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Back Country Navigation for the Hunter
Compass and Direction Finding, page 2 of 2
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Compass Deviation
Electrical sources and metal objects with magnetic properties such as steel or iron can influence your compass; power lines in particular nearly always have a serious affect. (This influence on your compass is commonly referred to as deviation.) The chart below provides the minimum distance you need to be from any of these objects to avoid deviation issues.

Power Lines .................................................60 meters/yards

Vehicles..... ..................................................25 meters/yards

Wire Fences..................................................10 meters/yards

Rifle... ............................................................1 meter/yard

Direction Finding - Compass and GPS
Now that we’ve worked our way through the “Three Norths” and covered compass deviation, we can begin making some practical applications in handling compass and GPS in conjunction for direction finding.

Some compasses are “declination adjustable;” that is, you can set them according to the True-to-Magnetic or Grid-to-Magnetic declination in the area of your map. The compasses that have been recommended, however - the British prismatics, the lensatic M-27, and the Brunton Combi - are not declination adjustable. This is not a drawback or a shortcoming, it’s simply an element of their design and actually simplifies things when using them in conjunction with a GPS.

The key point is this: your GPS’s “North Reference” setting must always match your compass. Since our compasses are “magnetic only,” your GPS must be set on “Magnetic” for its “North Reference.” (Conversely, if you were working with a declination adjustable compass set for True North, you would want your GPS’s “North Reference” setting to be “True North.”)

I like to use examples in my training seminars, so let's use one here. You're on a Wyoming elk hunt and the UTM coordinates for your camp on Soda Fork in the Teton Wilderness are 12 05 74 073 E, 48 69 416 N. You've got an elk down in a meadow on a timbered bench at 12 5 73 118 E, 48 68 939 N, and you've plotted both on your topo map (see Figure 24). (This, by the way, is a good habit to get into; that is, plotting important points on your map as you go instead of just saving them in your GPS and letting it go at that - it can pay big dividends later.

Your bull is cleaned out and cooling and it's time to head for camp, which your GPS tells you is a little over a kilometer (about 6/10s of a mile) away on a bearing of 051 degrees magnetic. (See Figure 25.) Follow that bearing - 051 magnetic - for just over a kilometer and you'll reach camp. Here is where we start to use compass and GPS in conjunction, because how you cover that 6/10s of a mile on a magnetic heading of 051 degrees magnetic is very important.

First - and always - examine your map to check the situation along your planned line of travel from "Point A" to "Point B;" in this instance, from the point marked "Elk" to the point marked "Camp." Cliffs, steep draws, and big, boggy meadows are nice things to know about before you encounter them, especially in the dark. And if there are such obstacles, use your map, compass and GPS to navigate your way around them.

A GPS-only approach requires you to leave the unit on just about the entire way, your eyes fixed on the screen most of the time as you constantly adjust your direction of travel - your heading - to keep yourself on course. This simply isn’t practical and can easily be dangerous, especially at night. There’s a much better way: I like to call it the “Turn It On, Turn It Off” approach to GPS-compass direction finding.

You've turned your GPS on and determined your bearing back to camp - 051 degrees magnetic. Next you turned your GPS off and studied your planned route, as shown in Figure 25; no apparent roadblocks; you can pretty much "straight line" this one.

You've determined that the bearing to camp is 051 degrees, and that bearing will now become your heading/course. (Throughout the essay I'll refer to "course" and "heading" - both of which mean your direction of travel - as "heading/course." The reason for this is most GPS's refer to your direction of travel as your "heading," while a few call it "course." This way we know we're talking about the same thing.)

The next step is to put your heading/course of 051 degrees magnetic on your British prismatic, U.S. Military M27, or Brunton Combi compass, as follows:

British Prismatic Compasses

Open the compass lid and extend the lid until it is parallel with the main compass body, as shown in Figure 26.

Note that the index ring is graduated clockwise through 360 degrees. Every 10 degrees are numbered, as indicated, with a five-degree mark in between each numbered graduation. Between each five-degree mark, you can readily "eyeball" single graduations (single degrees); in addition, when the index ring clamp screw is backed off slightly from a tightened position, the index ring clicks audibly in graduations of 1.5 degrees.

Loosen the index ring clamp screw and turn the index ring to 51 degrees by setting the "51" along the index line (also called the "lubber's line"), then tighten the index ring clamp screw. Note that there is a short luminous line on the index ring's glass face and another short luminous line on the compass card's north marking. (Figure 27.)

With the compass held level at belt level, lid pointing away, as shown in Figure 27A, turn the entire body until both luminous lines are aligned. (See Figure 28.)

The lid's luminous direction-of-travel line is now pointing along a heading/course of 051 degrees magnetic. Camp is along this imaginary direction-of-travel line, about 6/10s of a mile away.

Now, it's no more practical to walk along staring at a compass dial then it is to try the same thing with a GPS. Instead, we use what the military calls "steering marks."

With the compass set and aligned on 051 as described, you look ahead along the line extending from the direction-of-travel line in the compass lid to a specific landmark - a distinctively-shaped tree, rock, or rock outcropping, for instance - this is your steering mark. Close the compass, and walk to the steering mark you selected. Once there, the process is repeated as many times as necessary until you reach camp.

A couple of observations here. Naturally, you'll seldom be able to travel in an absolutely straight line the entire way; there will be times when you'll be compelled to depart from your straight-line heading/course, sometimes by quite a way. When this occurs, simply fire up your GPS, get a new bearing to camp (or whatever your objective is), and begin the process again.

Second observation. It's best to use steering marks that are between you and your objective; that way, if you have to depart from your base heading/course for a time, it doesn't matter, because you'll be picking it up again once you reach the mark. If, on the other hand, you use a steering mark that's beyond your objective - a distinctive peak or point on a ridgeline, for instance - you'll need to use your GPS to check your correct course/heading more often.

Using a British prismatic, the whole process is equally workable in darkness. Prismatics are fitted with five tiny Tritium lamps that glow 24-7 and need no "flashlight shot." The lamps are positioned so that the entire process - putting the heading/course on the index ring, locking it in with the index ring clamp screw, determining the direction of travel, etc. - can all be done in total darkness without any additional light source. Covering the ground is a much slower process, of course; your steering marks, by necessity, are usually much closer and you're hiking by flashlight (used sparingly). On the other hand, moonlight can provide a surprising amount of navigation light, especially when there's snow on the ground. In any event, it's important to factor in a lot more hiking time to reach an objective and to be patient; simply take it one leg at a time, move deliberately and carefully, and accept the fact that it's going to take longer to get there.

Model 27 United States Military Lensatic Compass Open the compass lid and extend the lid to its fullest; when so extended it is parallel with the main compass body, as shown. (Refer to Figure 29.) Move the lens/rear sight to its rearmost position, which will allow the compass card to float freely. (The M-27 is not liquid-dampened, as are the British prismatics and the Brunton Combi.) The M-27's compass card is graduated in degrees along its inner edge and in mils along the outer edge. The degree graduations run clockwise through 360 degrees in 5-degree increments; single-degree readings have to be "eyeball estimated."

With the compass held level at belt level, lid pointing away, as shown in Figure 29a, turn the entire body until our heading/course of 051 degrees can be read on the compass card directly beneath the black index line, (also called the "lubber's line"), as shown in Figure 30. Note that there is a short luminous line on the index ring's glass face and another short luminous line on the compass card's North marking.

While holding the compass in place, reading 051 degrees, rotate the index ring until both luminous lines are aligned, as shown in Figure 31.

The lid's luminous direction-of-travel line is now pointing along a heading/course of 051 degrees magnetic. Camp is along this imaginary direction-of-travel line, about 6/10s of a mile away.

Now, it's no more practical to walk along staring at a compass dial then it is to try the same thing with a GPS. Instead, we use what the military calls "steering marks"

With the compass set and aligned on 051 as described, you look ahead along the line extending from the direction-of-travel line in the compass lid to a distinctive landmark - a distinctively-shaped tree, rock, or rock outcropping, for instance - this is your steering mark. Close the compass, and walk to the steering mark you selected. Once there, the process is repeated as many times as necessary until you reach camp.

Putting a course/heading on an M-27 in total darkness is a very risky proposition; it's just too difficult to read the numbers on the compass card and too easy to make mistakes. You'll need to set the compass while there's still some daylight or use a flashlight, being very certain that the flashlight isn't causing any deviation problems. Once it's set, the M-27 works well in darkness. Its other main drawback is that its index ring can't be clamped down like the British prismatics, but this seldom presents a problem.

If your M-27 is the Tritium version, it's luminous 24-7. If it's conventionally luminous, you'll need to give it a "flashlight shot" once in a while.

Brunton Model 54LU Combi Refer to Figure 32 and note that the Combi's index ring is graduated clockwise through 360 degrees. Every 20 degrees are numbered, with graduations every two degrees. [The degree graduations are coarse enough to easily set "by eyeball" individual (single) degrees between the two-degree graduations.]

Turn the index ring to 51 degrees, as indicated, setting "51" along the index line (also called the "lubber's line"), as shown in Figure 33.
With the compass held level at belt level, direction-of-travel arrow/line pointing out, as shown in Figure 33a, turn the entire body until the red "Magnetic North" needle (with the white 'N') on the compass card is aligned with the red "North" arrow on the index dial, as shown in Figure 34.
The Combi's luminous direction-of-travel arrow/line is now pointing along a heading/course of 051 degrees magnetic. Camp is along this imaginary direction-of-travel line, about 6/10s of a mile away. [Note that when the red "Magnetic North" needle on the compass card is aligned with the red "North" arrow on the index dial, the luminous dot on the "Magnetic North" needle is positioned between the two luminous dots on both sides of the "North" arrow on the index dial. In addition, there is a luminous patch beneath the index line (also called the "lubber's line"). These luminous points make the Combi completely functional in the dark, once it's been given a "flashlight shot."]

Now, it's no more practical to walk along staring at a compass dial then it is to try the same thing with a GPS. Instead, we use what the military calls "steering marks"

With the compass set and aligned on 051 as described, you look ahead along the line extending from the direction-of-travel arrow/line on the compass baseplate to a specific landmark - a distinctively-shaped tree, rock, or rock outcropping, for instance - this is your steering mark. Walk to the steering mark you selected. Once there, the process is repeated as many times as necessary until you reach camp.

Using a Combi, the whole process is equally workable in darkness. Though it must be given a "flashlight shot" from time to time to freshen it up, the entire process - putting the heading/course on the index ring, determining the direction of travel, etc. - can all be done in total darkness without any additional light source. Covering the ground is a much slower process, of course; your steering marks, by necessity, are usually much closer and you're hiking by flashlight (used sparingly). On the other hand, moonlight can provide a surprising amount of navigation light, especially when there's snow on the ground. In any event, it's important to factor in a lot more hiking time to reach an objective and to be patient; simply take it one leg at a time, move deliberately and carefully, and accept the fact that it's going to take longer to get there.

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