part of
by Rick Curtis
published by Random
House
spring 1998
This material is taken from Chapter 6 - Wilderness Travel from The Backpackers Field Manual by Rick Curtis. For more details on this exciting book check out The Backpacker's Field Manual Page.
This material is provided by the author for educational use only and is not a substitute for specific training or experience. Princeton University and the author assume no liability for any individual's use of or reliance upon any material contained or referenced herein. When going into outdoors it is your responsibility to have the proper knowledge, experience, and equipment to travel safely. This material may not be reproduced in any form for commercial or Internet publication without express written permission of the author. Copyright © 1999, all rights reserved, Random House Publishing & Rick Curtis, Outdoor Action Program, Princeton University.
Traveling anywhere in the wilderness means determining where you want to go. Maps and guidebooks are the fundamental tools both for trip planning see (Chapter 1 - Trip Planning) and while you are out on the trail.
A map is a two-dimensional representation of the three-dimensional world you'll be hiking in. All maps will have some basic features in common and map reading is all about learning to understand their particular "language." You'll end up using a variety of maps to plan and run your trip but perhaps the most useful map is a topographic map. A topographic map uses markings such as contour lines (see page 00) to simulate the three-dimensional topography of the land on a two-dimensional map. In the U.S. these maps are usually U.S. Geological Survey (USGS) maps. Other maps that you'll find helpful are be local trail maps which often have more accurate and up-to-date information on specific trails than USGS maps do. Here's a brief overview of the basic language of maps.
Maps are drawn based on latitude and longitude lines. Latitude lines run east and west and measure the distance in degrees north or south from the equator (0° latitude). Longitude lines run north and south intersecting at the geographic poles. Longitude lines measure the distance in degrees east and west from the prime meridian that runs through Greenwich, England. The grid created by latitude and longitude lines allows us to calculate an exact point using these lines as X axis and Y axis coordinates.
Both latitude and longitude are measured in degrees (°).
1° = 60 minutes
1 minute = 60 seconds
Therefore:
7 ½ minutes = 1/8 of 60 minutes = 1/8 of a
degree
15 minutes = ¼ of 60 minutes = ¼ of a degree
All maps will list their scales in the margin or legend. A scale of 1:250,000 (be it inches, feet, or meters) means that 1 unit on the map is the equivalent of 250,000 units in the real world. So 1 inch measured on the map would be the equivalent of 250,000 inches in the real world. Most USGS maps are either 1:24,000, also known as 7 ½ minute maps, or 1:62,500, known as 15 minute maps (the USGS is no longer issuing 15 minute maps although the maps will remain in print for some time).
Standard topographic maps are usually published in 7.5-minute quadrangles. The map location is given by the latitude and longitude of the southeast (lower right) corner of the quadrangle. The date of the map is shown in the column following the map name; a second date indicates the latest revision. Photo-revised maps have not been field checked.
| Map Size | Scale | Covers | Map to Landscape | Metric |
|---|---|---|---|---|
| 7 ½ minute | 1:24,000 | 1/8 of a degree | 1 inch = 2,000 feet (3/8
mile) 2.64 inches = 1 mile |
(1 centimeter = 240 meters) |
| 15 minute | 1:62,500 | ¼ of a degree | 1 inch = ~1 mile | (1 centimeter = 625 meters) |
Map Legend
The map legend contains a number of important details. The figures below display a standard USGS map legend. In addition, a USGS map includes latitude and longitude as well as the names of the adjacent maps (depicted on the top, bottom, left side, right side and the four corners of the map). The major features on the map legend are show in Figure 6.3 and labeled below.
Contour lines are a method of depicting the 3-dimensional character of the terrain on a 2-dimensional map. Just like isobars in the atmosphere depict lines of equal atmospheric pressure, contour lines drawn on the map represent equal points of height above sea level.
Look at the three-dimensional drawing of the mountain below. Imagine that it is an island at low tide. Draw a line all around the island at the low tide level. Three hours later, as the tide has risen, draw another line at the water level and again three hours later. You will have created three contour lines each with a different height above sea level. As you see in Figure 6.4, the three dimensional shape of the mountain is mapped by calculating lines of equal elevation all around the mountain, and then transferring these lines onto the map.
On multi-colored maps, contour lines are generally represented in brown. The map legend will indicate the contour interval—the distance in feet (meters, etc.) between each contour line. There will be heavier contour lines every 4th> or 5th contour line that are labeled with the height above sea level. Figure 6.5 illustrates how a variety of surface features can be identified from contour lines.
There are a number of ways to measure distance accurately on a map. One is to use a piece of string or flexible wire to trace the intended route. After tracing out your route, pull the string straight and measure it against the scale line in the map legend. Another method is to use a compass (the mathematical kind) set to a narrow distance on the map scale like ½ mile and then "walk off" your route. It is a good idea to be conservative and add 5-10% of the total distance to take into account things like switchbacks that don't appear on the map. It's better to anticipate a longer route than a shorter one.
The compass consists of a magnetized metal needle that floats on a pivot point. The needle orients to the magnetic field lines of the earth. The basic orienteering compass is composed of the following parts: (See Figure 6.6)
No, this is not a silly question, there are two types of north.
You can see that location makes a great deal of difference in where the compass points. The angular difference between true north and magnetic north is known as the declination and is marked in degrees on your map as shown in Figure 6.7. Depending on where you are, the angle between true north and magnetic north is different. In the U.S., the angle of declination varies from about 20 degrees west in Maine to about 21 degrees east in Washington. (See Figure 6.7). The magnetic field lines of the earth are constantly changing, moving slowly westward (½to 1 degree every five years). This is why it is important to have a recent map. An old map will show a declination that is no longer accurate, and all your calculations using that declination angle will be incorrect. As you will see, understanding this distinction becomes important when navigating with a map and a compass.
| Tricks of the Trail |
|---|
| Buy Your Compass for the Right Area: As well as the magnetic deviation east or west, compasses also show a vertical "dip" up and down. This dip varies in different parts of the world and compasses are specially calibrated for that dip. So you can't take a compass made for North America and use it in South America and get accurate readings. |
So we have two types of north to contend with. When you look at your map, it is drawn in relation to true north;, when you look at your compass, it points to magnetic north. T to make the map and compass work together you must decide on one North as your point of reference and base all your calculations on that. As you can see the following chart, failure to take declination into account can put you way off target.
| Declination or Degrees Off Course | Error Off Target after Walking 10 Miles |
|---|---|
| 1° | 920 feet (280meters) |
| 5° | 4,600 feet (1,402 meters) |
| 10° | 9,170 feet (2,795 meters) |
Even after years of using a map and compass I could never remember how to correct for declination. Do I add declination or subtract it? What if I'm out west versus in the east? While navigating through dense fog on a sea kayaking trip, I finally came up with an easy way to remember. As long as you remember the basic principles, you can easily work it out in your head.
The first thing you need to know is where you are in relation to magnetic north. You can find this information by looking on your map legend. If you look at the map of North America in Figure 6.8 you will see the line roughly marking 0° declination. If you are on the line where the declination is 0 degrees, then you don't have to worry about any of this, since magnetic north and map north are equivalent. (Wouldn't it be nice if all your trips were on the 0 degree of declination line?) If you are to the right of that line, your compass will point toward the line (to the left) and hence the declination is to the west. If you are to the left of the line, your compass will point toward the line (to the right) and hence the declination is to the east.
The compass is used primarily to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic north or true north) to some point (either a point on a map or a point in the real world) (see Figure 6.8). Bearings are used to accurately travel to a destination or to locate your position. If you are working from your map, it is called a map bearing and the angle you are measuring is the angle measured clockwise from true north on your map to this other point on the map. If you are taking a bearing off a real point on the landscape with a compass, you are using your compass to measure the angle clockwise from magnetic north to this point on the landscape. This is called a magnetic bearing. Remember that the bearing is measured clockwise. If you think of true north as 12 o'clock then a bearing to the right of that (1 o'clock) is greater than true north and a bearing to the left of True north (11 o'clock) is less than true north.
If, you think about your map as an artist's rendition of the world. It displays true north, but it doesn't include magnetic fields as the real world does, so you need to make accommodations when going from your map to the real world. The real world doesn't have a true north—it's merely a construct of the map—so you have to make accommodations when going from the real world to your map.. The basic principle is this: to correct for declination, you want the map bearing and the magnetic bearing to be equivalent. If you are lucky enough to be on the line where the declination is 0°, both are already equivalent, or if you orient your map with your compass (see page 00) then you have made the two equivalent. Otherwise, you will need to make your own bearing corrections by adding or subtracting the declination amount. That gives us 4 possible permutations to work with:
If your declination is west, then magnetic north is less than true north and the map bearing is less than (<) the magnetic bearing . You need to make the two bearings equivalent by adding or subtracting the declination. This is illustrated in Table 6.2 and Figure 6.8b.
If your declination is East then magnetic north is greater than true north the map bearing is greater than the magnetic bearing. You need to make the two worlds equivalent by adding or subtracting the declination. This is illustrated in Table 6.2 and Figure 6.8a.
| If the declination is... | Then... | Map Bearing to Magnetic Bearing |
Magnetic Bearing to Map Bearing |
|---|---|---|---|
| West | Magnetic North < True North Map Bearing is < the Magnetic Bearing | Map Bearing + Declination = Magnetic Bearing. | Magnetic Bearing - Declination = Map Bearing. |
| East | Magnetic North > True North Map Bearing is > the Magnetic Bearing | Map Bearing - Declination = Magnetic Bearing. | Magnetic Bearing + Declination = Map Bearing. |
Another way to deal with declination is to adjust your compass. Some compasses have an outer degree ring that can be unlocked either with a set screw or a latch. This allows you to reset the compass to account for declination. For example, if the declination were 14 degrees East, you could rotate the degree dial to the right so that the magnetic needle was pointing to 14 degrees instead of 360 degrees. Once you do this, you will no longer have to add or subtract for declination because your compass is aligned to true north. Now when the compass needle is inside the orienting needle, the compass bearing that you read off your compass will be in relation to true north instead of magnetic north. If you have a fixed-ring compass, you can mark the declination angle on the compass ring with a piece of tape.
Navigation in the wilderness means knowing your starting point, your destination, and your route to get there.
Make it a habit of keeping your map and compass handy and refer to them every hour or so to locate your position (more often in low visibility). Keep track of your starting time, rest breaks and lunch stops, and general hiking pace. This will also give you an idea of how far you have traveled and whether your Time Control Plan is accurate (see Planning Your Day, page 00).
It is easiest to read a map if the map is oriented to the surrounding landscape. If you see a valley on your left, then the valley shows on the left on the map. You can do this by eye or with your compass.
until the north end of the magnetic needle points to the N on the compass housing (i.e. the red north end of the magnetic needle and the orienting arrow align). This is often referred to as "boxing the needle" since the magnetic needle is inside the "box" formed by the orienting arrow. The map is now oriented with respect to magnetic north. This means that the compass needle direction north is the same as true north on the map. You can also place the compass on the map so that the edge of the baseplate lies along the magnetic north indicator line on the map legend at the bottom and rotate the map as described above. This may give you a more accurate orientation for your map.
With the map oriented, look around for prominent features landscape features such as mountains, valleys, lakes, rivers, etc. Make a mental note of the geographical features you will be traveling along and seeing during the day. If you keep the terrain in your mind, you will usually have a general idea of where you are just by looking around.
| Tricks of the Trail |
|---|
| Orient Your Map: You can eliminate the need to correct for declination if you use your compass to orient the map each time. As long as the map is oriented with respect to magnetic north, any bearings you take from map to compass or compass to map will be the same. For this reason, it's a good idea to always take the time to orient your map. It will make your life much easier. It also means that each time you use your map, your will need to re-orient it with your compass. |
Let's look at some common backcountry scenarios and see how you can use your map and compass to navigate.
Okay, you hike in along the trail and then bushwack off trail to a nearby alpine lake to camp. When you wake up the next morning, you are fogged in. You know where you are on the map, but you can't see to find your way out. What you need to do is take a bearing on your map from your known campsite back to a known point on the trail that you can identify on the map. Then follow your bearing through the fog (or you might decide to wait out the fog if there is difficult terrain to traverse - see Chapter 7: Safety and Emergency Procedures: Dynamics of Accidents page 00). Here's your procedure:
You have been hiking along the trail and found a good campsite that is marked on the map. You see a summit ridge above treeline that looks like a great place for photographs, but there's a valley thick with Douglas fir between you and the summit. What you need to do is take a bearing from your current position to the summit and use that to travel through the forest. Here's your procedure:
To walk a bearing taken from the map, you may need to correct for declination if you did not orient the map to magnetic north before you took your bearing. Once you have corrected for declination, follow the same procedure as indicated above for walking a bearing taken from the land.
Sometimes the terrain isn't always so cooperative to let you just follow your bearing in a straight line so there are a number of techniques to use when traveling on a bearing.
You got to the summit and got some great photos, even one of a baby mountain goat. Now it's time to get back to your campsite. You could just follow your back bearing (see below) back to your location, but there is bound to be some error, when you hit the trail where will you be in relation to your campsite? The best bet is to intentionally aim off. Here's your procedure:
You're doing this incredible bushwack and you've been diligently following a compass course, sighting from tree to tree. Up ahead there is a clearing, when you enter it you discover a bog. There's no way you can go straight through on your compass course. Now what? Here's your procedure:
You're hiking off trail through the broad alpine valleys and your having this deep philosophical conversation about the connection of man with nature, so deep that you have lost some of your connection with nature. You look around and you don't know where you are. One alpine valley looks a lot like the last one you came through. Okay, so you're lost. Now what? Here's your procedure:
Triangulation is used to locate your position when two or more prominent landmarks are visible. Even if you are not sure where you are, you can find your approximate position as long as you can identify at least 2 prominent landmarks (mountain, end of a lake, bridge, etc.) both on the land and on your map. (See Figure 6.15).
An altimeter can also be a useful navigation tool. An altimeter measures the local atmospheric pressure of the air just like a barometer. This is usually expressed in inches or millibars of Mercury. The altimeter displays the current altitude on a dial with a needle or with a digital display. Since atmospheric pressure is constantly changing due to weather (see Chapter 7 - Natural History: Weather page 00), you must calibrate the altimeter by first setting it when you are at a known elevation. Say you arrive at the trail head parking lot which the map indicates is at 2,400 feet (730 meters). Set your altimeter for 2,400 feet (730 meters). As you hike the altimeter shows the current altitude as your elevation increase or decreases. In order to maintain accurate readings you should recalibrate your altimeter several times each day. One good trick is to recalibrate or at least look at your altimeter reading before you go to bed. If the altimeter reads higher the next day, then the atmospheric pressure has gone up during the night (typically indicating stable or improving weather). If the altimeter reads lower, then the atmospheric pressure is falling (indicating potential stormy weather).
You can use your altimeter in navigation as another information source to help locate your position. If the altimeter is properly calibrated, you know that you are at a specific altitude. Think of this altitude as corresponding to a particular contour line on your map. This may be enough to give you a very accurate fix on your location. If you are hiking up a trail and it crosses a particular altitude (contour line) at only one point, then you know exactly where you are. In other situations, you know that you are somewhere along a contour line that lies at that altitude (elevation). Other clues may help pin down exactly where along that contour line you are.
Inexpensive altimeters are available for under $50 and are also prone to inaccuracies due to temperature. To minimize temperature problems it is best to let your altimeter adjust to the ambient air temperature before taking a reading. More expensive altimeters that automatically correct for temperature changes can run over $200. A digital watch with an altimeter/barometer is an item that combines two useful tools.
Wearing a watch in the backcountry is a point of personal wilderness ethics. Many people like to let nature set the pace of the day rather than a watch. I may not wear my watch, but I always bring one along. There are too many times when I have needed a watch. For example, to get an accurate check on how fast I am hiking to see if my Time Control Plan is correct (see Chapter 1 - trip Planning: Planning Your Day page 00), and especially in first aid and emergency situations where timing vital signs and knowing the exact time that things are occurring may be essential in proper diagnosis and treatment (see Chapter 9 - First Aid & Emergency Care: Patient Assessment page 00). Watches can also be used to determine basic direction (see page 00).
The Global Positioning System is a network of satellites in orbit above the earth. A GPS unit is basically a radio receiver. The satellites transmit to the GPS unit which interpolates the signals into latitude and longitude which are displayed on the unit. Typically signals from three satellites are needed to identify a specific position and a fourth to interpolate altitude. GPS units are accurate to within a few hundred feet of your actual location. Although they can be used to very accurately determine your location and establish compass courses, don't rely on a GPS unit in place of solid knowledge of map and compass. Battery failure, damage to the GPS unit, or even leaving it behind at a rest stop could leave you lost if you don't have good map and compass skills. GPS units are particularly useful in locations where there a few landmarks to identify your location (for example long canoeing trips in northern Canada). GPS units are available as hand held units easily transportable in the backcountry.
The Outdoor Action World Wide Web Site includes a range of Internet resources accessible from this Home Page. The information provided here is designed for educational use only and is not a substitute for specific training or experience. Princeton University and the author assume no liability for any individual's use of or reliance upon any material contained or referenced herein. When going into outdoors it is your responsibility to have the proper knowledge, experience, and equipment to travel safely. The material contained at the Web Site may not be the most current. Copyright © 1997, all rights reserved, Random House and Rick Curtis, Outdoor Action Program, Princeton University.