "magnetic" or "true."
	Useful direction notation The user, noting your designation, will know whether or not to apply magnetic error corrections when marking his or her map. To Walk A Course with a Simple Compass
	(1) Hold your compass directly in front of your body. ...then For Needle Compasses: (2) Look down at the compass and turn it until the course you want to walk points directly forward. (3) Hold your compass steady and turn your whole body to box the needle. For Card Compasses: (2) Look down at the compass and turn it until the lubber's line points directly forward. (3) Hold the compass steady and turn your whole body until the course on the card aligns with the lubber's line. ...then (4) Look up. Find a landmark directly in front of you as far away as possible. Walk toward the landmark without referring to your compass. When you arrive at the landmark, repeat this process for a new landmark.
	Where am I on the earth? (Making a Fix) To demonstrate, let's use the bearings to Dog Rock and Bare Light we used in Part Two: "Dog Rock bears 270°; Bare Light bears 195°." Let's assume magnetic direction. There is only one place where you can obtain those two bearings: the intersection of the lines of bearing for Dog Rock and Bare Light. That point is the fix for your location. Making a fix is simply determining the bearings on two or more landmarks that you can see at the same time. For better fixes, choose three or more prominent landmarks at least 30° apart.
	To Return to a Fixed Location Relocating the intersection of several lines of bearing sometimes seems complex in the field, but if you handle it in an organized way it can be fairly simple. The method works with either direct magnetic fixes, from landmarks, or with fixes derived from map objects. Let's use Dog Rock and Bare Light again:
	(1) Move to a location where Dog Rock bears 270° magnetic (A, below). Mark your starting point with something visible. (2) Move along that line of bearing (either course 270° M or 090° M) until you can see Bare Light. Place a visible marker (B, below). (3) Continue moving along your course until Bare Light bears 195° M (C, below). Place a visible marker—a flag on land, a buoy on water. (4) Double check both bearings. If they're correct, that point is your fix. Adjust your last marker.
Making a visible marker Placing a marker	Using Visible Markers Unless I am hiding something or trying to find something hidden by someone else, there is no logical reason for me to work without visible markers. For about $2.50 I bought a large roll of fluorescent engineering tape designed to make very visible terrain markers (crèpe paper ribbon is OK, too; and it's bio-degradable). As I take bearings, walk courses, or fix landmarks, I mark important points with the tape so I can see them easily from a distance. I use a ball-point to write ID info on the tape as a reminder. Life becomes so much easier when I can see what I'm looking for from a distance. To protect the environment, on my way out I retrieve markers I won't use again. I frequently stash 2-liter soda bottles of drinking water as markers along a hike. I flag them with a long ribbon of international orange engineering tape and hang them from trees or bushes along the way. If someone else sees them, OK; they may be thirsty, too.	Why make water hard to find?
	Simple Compasses, Summary A simple compass will do everything a compass is supposed to do: show you the direction to a landmark (determine a bearing); show you where you want to go (walk a course); show you where you are (take a fix); orient the map to match your surroundings (more about that later); help you find your position on a map (also later).
		A typical boat compass
	Auto and boat compasses work the same way; they're simple card compasses with magnetic compensating provisions. When calibrated properly the lubber's line on these compasses coincides with the vehicle centerline, and the compass indicates your course. Determining a landmark's bearing can be done with a car or boat compass, but it is difficult except on flat land or open sea.

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