In poor visibility, solo walkers may have to walk on their bearings by holding the compass and literally following the arrow. Two or more walkers can send one person ahead to the limit of visibility. The scout stops so another walker can check the position with the compass and direct the scout to move left or right until in line with the bearing. Then everyone else joins the scout and the process is repeated. It’s a slow but very accurate method of navigation, particularly useful in whiteout conditions. I’ve used it many times when skiing.
Taking a compass bearing from a map. Align the compass with your objective on the map (left), then rotate the housing to line up the orienting arrow with the north-south grid lines (right).
Taking a map bearing with an orienteering compass from the trail to Tomahawk Lake. Place an edge of the baseplate on the point on the trail at your location and line up the edge with Tomahawk Lake. Rotate the compass housing until the orienting arrow is aligned with north on the map. Remove the compass from the map and turn it—without rotating the housing—until the magnetic needle and orienting arrow are aligned. The direction-of-travel arrow now points to Tomahawk Lake.
If you know where you are but not which way you need to go to reach your destination, then you need to take a bearing off the map. To do this, place an edge of the baseplate on the spot where you are, then line up the edge with your destination. Rotate the compass housing until the orienting arrow is aligned with north on the map. Remove the compass from the map and turn it, without rotating the housing, until the magnetic needle and orienting arrow are aligned. The direction-of-travel arrow now points in the direction you want to go. The number on the compass housing at this point is your bearing.
This process is straightforward, but you must account for magnetic variation. Most topographic maps have three arrows somewhere in the margin showing three norths. Grid north can be ignored for compass navigation (though not for GPS navigation—see UTM sidebar, pages 368–70); the other two are very important. One is magnetic north, the direction the compass needle points. The other is true north. The top of the map is always true north, so if your map has no grid marked on it, you can use the margins. Because compasses point to magnetic north and maps are aligned to true north, the difference between them should be taken into account when using the two in conjunction. This angle is measured in degrees and minutes (60 minutes equal 1 degree) and is called the magnetic variation or declination.
Aiming off.
Since magnetic north lies in the Far North of Canada, true north can be either east or west of magnetic north. In parts of Michigan, Indiana, Ohio, Kentucky, Tennessee, and North and South Carolina, however, magnetic north and true north coincide. In areas of North America to the east of those states, true north is west of magnetic north, but to the west of them, true north is east of magnetic north. The declination is usually marked on topo maps. For example, the Tom Harrison Trail Map for the Mono Divide High Country in California gives the declination in 2002 as 14.5 degrees east, while the Appalachian Mountain Club map of the Presidential Range in New Hampshire gives the declination in 2003 as 17 degrees west. Just to confuse matters further, magnetic north isn’t stationary but moves in a (luckily) predictable pattern. The declination on an old map will not reflect the current position of magnetic north. Some maps list the rate of change so you can work out the current figure. If your map isn’t new and doesn’t show the rate of change, you may be able to find it in a trail guide. You also can calculate the magnetic variation yourself by taking a bearing from one known feature to another, recording the bearing (without taking any declination into account) and then taking the same bearing from the map. The difference between the two is the current declination. If you stick a piece of tape on the compass as a declination mark, you won’t have to recompute it every time you use the compass. You will have to remember to move it if you visit different areas, though. Some compasses come with adjustable declination marks.
In the eastern United States, because magnetic north lies west of true north, when you take a bearing from the map you add the declination figure. However, if your bearing is taken from the ground and transferred to the map (not something you’re likely to do often), you subtract the declination. A mnemonic for remembering this is “empty sea, add water”—MTC (map to compass), add. Of course, in western states the opposite applies; you subtract declination when taking a bearing from the map and add it when taking one from the ground.
There’s always an element of error in any compass work. If your bearing is 5 degrees off, then you’ll be 335 feet off the correct line of travel after walking 0.6 mile, 650 feet after 1.2 miles, and more than half a mile after 6.2 miles. This makes it difficult to find a precise spot that lies some distance from your starting point unless you can take a bearing on some intermediate feature. One of the few compass techniques I use, other than straightforward bearings, is aiming off, which is especially handy in poor visibility. If your destination lies on or near an easy-to-find line such as a stream, you can make a deliberate error and aim to hit a line on one side or the other of your destination. When you reach the line, you know which way to turn to reach your destination. I used aiming off on a large scale in the Canadian Rockies. I knew that hundreds of miles somewhere to the northwest was the little town of Tumbler Ridge, which I wanted to reach, and that a road ran roughly east-west to the town. By heading north rather than northwest, I knew I’d hit the road east of Tumbler Ridge, and after a week’s walking I did—and found myself 46 miles away. But I knew where the town was, and, more important, where I was.
The compass has other, more complex uses; you’ll find instructions for them in good navigation books. Don’t rely on your compass blindly, though. There are areas high in iron ore where a compass won’t work.
Keep your compass where you can reach it easily; otherwise you might be tempted to skip checking it when you’re unsure of your direction. As I’ve learned, this can mean retracing your steps a considerable distance. Like most people, I have a loop of cord attached to my compass (most come with small holes in the baseplate for this purpose), though I don’t often hang it around my neck. I may tie the loop to a zipper pull on a garment pocket so that I don’t lose the compass and can refer to it quickly whenever I want to.
GPS
GPS (global positioning system) is a government-operated system that consists of twenty-eight satellites 12,000 miles up, four of them used for backup. Each satellite orbits earth twice a day and sends out a continuous signal, giving its position and the time. By locking onto a minimum of four satellites, a GPS receiver on the ground can triangulate its position. An accurate fix may not be obtainable if the satellites aren’t in the right alignment, however. Also, there must be a line of sight between the receiver and the satellites; it can be impossible to get a fix in dense forest or below steep cliffs.
UTM COORDINATES AND GPS
All GPS receivers give positions as a set of numbers called coordinates. Latitude and longitude is the standard coordinate system. However it isn’t the easiest to use. Much simpler and the best for use with GPS is Universal Transverse Mercator (UTM), a metric system that appears on all U.S. topo maps and many other maps. The UTM system divides the earth into sixty zones, each with its own number. Zones 5 to 22 cover North America. Each zone is then divided into a rectangular grid. The UTM grid is superimposed on a flat projection of the earth’s surface rather than on a globe like latitude and longitude (which means a degree of longitude varies in size according to distance from the equator). This means the grid squares are identical in size, which makes plotting a position easy. (Using a flat projection does introduce some inaccuracies but these are so small—less than 0.4 percent—that they can be ignored. The discrepancy is marked on topo maps as the difference between grid north and true north.) Using the UTM grid you can give a position in numbers of meters from the horizontal and vertical grid lines on a map. The grid lines are 1 kilometer apart on USGS topo maps. (Always check the map key for the distance bet
ween UTM marks—on the Map Adventures White Mountains of New Hampshire and Maine Trail Map and the Trails Illustrated Sequoia and Kings Canyon National Parks map they are 5 kilometers apart for example.)
USGS topo map with UTM grid penciled in.
To plot positions accurately using UTM you need a plastic measuring device called a roamer, which is transparent and has sets of lines at right angles to each other for different map scales such as 1:24,000, 1:50,000 and 1:63,500. Roamers are found on many protractor compasses (see illustration page 364) and also separately, such as the UTM Grid Reader and the Topo Companion. Alternatively you can use a grid tool, with which you can subdivide a map grid into tenths. As the grid square on a 1:24,000 USGS topo covers 1,000 square meters a grid tool will let you locate a 100-meter square. Grid tools come in different map scales.
Unfortunately on USGS topos the grid is not drawn on the map. Instead blue ticks round the edges of the map mark the position of the grid. To find positions on the map you need a straight edge. Eyeballing positions away from the edge of the map is difficult. Using a straight edge in the field can be hard too, especially when it’s windy or raining and there is no level surface on which to place the map, so it’s best to draw the grid in place at home with a fine pen or pencil. As the grid may not be exactly in line with the edges of the map make sure you link up the blue ticks on either side.
To plot a position as a UTM coordinate on a map you need to measure the distance from the grid lines that border the position. UTM is designed to be read to the right and from the bottom up so the grid numbers increase from left to right (east to west) and from bottom to top (south to north). The vertical (north-south) grid lines to use are those to the west (left) of the position. The horizontal (east-west) grid lines to use are those to the south of the position. To emphasize: the grid lines to use are those on the left (western) and bottom (southern) sides of the square in which the position lies. Using a roamer you can measure tenths of a square to give a Grid Reference (GR). The distance of a point east of the vertical grid line is called an easting. The distance of a point north of the horizontal grid line is called a northing. Coordinates are always stated with eastings first.
An overview of the UTM grid system.
As an example let’s take a typical USGS 1:24,000 topo map, North Peak, Arizona, which shows part of the Mazatzal Mountain range (see opposite), with UTM grid lines penciled in. The information in the lower left margin of the map tells us that the map is in UTM zone 12. That will be the figure that first appears on a GPS reading for anywhere on this map (sometimes with a letter after it—N, S, or U—this can be ignored). Along the sides of the map are the UTM blue ticks, with numbers alongside with the first figure or figures printed small, the second ones printed large, such as 463. The larger printed figures are the ones that mark kilometer distances on the grid, the smaller ones aren’t relevant (they refer to the distance from the zone meridian, a line running down the center of each UTM zone, and the equator). Let’s say we want to put the coordinates of the Pasture Tank into a GPS receiver (which, as a water source, could be a crucial feature). The Pasture Tank lies in the square 63 (easting) 88 (northing). Using a roamer we can divide the square into tenths. The pasture tank is 9/10 of a square east of line 63 and 1/10 north of line 88, giving Grid Reference 639 881. Six-figure coordinates like this are for a 100-meter square, normally accurate enough for practical purposes. GPS receivers usually give more precise readings than these, often ten-figure ones accurate to a square meter. However I find six-figure coordinates—three for the easting, three for the northing—accurate enough and easy to use. When entering a coordinate into a GPS receiver, zeros can be used for the final three figures if these are required. Sometimes a GPS receiver shows a zero at the start of a seven-figure easting. This can be ignored.
Further information on using UTM and GPS can be found in Michael Ferguson’s GPS Land Navigation and Lawrence Letham’s GPS Made Easy and at the maptools.com website.
Map Tools’ grid tool (left). Pocket corners and a roamer, also from Map Tools.
Using electronic gadgets like GPS in the wilderness is frowned on by some. Whatever you think about the place of electronics in the wilds, one thing is clear: they won’t go away. Indeed, their use will increase as more devices designed for wilderness use are developed. It is futile to debate whether the wilds are an appropriate place for such technology. The point is to look at electronic devices exactly as we would any other new gear: to consider what purpose they serve and just how useful they are. After all, they’re only tools, like packs or stoves.
I’ve tried a number of GPS units, but this is an area of rapid development, so before buying a unit I’d recommend getting up-to-date expert advice. GPSinformation.net is the Web site for such information.
For several reasons, GPS should not be considered a substitute for a map and compass (and the skills to use them) but rather as complementary. On a few occasions I’ve been unable to confirm my position with GPS because the receiver wouldn’t give a reading; each time I was very glad I’d been checking my progress on the map.
Once a receiver has a fix, it will show the position on its screen. This can be in latitude or longitude or UTM (Universal Transverse Mercator), the grid used on USGS topo maps. To translate this reading onto the map, you must be able to plot grid references accurately (see sidebar). All receivers can be set to give positions for the maps of different countries. These are known as map datums. It is very important to set the correct map datum, as an incorrect one will result in an inaccurate reading. The most common map datums for North America are North American Datum 1927 (NAD 27) and World Geodetic System 1984 (WGS 84).
Anyone comfortable with computers should have no problem with GPS. Technophobes and those who have trouble programming a video recorder might find GPS receivers difficult. Studying the manuals carefully (which takes several hours) and practicing with the receiver at home are essential if you are to make full use of a unit in the field.
GPS receivers have multiple functions. In addition to showing your position, they tell you the approximate altitude, and they can be programmed to guide you to a specific destination by a series of legs—they can store from dozens to hundreds of locations, called waypoints—or take you back to your starting point. They can tell you how far you have to go to reach your goal, which direction to go, how fast you’re traveling, how long it will take you to get there, and the estimated time of arrival. If you stray from the route they’ll warn you of that, too.
Using all these functions requires power, and that means batteries. Most receivers run off AA batteries, some off AAA; and though figures for battery life are impressive (twenty hours and more), these are usually for temperatures of about 70°F (21°C). In colder temperatures (I’ve used receivers at 0°F [−18°C]) battery life is much shorter, even when the receiver is kept warm inside your clothes and you use lithium batteries. Carrying spares is advisable.
The most common use of GPS is to guide you toward a destination. To do this you first enter the grid references of the destination and waypoints en route into the receiver’s memory (most easily done by downloading them from a computer mapping program), then follow the direction indicators on the screen to each point or transfer the bearings given to your compass. Of course, the receiver can direct you only in straight lines, though most will indicate how far off your line of travel you are.
A GPS receiver. Brunton/Silva Multi-Navigator.
Using GPS like this means walking with the receiver in your hand and consulting it regularly. I tried this on a cross-country walk in viewless forest, and it worked well. However, I don’t go for walks in order to stare at a screen; I do enough of that when I’m writing. There are times and places—when I was unsure of my whereabouts for nearly a week in the Canadian Rockies, for example, or in a whiteout in winter anywhere—when I could see the usefulness of following a preset course. But for most walking it’s unnecessary and a waste of batteries. I think GPS is better used
as a means of checking your position when it’s important you know exactly where you are. I’ve used it for this in trackless, snow-covered terrain in poor visibility and felt reassured when the receiver confirmed that I was where I thought I was. For this a simple tiny, lightweight receiver is adequate, like one of the 3-ounce Garmin Gekos.
GPS: MY CHOICE
My current GPS unit is a Brunton Multi-Navigator, a powerful waterproof device that weighs 10 ounces with two AA batteries. The Multi-Navigator has a compass that automatically compensates for local declination and can be used with the GPS turned off, as well as an altimeter/barometer. It can hold ten routes and one thousand waypoints with names of up to eight characters and record tracks for downloading to a computer mapping program. It does far more, but all I use it for on the rare occasions I take it out—mainly on ski tours—is pinpointing my position, usually in a whiteout or dense mist or in featureless terrain. For this a much less sophisticated GPS unit would be fine, and if I were purchasing a new one it would be the tiny Garmin Geko 201, which weighs 3.01 ounces with two AAA batteries. At that weight I might carry it more often than the Multi-Navigator.
The GPS receivers I’ve tried perform well once you master the necessary procedures and the technical jargon of the instructions. Besides Brunton and Garmin, good units are made by Magellan and Trimble.
Chris Townsend Page 54