Maps/Navigation #4, 11/26/99: Orientation, Part 4, Using Compasses with Maps

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"Jesus, Savior, pilot me
Over life's tempestuous sea;
Unknown waves before me roll,
Hiding rock and treach'rous shoal;
Chart and compass came from Thee;
Jesus, Savior, pilot me."—Edward Hopper

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

Orientation Part 4

(Maps/Navigation #4, November 26, 1999)

I'd like to remind you of a very important compass fact that I neglected to mention last time. Compasses are magnetic! Not so earth-shattering, right? Well it can ruin your whole day if you forget and put your compass in the same pocket with a computer disk, a credit card, a bank card, a magnetic ID card; near a camera or light meter, a television set or a computer monitor; etc., etc., etc. Not only can a compass severely damage passive magnetic media; it can be damaged by active electronic devices that produce strong magnetic fields. Effects are totally unpredictable—take care!

Here are some definitions that will make it easier to communicate:

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.
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 0º on the azimuth circle as a lubber's line. On sighting compasses the sight centerline is the lubber's line.
magnetic declination: easterly or westerly angular difference between the direction to the earth's geometric and magnetic poles.
magnetic inclination: the vertical component of the earth's magnetic field which causes compasses needles to dip.
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.
orienteering compass: an adjustable compass with special features that make it more convenient to use in the field with maps.
orienting lines: visible lines in the capsule engraved parallel to the boxing mark; used as reference lines when marking angles on the map with an orienting compass.
representative fraction (RF): a numerical indication of the relationship between distances on the map and distances on the ground. On a map with an RF of 1/24,000, one inch on the map represents 24,000 inches (2,000 feet) on the ground.
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.
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.

Using Compasses with Maps

Map and compass, a powerful combination

Compasses work best with maps. The main tasks are to orient the map correctly with the terrain, and to transfer bearings, courses, and locations to and from the map and compass. The security of having a clear picture of where you are turns map and compass into a powerful navigational system. Coupled with GPS and visible markers it's an unbeatable combination for a hiker/explorer.

Maximum navigation system

	Magnetic Errors Magnetic errors cause inaccuracy in magnetic compass readings. There are several important magnetic errors to be constantly aware of in the field:
Magnetic Declination	Magnetic declination is an easterly or westerly error caused by the difference between the locations of the earth's north and apparent magnetic north poles as seen from your location. Since this error is primarily a difference between true direction and magnetic direction, it is only important when mixing the two, as when transferring angular dimensions between map and field. Declination is fairly regular, predictable, and easy to correct.
Annual Magnetic Variations	Annual magnetic variations are periodic magnetic errors peculiar to geographic locales. They may result from the difference between the spin rate of the earth's crust and that of its molten core. They change fairly regularly over time. Annual magnetic variation behaves like magnetic declination and can be corrected in the same way, at the same time.
Magnetic Inclination	As one nears a magnetic pole the lines of magnetic force become more vertical. This causes a compass needle to dip at one end. If inclination becomes pronounced enough, the needle will drag inside its bearing or on the inside top or bottom of the capsule, reducing its reliability. This is not normally a problem in the United States, and it's easy to correct as long as it's mild: just hold the compass at a slight angle so the needle is free to swing. Compass manufacturers have special versions of their compasses for use in areas of more extreme inclination. Models with special inclination-proof bearings are also available—at greater cost, of course.
Local Magnetic Effects Cars are magnetic, too.	Local magnetic effects, such as deposits of natural magnetic minerals, masses of buried metal, underground pipes, electrical wires, nearby magnetic objects like cars, sheds, or tanks, and many others, even objects in your pockets, are completely unpredictable in their effects and do not respond to simple correction. Their effects on your compass can be unnoticeable to severe. The only protection against local magnetic effects is being informed or warned of them and being very observant of your compass behavior. The only correction is to move away from the effected area. After spending some time using magnetic compasses I've become convinced that local magnetic effects are the most subtle of the errors. I'm not alone in this; the U. S. Army cautions its soldiers to keep these distances from several common local magnetic effects when using compasses: power lines 20 meters cars or trucks 20 meters barbed wire or phone wires 10 meters rifle or shotgun 1 meter Add to that one of my own: concrete reinforcing rod 2 meters

Where does the map say I am?
(Orienting Your Map with a Compass)

Orienting a map is aligning it with the terrain so map objects appear in the same relative positions as the landmarks they represent; in other words, pointing the map's north grid lines toward true north. The map's basic reference is grid north and yours is net magnetic north. The difference between the two can be ignored when working with either map or compass; but it must be considered when using map and compass together; as when transferring bearings, courses, or fixes from map and compass. Orienting your map corrects these errors.

To correct net magnetic error determine the direction and amount of the error and apply a correction, or offset, to all magnetic bearings when transferring them to or from a map:
(1) Declination error is the difference between true and net magnetic norths on the declination diagram in the map margin.
(2) Error direction is the direction from true north to magnetic on the map declination diagram. Easterly is clockwise, and can be thought of as + error; westerly is counterclockwise, a - error.
(3) Adjust the final offset with annual variation if required. Multiply any annual variation given in the map margin notes by the number of years since the release of the map and add it to the declination for the final value.
(4) To make the correction, add the magnetic declination to the observed bearing when converting magnetic bearings to true. Add an equal offset in the opposite direction when converting from true bearings to magnetic. Here's an example:

On my 1953 map of Laguna Peak, NM (revised in 1979), magnetic north is 12-1/2° east of true north. Annual magnetic variation is not mentioned in the margin notes, so it can be ignored. So magnetic error is 12-1/2° easterly (clockwise, or +). To correct the easterly error, offset it by adding the error to any bearings transferred from the field to the map (magnetic to true), or by adding an equal westerly (clockwise, or -) to bearings transferred from the map to the field (true to magnetic).

To correctly transfer our Dog Rock and Bare Light compass bearings to our map, we'd get:

Dog Rock bears 270° M
              + 12-1/2° (easterly correction)
Dog Rock bears 282-1/2° T,



Bare Light bears 195° M
                + 12-1/2° (easterly correction)
Bare Light bears 207-1/2° T.

Similarly, when looking for Dog Rock and Bare Light using map information:

Dog Rock bears 282-1/2° T
            + (-12-1/2°) (westerly correction)
Dog Rock bears 270° M,



Bare Light bears 207-1/2° T
              + (-12-1/2°) (westerly correction)
Bare Light bears 195° M.

But what if your map doesn't have magnetic declination information included in its margin notes?

Measuring Declination Directly

This direct method is described by Chris Goulet, an experienced Canadian adventurer, and has the advantages of always providing up-to-date declination data and accounting for all magnetic errors simultaneously:

Locate your position on your map. You may use direct observation or position yourself at an unambiguous landmark with a corresponding map object;
Locate another unambiguous landmark and its corresponding map object;
Measure the compass (magnetic) bearing to the landmark. Write the bearing down;
Measure the map (true) bearing to the landmark using your compass as a protractor. Write the bearing down;
The difference between the true and the magnetic bearing is the magnetic declination at your location.

Increase the reliability of your measurement by using multiple landmarks and averaging your results, or by using one landmark and changing your location a measured amount to take another bearing. Again, average your results.
Another interesting method suggested by Mr. Goulet is to use Polaris instead of a landmark/map object. This does not require you to know your exact location on the map, and that's good. It does require you to be able to take accurate magnetic bearings on Polaris, maybe in the dark, and that's—well—interesting.

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