How to Read Maps for Navigation
Map reading and navigation while hiking or hammock camping is a skill. Global Position Systems (GPS) are convenient. However it is important you learn the skill of map reading. This ability allows you to determine your own position and the location of your destination on a map should your GPS fail.
The ability to navigate accurately in all weather conditions is fundamental to your safety. If you are in a group a leader needs to have a thorough understanding of the basics of map reading and navigation.
In theory navigation is an exact science, in practice it is somewhat different. A number of practical considerations which the map sometimes provides no guidance need to be taken into account.
These include the ability of the group, the availability of shelter and water, the type of hiking terrain which often cannot be judged accurately from the map and weather conditions. Your planning must therefore be flexible and you must be able to use the map to work out alternative routes.
To Navigate Competently, Know the Following
Understand map distance, symbols and scale.
Interpret terrain by various contour configurations.
Supply a grid reference.
Orient a map visually and by compass.
Use a map and compass to determine your position.
Take a true or magnetic bearing and convert
Calculate a back-bearing.
Walk on a bearing in any weather.
Navigating around obstacles.
Navigation can be complicated by
Unfamiliarity with the terrain.
Short line of sight typical of the mountains.
Adverse weather conditions.
Limited number of possible routes.
The more unfamiliar the terrain, the more important it is for the leader of the group to be thoroughly versed in the use of the two most important navigational aids, the map and the compass.
What is a Map?
A map is a symbolic representation, on a flat surface according to a specified scale of an area and its terrain or a part of and the natural and artificial features.
Different kinds of maps are used for different applications, for example road maps, geological maps, oceanographic and topographic maps. The maps most commonly used by mountaineers and hikers alike are topographic maps.
What is a Topographic Map?
Topographic maps provide an accurate representation of the area covered by the map and are usually drawn to a scale of 1:24 000 in the US.
They are drawn from aerial photographs and checked by field survey or satellites. They are well detailed and use conventional map symbols that are easy to use and understand in a map key.
What is Map Scale?
The scale of a map is the ratio of the distance between two points on the map and the actual distance between these same two points on the ground. Because scale is so important, it is usually indicated in more than one place and in different ways on a map.
Three Ways of Indicating Map Scale
In written word
As a representative fraction, or R.F. (e.g. 1:24 000).
By a scale line.
The U.S. Geological Survey (USGS) publishes maps at various scales. The scale used for most U.S. topographic mapping is 1:24,000. USGS maps at this scale cover an area measuring 7.5 minutes of latitude and 7.5 minutes of longitude and are commonly called 7.5 minute quadrangle maps. Map coverage for most of the United States has been completed at this scale.
To Calculate the Straight-Line Distance Between Two Points
- Use a ruler or pair of dividers to measure the distance on the map.
- Compare the map distance with the scale line, or multiply this distance by the representative fraction.
To Calculate a Distance Along a Winding Path or River
Another way of calculating distance on the map involves the use of a pencil and paper. Divide the distance into a series of straight lines from bend to bend. Mark each straight section along the edge of a piece of paper, rotating the paper at the end of each section. Now measure the distance obtained by comparing the paper with the scale line. This method however is not very accurate over long distances.
Conventional Map Symbols
All topographic maps in North America use the same symbols to depict features on the ground. These symbols are also color coded to simplify map reading.
Black symbols are for man-made features such as buildings, power lines, telephone lines, fences, paths, boundaries, etc.
Green symbols are used to indicate agricultural and natural features of vegetation, such as cultivated land, forests, grassland, etc.
Blue symbols are used to indicate coastal and water features.
Brown is used for contour lines, rocky outcrops and secondary roads.
The key to these symbols is found in the bottom margin of the map.
Topographic maps are usually printed with north at the top of the map. The left and right edges therefore run in a north-south direction, but this is not invariably the case.
An arrow indicating true north is always printed somewhere, usually in the left margin on the map. A second arrow, with the same origin as the first one, indicates magnetic north.
The angle between these two arrows is called the magnetic declination. Maps are usually printed using true (geographic north) as the reference direction, but the compass needle points to the magnetic north pole, which is a point somewhere in Canada, west of true north.
Unless magnetic declination is taken into consideration when you use a compass and map together, your bearings will be out by the number of degrees represented by magnetic declination in your area.
Grids and Grid References
The topographic maps generally are not “overprinted” with a grid. In order to simplify navigation and to increase accuracy it is a good idea to draw your own grid, particularly on maps you use often.
If the grid is drawn parallel to true north (i.e. the side of the map), this will enable you to determine bearings from the map, without first having to orient the map.
Should you draw the grid parallel to the magnetic north line on the map, the need to calculate true bearings from magnetic bearings, and vice versa will be eliminated.
What are Contour lines?
A contour line is an imaginary line joining all points of the same height above sea level. A contour line does not have a beginning or an end, but may run off the edge of a map onto an adjoining map.
Contour Line Representation
Contour lines represent the most accurate and the most easily interpreted method of indicating relief on a map. It is very important to know exactly what kinds of land form are represented by various groupings of contour lines; this allows you to choose the least strenuous and safest route between two points.
On any given map the height difference represented by the space between two adjacent contour lines is always the same (e.g. 50 ft., 20 m, etc.), and this difference or interval is called the contour interval. It is usually indicated in the bottom margin of the map.
Every fifth contour line is drawn thicker than the intervening contour lines to assist with the interpretation of the features and to make it easier to judge height differences. These thicker contours are usually labelled with the height above sea level in feet (or metres).
Because the vertical distance represented by the distance between any two adjacent contour lines never changes, an idea of the relative steepness of a slope can be estimated by considering the distance between the contour lines on a map.
The closer they are together, the steeper the slope. Where contour lines merge, they indicate a vertical cliff or an overhang.
An experienced map reader will be able to form a fairly accurate idea of land forms by just looking at the map, but when in doubt a profile can be drawn.
To Draw the Profile Of A Cross-Section Of A Slope
- Draw a pencil line across the slope on the map.
- Place the edge of a sheet of paper along the line.
- Mark the crossing point of the contours and label them with their height.
- Draw a base line equal to the length of the section. Choose a vertical scale at a right angle to the base line and mark the heights corresponding to the contours crossing the section.
- Join the points with a line, (Note: This method exaggerates the vertical relief.)
What is an Altimeter?
When used in conjunction with a topographic map, an altimeter, an instrument which is used to determine your height above sea level, can help you navigate accurately, particularly in thick mist in high mountains. If, for example, you are going up or down a well-defined ridge, a compass bearing is unnecessary if you keep to the crest of the ridge.
If visibility is poor it can be difficult to tell how far along the ridge you have gone, unless you have an altimeter. While you are traversing round a hill in bad visibility, an altimeter can also help you stay at the correct height and locate crucial points such as a neck or saddle.
Using Your Maps
Hikers, climbers and hammock campers use maps for three main purposes.
Determining your own position in the area represented by the map.
Planing a route between different points or to the next destination from their present position, taking into account the terrain.
To determine distances.
Using a Map to Determine Your Own Position
Before you can determine your own position on the ground using a map, you need to orient, or set the map. This simply means that you need to hold the map in such a way that the true north arrow of the map points to true north (or north on the map points to north in the actual countryside).
The features depicted on the map and the same features on the ground will then lie in the same orientation relative to you. A map can be oriented visually or with a compass.
Visual Orientation of a Map
- Observe at least two outstanding terrain features, such as two hills, which should have an angle of at least 60° between them (alternatively, the features can lie behind each other in a straight line, relative to you).
- Locate these terrain features on the map.
- Turn the map until the map features and the terrain features are aligned in the same direction relative to your north on the map will now be approximately aligned with true north.
Once the map has been oriented you can determine where you are from the relative position of other terrain features around you. This is not very accurate, particularly if the terrain features are not clearly defined, or if visibility is very poor. However, visual orientation of the map is an essential, basic procedure.
Orient the map at regular intervals while walking, thus ensuring that you always know exactly where you are on the map.
Magnetic Orientation of a Map
- Lay the map out flat.
- Set the magnetic bearing of true north on the compass and place it on the map so that the direction of travel arrow is on the true north grid line or a grid line in the map margin (or aligned with it, if you are using a compass with a non-transparent base).
- Gently rotate the map and compass together until the compass needle coincides with the orienting arrow. North on the map will now be aligned with true north.
Having used a compass to orient the map, you can now visually locate your own position on the map by comparing features on the map with terrain features around you.
Maps are subjected to a lot of wear and tear. The ink used on maps is not always waterproof and maps can therefore soon become useless in the rain. Maps tear easily along folds when they have been folded repeatedly. They can also become so dirty that they can no longer be used.
Ensure you can protect your map. Cover it with a clear adhesive plastic, laminate covering or spray it with the clear lacquer sprayed. Some maps can be bought pre-coated, but the coating used is not always easy to clean. There are many services for laminated coating.
Whats a Good Compass?
What is a Bearing?
A bearing is the angle between north and a specific point or course measured from a given position. It is always measured in degrees clockwise from true or magnetic north to give a true or magnetic bearing, respectively.
To take a magnetic bearing to a visible object.
- Face the feature or point of which the bearing is to be determined, holding the compass level in one hand so that the direction-of-travel arrow points directly ahead of you.
- Point the direction-of-travel arrow at the object.
- Turn the calibrated ring, or scale of degrees, (while holding the baseplate still and level) until the north end of the compass needle points to north (0°) on the calibrated ring.
- Read off the magnetic bearing on the calibrated ring at the index mark.
Walking on a Bearing
- The magnetic bearing to an object such as a the top of a hill in a general direction you wish to walk is set on the compass by turning the calibrated ring until the required bearing coincides with the index mark.
- Holding the compass level, with the direction-of-travel arrow pointing directly in front of you, turn until the compass needle aligns with the orienting lines, with the north end of the needle pointing to 0°. Look down the direction-of-travel arrow and select a distinct feature between you and the hill top, e.g, a single tree or large rock.
- Once you have reached that object, select another object on the same bearing and repeat the process until you reach your objective.
The ability to walk accurately on a bearing is the most important part of map and compass work and will be considered in greater detail later.
Map “True or Grid” Bearings
To measure a grid bearing to a point
- Draw a thin pencil line on the map from your position to the point.
- Measure the angle between this line and a north-south grid line (an easting) with a protractor.
- Draw a line from your position to the point on the map.
- Place a long edge of the compass along this line with the direction-of-travel arrow pointing towards the point of which the bearing must be determined.
- Rotate the calibrated ring (while holding the baseplate firmly) until the orienting lines align with a north-south grid line, pointing north.
- Read off the bearing on the calibrated ring at the index mark. This is a true north bearing and can be converted to a magnetic bearing by adding the magnetic declination.
If the map has a set of grid lines, it need not be oriented to determine the bearing between the two points. The magnetic needle is then also ignored while using the compass as a protractor.
To compensate for magnetic inclination, compass needles are minutely weighted so as to keep them close to the horizontal in the appropriate ‘zones of inclination’. A compass weighted for the northern hemisphere will not necessarily function effectively in the southern hemisphere, and vice versa.
Incorrect readings can be obtained using a compass weighted for a different zone of inclination, as the needle might not swing freely. Reputable compasses bought locally are appropriately weighted, be careful of compasses you purchased from foreign countries.
Magnetic Declination ‘variation’
This is the angle between the bearing from your position to true (geographic north) and the bearing to the magnetic north pole (magnetic north) indicated by the compass needle. In other words, it is the variation between ‘true’ and ‘magnetic’ north. This magnetic anomaly occurs in the horizontal plane.
Magnetic Inclination ‘dip’
This little known anomaly occurs in the vertical plane. It is the angle between the direction of the earth’s magnetic field and the horizontal. This phenomenon is used to locate the precise position of the magnetic north pole an inclinometer will point straight down at the magnetic north pole and will be horizontal at the equator.
Conversion of Bearings
Map to Compass
To convert a true (map) bearing to a compass bearing, add the magnetic declination to the true bearing.
Since the compass needle points to a point on the earth’s surface some 10° west of true north, you will bypass your objective if you walk on a true (map) bearing without first adding the magnetic declination to it.
Compass to Map
To obtain the true (map) bearing, which can be drawn in on the map, subtract the magnetic declination from the compass bearing.
Two useful tips for remembering when to add or subtract the magnetic declination are,
UPMA – Up from the map = add & DOMS – Down to the map = subtract
Position on a Map Using a Compass
Resection is a method for locating your position on a map with precision by using a compass. It can only be used when at least two terrain features can be observed which can also be identified on the map.
Find Your Position on the Map Using Resection
- Identify two or more landmarks on the ground and on the map.
- Take magnetic bearings to the landmarks.
- Subtract the magnetic declination to obtain true bearings.
- Set the true bearing to a landmark on the compass and pencil in a line on the map on that bearing, with the line passing over the landmark. Repeat this process for each landmark with the calculated bearing.
- The lines will intersect close to your position.
Determining your own position by resection. Less accurate map and compass work will produce a larger ‘cocked hat’.
Consider these steps in greater detail.
- The terrain features, or landmarks, should be distinct, some distance apart, and preferably at right-angles to each other.
- At least two landmarks are required, so that you have cross-bearings to fix your position.
- The magnetic bearings must be converted into true bearings by subtracting the magnetic declination.
- Set the true bearing to the first landmark on the compass. Place the compass on the map so that the edge of the base plate intersects the first landmark. Keep the edge of the plate on the landmark and, without disturbing the setting, swivel the compass on the map until the orienting lines are parallel to the grid lines (or the sides of the map) and the orienting arrow points to north on the map. Ensure that the edge of the compass base plate still passes over the landmark and then pencil a line on the map along the edge of the compass. Your position is somewhere along this line. Repeat this procedure for the compass bearings to the other landmarks.
- If you take bearings to three landmarks it is unlikely that the lines will intersect in exactly the same place. It is more likely that there will be a triangle of error (also called a ‘cocked hat’), with your position somewhere in this triangle. The more accurately you work, the smaller the triangle will be.
Identifying Unknown Points with a Map and Compass
An unknown peak or feature can be identified if it appears on the map and if you can identify your own position on the map.
- Find your own position on the map by resection.
- Take a bearing to the unknown feature and calculate the true bearing by subtracting the magnetic declination.
- Estimate the distance to the unknown feature, especially if there is more than one feature in the general direction of the unknown one.
- Draw a line from your position on the map at the angle of the calculated true bearing. This line should pass directly over the unknown feature which can then be identified from the map. (The distance is estimated to ensure that the correct feature is chosen on the map.)
What is a Backbearing?
A backbearing is the bearing in the opposite direction to your objective.
To Calculate A Backbearing
- Take a map bearing or a bearing to a landmark, using a compass.
- If the bearing is less than 180°, odd 180°.
If the bearing is more than 180°, subtract 180°.
Bearing 60° Backbearing = 240° (60° + 180°)
Bearing 295° Backbearing = 115° (295° – 180°)
A simple means of checking, there are only 360° in a circle, if the backbearing is greater than 360° you have made a mistake.
Backbearings are Useful
- If, in the middle of a flat plain which you are crossing on a given bearing, e.g. 120°, you have no object ahead of you to aim towards, you know that if you keep a visible object or landmark behind you on a constant bearing of 300° you will be walking in the right direction.
- If mist obscures the point ahead of you, while a landmark behind you is still clear.
- If you are lost you can retrace your route along the backbearing, this is called backtracking.
If you need to locate a particular place where, for example, you left an injured person to seek help backtracking is used as follows,
- Take a bearing to a landmark in the direction in which you will go for help and which you will easily be able to identify again when you return.
- Pace off the distance to this point, counting one for every pace you take with your left foot.
- When you return, walk on the backbearing from the distinct landmark originally chosen, checking the number of paces.
Walking a Route
The ability to walk on a compass bearing and arrive at your destination in all weathers, day or night, summer or winter, is the most important part of map and compass work. The best way to achieve this is to proceed by ‘legs’, or stages, following the same procedure for each stage.
Walking a Route in Good Weather
- Identify a distinct landmark in the distance which lies in the direction you must walk.
- Look for one or more smaller landmarks somewhat closer but in line with the distant landmark.
Poor navigation can have disastrous consequences, simply walk from one feature to the next. This permits minimum use of the compass and makes allowance for contouring around ridges and obstacles. You can deviate from the direct route if you need to and return to the original route by realigning the landmarks that you originally identified and using the relative positions of the most distinct features that you pass.
This method works very well when the visibility is good and you can see distinct landmarks relatively far away. Remember to look for landmarks behind you as well as ahead of you. Occasionally use backbearings to confirm that you are still heading in the right direction.
Walking a Route in Bad Weather
- It is much more difficult to stay on course when visibility is limited.
- The same method as described above is used, but the landmark chosen will be much closer. If no landmark can be seen, a person can be sent ahead to act as a landmark. He can move left or right until he is standing in the right direction and the group then moves up to him.
- The process is then repeated. Another way to keep on course is to let the first and last person in the group walk by compass.
- The last person must check that the person in front and the rest of the group is moving in the right direction.
In thick mist it is very important to keep your compass handy. This is so that you can take a bearing to a landmark if the mist should clear partially even for a few seconds. Keep in mind, if visibility is extremely bad it is better to sit out the bad weather and to move only once visibility has improved sufficiently to allow you to keep moving safely. To blindly move on, not knowing whether you are on course or not, is foolish and dangerous.
Navigating Around Obstacles
Occasionally, obstacles such as a rocky outcrop or dense thorny thickets will require you to change course and walk around the obstacle.
If You Can See a Prominent Feature
- If you can see a prominent feature like a tall tree in line with your bearing on the other side of the obstacle, walk around the obstacle towards the feature.
- Continue walking on your bearing to the landmark. If necessary, double-check by taking a backbearing to the point you came from.
If You Cannot See The Other Side Of The Obstacle?
- On reaching the obstacle, change direction by 90° and walk until you are clear of it; count the number of paces you take.
- Return to your original bearing and walk until you have passed the obstacle.
- Change direction back again by 90° and walk the same number of paces back to your original route, continue walking on the original bearing.
Common Navigating Errors
The two most common types of navigating error are,
- Errors of distance
- Errors of direction.
Distance errors are usually due to inexperience, difficult terrain, taking supposed short-cuts, fatigue, or a combination of these.
Errors of direction are usually caused by using incorrect bearings.
Mistakes with Bearing Direction
Interference by the earth’s magnetic field, caused by metallic deposits in the ground.
Metal objects in your pockets, fences, power lines, backpacks, even spectacles can effect the direction indicated by the compass.
Not holding the compass level or holding it upside down. The compass needle will not be able to move freely to align with the earth’s magnetic field.
Aligning the wrong end of the compass needle with north, this 180° error is often made by beginners.
Using magnetic bearings instead of true bearings, or vice versa.
The incorrect calculation of true bearings from magnetic bearings, or vice versa.
An error of 4° over a distance of 1100 yards will give a possible error of 230 feet (an inexperienced person usually makes an error of this magnitude). An error of 180° results in the hiker going in exactly the opposite direction to the intended direction and therefore gives an error of 1.2 miles after only half a mile has been walked, another mistake commonly made by beginners.
The magnitude of these errors increases with the distance walked. This implies that even experienced hikers can make some impressive blunders if they do not regularly check the accuracy of their navigation.
Night navigation is difficult, even for experienced hikers, since landmarks usually cannot be observed and bearings therefore cannot be taken. In addition, it is very difficult to estimate distances at night.
Plan a route which can be divided into stages which end at very distinct features, such as a river. Avoid heading towards dangerous points, such as the edge of an escarpment, and keep the stages short.
Prior to a night hike, or even before planning the various stages of a night hike, you should know your exact location on the map. Work out compass bearings and distances for each leg of the hike and write them on the map.
Set the first bearing on the compass and proceed, keeping as accurate a pace count as you can. At the end of each stage you should confirm that you have in fact reached the point you were aiming for, before continuing with the next stage.
Common Sense Navigation
Above all, you should try to know at all times where you are on the map.
Trust your compass, not your sense of direction, especially when tired, pressed for time, or in bad weather.
Try to stay clear of metal objects when using a compass.
Maps age, check the date of the survey if you suspect that the map does not show all the man-made terrain features.
Plan your route ahead.
Observe the terrain around you and form an idea of the relative position of landmarks around you, try to anticipate when certain landmarks will become visible.
Plan and execute your route in stages, keep direction by using landmarks in the far distance with a few closer ones in between.
Use an off-route landmark that can be seen from almost anywhere along your route. This gives a good reference point at any time or place along your route.
Remember that you do not always see the true top of a peak when lower than the summit.
Contouring is often (but not always) easier and faster than a direct route.
As noted map reading and navigation while hiking or hammock camping is a skill. Global Position Systems (GPS) are convenient but can fail. It is important you learn the skill of map reading. This ability allows you to determine your own position and the location.
Always apply common sense, it is best to sit out conditions in which visibility is too poor to allow you to take any bearings. ‘when in doubt, sit it out’.