Back Country Navigation 
Compass and Direction Finding 
Part 1 and 2 

by Dick Blust, Jr., photographs by Mark Furman
Click here to go to page 2

Before there was GPS, there was map and compass.

Map and compass are an integral part of a comprehensive approach to back country navigation and, at the same time, the right compass provides a means for position plotting, either as a stand-alone or as backup in the event of GPS failure or loss.

As the essay moves along, instructions will be provided that are specific for each of the three compasses recommended, first in direction finding, and later in position plotting.

Compass Selection
There are a lot of compasses out there on the market, and they range from the meticulously crafted to the downright useless. Fortunately, the best choices for the hunter can be boiled down considerably. The key factors are accuracy, durability, and night operation capability. I put special emphasis on night capability because for a serious hunter the ability to navigate before dawn and after dark are absolutes. The top choices in order of expense:

British Prismatic Compasses
The British prismatics are top of the line, and their price tags - usually about $400 to $450 - reflect it. Both manufacturers - SIRS Navigation ( www.sirs.co.uk) and Pyser-SGI ( www.pyser-sgi.com) - are located in Kent, near London. SIRS calls their compass the Model G150 Prismatic Marching Compass and Pyser-SGI markets theirs as the Francis Barker M-73 or M-88, but they’re virtually identical to each other in terms of design and features, including sight readings down to1/4 degree under ideal conditions. The Francis Barker M-73 and the SIRS Model G150 weigh in at about 7.4 ounces. This is due to a lot of brass in their construction, plus they're fluid-dampened. At 4.4 ounces, the Francis Barker M-88 is the lightweight version of the M-73 with less brass and more aluminum in the design. Otherwise, its features, accuracy, etc., are identical to those of the M-73.
 

These British prismatic compasses are the contemporary versions of what the Brits have been using, with constant improvement and refinement, since the Boer War. Fitted with five tiny Tritium lamps, they are not only extremely accurate but are completely functional in even total darkness. Though pricey, they’re worth every nickel. (See Figure 12, a Francis Barker M-73.)

Note: There are many cheap knockoffs of these compasses in circulation, manufactured mostly in the Middle East. Exercise caution if you’re not buying yours directly from the manufacturer or a reputable dealer.

Model 27 United States Military Lensatic Compass
Manufactured since 1992 by Cammenga, ( www.cammenga.com), this compass has been official American military issue for decades. Durable and accurate, though not as “finely tuned” as its British cousins, the Model 27 is a good compass that runs about $90 in the tritium-illuminated version. (There is a conventionally-illuminated, non-tritium version for about $30 less, but my advice is to stick with the tritium.) Prior to Cammenga, these were made by a firm called Stocker & Yale in New Hampshire; these units will be so marked and are just as good as the Cammengas. Weight: 5.3 ounces. (See Figure 13.)

Note: There seems to be an endless supply of lame lensatic compasses on the market running from $10 to $25 or so. Do yourself a favor and stick with the real thing made by either Cammenga or Stocker & Yale.

 

Brunton Model 54LU Combi
An extremely clever, reliable, and accurate design that combines precise prismatic sight readings - down to ½ of a degree - with the features of a baseplate orienteering compass, all for about $76.00. Though it’s luminous - operating in the dark, you have to give it a “flashlight shot” from time to time - it’s not fitted with tritium lights, though there’s a Silva version marketed in Australia that is.

A product of Brunton of Riverton, Wyoming (www.brunton.com), the Combi also features a magnifier and 1:25,000, 1:50,000 and 1:63,360 UTM map scales built into the baseplate. Weight: 1.4 ounces. (See Figure 14.)

There are other night-friendly compasses on the market made by Brunton, Silva, Suunto, and others, but I’ve used these three extensively and recommend them highly for hunters.

(I'd be remiss if I didn't make an observation at this point. It is often the case with equipment that cheaper means poorer quality, but that is not so here. The Brunton Combi represents an ingenious design that is multi-functional yet extremely light in weight - a great instrument. If I were starting from scratch today, I'd go with this one.)

 

     

Expressing Direction
A means of expressing direction that is accurate and precise is essential, and for this we use the compass rose. A simplified compass rose is shown in Figure 15 and depicts the cardinal directions - north, south, east, west - and the semi-cardinals - northeast, southeast, southwest, and northwest. Also shown are the compass headings for these points, expressed in degrees:

North 0 or 360

Northeast 45

East 90

Southeast 135

South 180

Southwest 225

West 270

Northwest 315

 

For the precision we need, the compass rose is further divided up into a full circle of 360 degrees, starting at north and moving clockwise, as shown in Figure 16. It is by this means that your GPS, map plot, and compass will express directions. (Your GPS, for instance, will tell you that your downed buck deer is 2.8 miles away on a heading of 268 degrees, or camp is 3.7 miles from your position on a heading of 135 degrees.)

(There is a second direction system those with a military background will probably have encountered - mils. Within this system, a full circle of 360 degrees is replacedwith a circle of 6400 mils. North is 0000 or 6400 mils, east becomes 1600 mils, south is 3200 mills, west is 4800 mils, and so on. Any GPS can be set to use mils instead of degrees, and the M27 military lensatic compass is dual-graduated to permit the use of either degrees or mils. In any event, the drill is exactly the same - you're simply substituting mils for degrees - but for simplicity's sake I recommend that hunters stick with degrees.)

 

The Three Norths
There are three norths to deal with in back country navigation: Magnetic, True, and Grid. The differences between the three are vital and can appear complicated, but, as we will see, are readily reduced to a single one- or two-digit number that we will be dialing into our navigation formula.

Magnetic North is, of course, what makes your compass work. Magnetic compasses "point" to the Magnetic North Pole - that point where the earth's magnetic field is directed vertically downward - providing a vital point of reference for the navigator. Presently located near the islands of northern Canada's Arctic Archipelago, it is about 600 miles from the True North Pole, that northernmost point on Earth where all meridians of longitude meet.

Magnetic North, then, is that line extending from any given position on the planet (or your topographic map) to the Magnetic North Pole.

True North is that line extending from any given position on the planet (or your topographic map) to the True North Pole. Lines of longitude, then, by definition, represent True North and actually are True North. Hence the east- and west-side map borders of every "store bought" topographic map are True North, because they are actual lines (meridians) of longitude.

Lines of Grid North are established by UTM and run very close to True North - they're usually within only a few degrees - but are not the same. The difference is a built-in because of UTM's core feature - instead of a spherical grid system (Lat-Lon) being applied to a flat surface, it's a flat grid system for a flat surface - hence creating a difference between True North and Grid North.

The difference between True North and Magnetic North or the difference between Grid North and Magnetic North at any point on Earth or for the area of any topographic map, expressed in degrees, is called "declination." Along a meandering line called the Agonic Line extending roughly - very roughly - up along the length of the Mississippi River, continuing up into Canada through Ontario, Manitoba, Nunavut, and points north, Magnetic North and True North are one and the same: declination is zero.

If you're west of that line, however, declination is east because Magnetic North is east of True North. If you're east of the Agonic Line, the opposite is true: declination is west because Magnetic North is west of True North. For Gannett Peak, Wyoming, for example, true to magnetic declination is 13 degrees east. The summit of Little Round Top at the Gettysburg battlefield in Pennsylvania is 11 degrees west declination, true to magnetic.  
     

A given map's declination is most often identified with a declination diagram on the map itself, as seen in Figure 17, a declination diagram drawn up for Gannett Peak, and in Figure 18, drafted for Little Round Top.

True North is usually identified with a star and/or an "N," Magnetic North with an "M" or "MN" and/or an arrow, and Grid North with a "G" or "GN," as depicted in Figures 17 and 18.

 

Why the fuss? The issue is accuracy. Consider this formula:

One degree of compass or map error equals nearly 100 feet of ground error per mile

To put this "error formula" into context, let's say you're hunting the Gannett Peak area and ignore the area's true-to-magnetic declination of 13 degrees east, as shown in Figure 17. Those 13 degrees of error result in 1,300 feet (over 400 yards) of error for every mile traveled; you'll be just under a mile off for every four miles covered. It's better, much better, to be as accurate as possible.

Because our maps generated by Terrain Navigator are UTM-gridded and thus the north-south lines that appear on the grid are Grid North, we use grid-to-magnetic declination rather than true-to-magnetic declination.

Figure 19 reflects a typical declination likely to be encountered in Wyoming or Colorado, with magnetic declination being 14 degrees east and grid declination being 2 degrees east of True North. What we’re after is our grid-to-magnetic declination, so in this instance we subtract the true-to-grid declination of 2 degrees from the true-to-magnetic declination of 14 degrees for a grid-to-magnetic declination of 12 degrees east.

 
     
Figure 20 could represent locations in some parts of Idaho or Washington, with magnetic declination being 19 degrees east and grid declination being 2 degrees west of True North. What we’re after is our grid-to-magnetic declination, so in this instance we add the grid-to-true declination of 2 degrees to the true-to-magnetic declination of 19 degrees for a grid-to-magnetic declination of 21 degrees east.  
     

In Figure 21 and Figure 22, we’ve moved east of the Mississippi - what’s more to the point, east of the Agonic Line.

Figure 21 reflects a typical declination encountered, perhaps, in West Virginia, with magnetic declination being 12 degrees west and grid declination being 2 degrees west of True North. What we’re after is our grid-to-magnetic declination, so in this instance we subtract the true-to-grid declination of 2 degrees from the true-to-magnetic declination of 12 degrees for a grid-to-magnetic declination of 10 degrees west.

 
Figure 22 represents a location in Maine. Here our magnetic declination is 18 degrees west and our grid declination is 1 degree east of True North. What we’re after is our grid-to-magnetic declination, so in this instance we add the magnetic-to-true declination of 18 degrees to the true-to-grid declination of 1 degree for a grid-to-magnetic declination of 19 degrees west.  
     

The key to arriving at the correct grid-to-magnetic declination is to look carefully at the declination diagram and visualize what you’re doing as you do the math - it helps a lot.

Determining Correct Declination
Virtually every topographic map has its own declination diagram printed on it, and for generations it’s been an article of faith to use the declination it indicates. This approach, unfortunately, is problematic.

First, many topos, particularly the “provisional editions,” do not provide Magnetic North or Grid North declinations at all, only a True North line. Others will indicate True North and Magnetic North, but not Grid North. And to make things worse, many of the declination diagrams printed on the maps are inaccurate. This inaccuracy stems from the fact that the Magnetic North Pole moves, and moves constantly at a rate of about 24 miles per year. In the decades since a particular map was printed, it may have shifted hundreds of miles, thus affecting declination in most areas. (The topo maps for most of southwest Wyoming, for instance, are about 3 degrees off.)

 

Fortunately, there are remedies. The first is on the Internet at www.topozone.com, a wonderful map site offering a host of features. Once at the site, all you need do to determine a current, accurate grid-to-magnetic and true-to-magnetic declination for any given area in the U.S. is tap in the name of a city, town, river, creek, lake, summit, or other landmark. TopoZone will seek it out, provide a small map window showing the landmark, and indicate the declinations, as shown in Figure 23.

 

Another solution is your GPS, which can be set for either True, Grid, or Magnetic, depending on the make and/or model. Once your unit has locked on, it can be used to determine the declination at its location. Garmins and newer Magellans simply indicate the declination; with an older Magellan, note the bearing to any stored landmark with your unit set on “True” then change it to “Magnetic” and check the bearing to the same landmark again - the difference is your declination.