Parts of a Compass
Most compasses have markings similar to these:
(1) Ruler/scales, (2) Direction of Travel Arrow, (3) Declination Arrow, (4) Rotating Bezel, (5) Housing, (6) Orienteering lines, (7) Bearing (Index) Line, (8) Orienteering Arrow, (9) Magnetic Needle, (10) Base Plate
Baseplate: the hard surface that the compass is mounted on. The edge is straight and used to lay lines on a map.
Bubble: some compasses contain a bubble which helps to ensure the compass is being held level.
Declination Scale: (or declination marks) is used to orient the compass an an area with a known declination.
Dial: moveable ring around the compass dial. It is turned to rotate the compass housing.
Direction of Travel Arrow: marked on the baseplate, used to point to the direction of travel.
Housing: the round housing, filled with liquid, that the compass needle rotates within.
Index Lines: parallel lines on the baseplate of the compass used to assist in alignment on the map.
Index Pointer: a marking between, and connecting, the direction of travel arrow and the compass dial. It is used to take degree readings.
Magnifier: used to magnify small markings on a map.
Mirror: some compasses include a mirror that let you see distant objects and a reflection of the compass at the same time. May also be used as a signaling device.
Needle: the magnetized piece of metal, one end painted red to indicate North, which rotates to mark the magnetic reading.
Orienting Arrow: a mark on the bottom of the compass housing which rotates when the compass housing is turned. You use the orienting arrow to orient the compass to the map.
Orienting Lines: (or meridian lines) parallel lines marked on the bottom of the compass housing and baseplate.
Ruler: many brands contain rulers to assist in map measurements
Scales: used with different map scales.
Sight: used to make aiming the compass easier.
The Topographic Map
A topographic map tells you where things are and how to get to them, whether you’re hiking, biking, hunting, fishing, or just interested in the world around you. These maps describe the shape of the land. They define and locate natural and man-made features like woodlands, waterways, important buildings, and bridges. They show the distance between any two places, and they also show the direction from one point to another.
Distances and directions take a bit of figuring, but the topography and features of the land are easy to determine. The topography is shown by contours. These are imaginary lines that follow the ground surface at a constant elevation; they are usually printed in brown, in two thicknesses. The heavier lines are called index contours, and they are usually
marked with numbers that give the height in feet or meters. The contour interval, a set difference in elevation between the brown lines, varies from map to map; its value is given in the margin of each map. Contour lines that are close together represent steep slopes.
Natural and man-made features are represented by colored areas and by a set of standard symbols on all U.S. Geological Survey (USGS) topographic maps. Woodlands, for instance, are shown in a green tint, waterways, in blue. Buildings may be shown on the map as black squares or outlines. Recent changes in an area may be shown by a purple overprint. A road may be printed in red or black solid or dashed lines, depending on its size and surface.
Maps are made to scale; that is, there is a direct relationship, a ratio, between a unit of measurement on the map and the actual distance that same unit of measurement represents on the ground. If, for instance, 1 inch on the map represents 1 mile (which converts to 63,360 inches) on the ground, the map’s scale is 1:63,360. Below is a listing of the scales at which some of the more popular USGS maps are compiled.
A convenient way of representing map distance is by the use of a graphic scale bar. Most USGS topographic maps have scale bars in the map margin that represent distances on the map in miles, feet, and kilometers. The table below shows the corresponding area of coverage for each scale and the linear distance that each scale represents in inches and centimeters.
To determine the direction, or bearing, from one point to another, you need a compass as well as a map. Most compasses are marked with the four cardinal points—north, east, south, and west—but some are marked additionally with the number of degrees in a circle (360: north is 0 or 360, east is 90, south is180, and west is 270). Both kinds are easy to use with a little practice. The illustrations on the reverse side show how to read direction on the map.
One thing to remember is that a compass does not really point to true north, except by coincidence in some areas. The compass needle is attracted by magnetic force, which varies in different parts of the world and is constantly changing. When you read north on a compass, you’re really reading the direction of the magnetic north pole. True North is static and located geographically about 800 miles north of the magnetic pole. Maps and directions are usually based on True North.
Magnetic declination is the angle between True North and Magnetic North. The amount of declination at any given point depends on the geographic location at that point on the continent. A diagram in the map margin will show the difference (declination) at the center of the map between compass north (magnetic north may be indicated by a MN symbol) and true north (polar north may be indicated by a “star” symbol). This diagram also provides the declination between true north and the orientation of the Universal Transverse Mercator (UTM) grid north (typically indicated by the GN symbol). The declination diagram is only representational, and true values of the angles of declination should be taken from the numbers provided rather than from the directional lines. Because the magnetic declination is computed at the time the map is made, and because the position of magnetic north is constantly changing, the declination factor provided on any given map may not be current.
Where True and Magnetic North are in the same direction, the declination is zero. At any point west of that line, your compass needle will point east of True North. This is called “Easterly Declination.” At any point east of that zero line, your compass needle will point west of True North. This is called “Westerly Declination.”
Taking a compass bearing from a map:
Draw a straight line on the map passing through your location and your destination and extending across any one of the map borders.
Center the compass where your drawn line intersects the map border, align the compass axis N-S or E-W with the border line, and read on the compass circle the true bearing of your drawn line. Be careful to get the bearing in the correct sense because a straight line will have two values 180° apart. Remember north is 0, east is 90, and so on.
To use this bearing, you must compensate for magnetic declination. If the MN arrow on the map magnetic declination diagram is to the right of the true north line, subtract the MN value. If the arrow is to the left of the line, add the value. Then, standing on your location on the ground, set the compass so that “zero degrees or North” aligns with the magnetic north needle, read the magnetic bearing that you have determined by this procedure, and head off in the direction of this bearing to reach your destination.
Find your way without a map
Find a heading (or field bearing)
- First, select a landmark along the route you want to travel. Make sure the compass is held level. Point the Direction of Travel Arrow at the landmark.
- Turn the compass dial until “N” (north) aligns with the red end of the magnetic needle. Read the heading in degrees on the index line. This is your “heading” to the landmark.
- Make sure to keep the magnetic needle aligned with “N” while travelling to the landmark. The Direction of Travel Arrow will point towards the landmark. Continue to check and repeat the procedure periodically (or when the landmark is back into view).
When you already know the heading
In some instances, you may be given the heading to travel and will need to move along the given heading.
- Turn the dial so the heading is set at the Index Pointer. Make sure the compass is held level and the Direction of Travel Line is pointing straight ahead.
- Turn your body until the red Magnetic Needle is aligned with “N” (north) on the dial.
- You are now facing the direction of travel. Pick out a landmark and travel towards the landmark.
- Check periodically and repeat the procedure.
Find Your Bearings with a Mirror Compass
Some compasses fold and typically have a mirror and sights to make sighting landmarks, while looking at the compass, much easier.
- Set the dial to the desired degree reading.
- Without changing the dial, move the compass so that the orienting arrow lines up with the magnetic needle.
- Hold the compass at eye level and adjust the cover to a 50º—70º opening. The mirror should reflect a top view of the compass dial. While looking in the mirror, move your sighting eye sideways until you see the sighting line intersect one of the two luminous points. Without changing the relationship between compass and eye, pivot yourself and compass together until you see, in the mirror, that the orienting arrow is lined up with the magnetic needle and the red end of needle is between the luminous points.
- Your direction or objective will now lie straight beyond the sight.
Be sure to keep the base plate level so the magnetic needle can turn freely. When sighting uphill or downhill, lower the sighting eye in relation to the compass. Also, a greater than 70º cover opening will increase the parallax effect and could cause as much as a 5º reading error. Should such a need arise, the mirror feature also functions as a signaling device.
Allow for Declination When Using a Map
When the paralleling method of declination adjustment with pre-drawn Magnetic North lines is not available, the Declination Scale gives a fast, sure method for compensating for the difference between True North and Magnetic North.
- Take your heading from the map by placing the Base Plate edge of the compass along your desired line of travel. The Direction of Travel Arrow points to your destination.
- Turn the compass Dial so that the Orienting Arrow and Orienting Lines are parallel to the map side margins. Your Map Heading can be read at the Index Line on the Dial.
- Hold the compass level, turn your body until the Compass Needle aligns with the Orienting Arrow. You are facing your Map Heading. Now turn yourself slightly until the Needle offsets against the Declination Scale to the appropriate degrees for your area. You now face your Magnetic Heading. Sight ahead to a landmark and walk to it. Repeat this process until you reach your destination.
Ignore Declination When Using a Map
If declination is slight in your location or if you are not referencing a map or if accuracy is not critical; you may use the compass without declination allowance.
- To locate your position, choose two landmarks and find them on your map. Label them L1 and L2.
- Point the Direction of Travel Arrow toward a landmark (L1) and rotate the compass Dial until the Red end of the Needle points to “N” on the dial. Read the heading at the Index Line.
- Place the compass on your map with Base Plate edge touching the landmark (L1) and pivot it until the Orienting Arrow or Orienting Lines align with the Magnetic North lines. Draw a line from the landmark (L1) along the side of the Base Plate across the map.
- Repeat this process with the second landmark (L2). Where the lines intersect is your location.
Compass readings are also affected by the presence of iron and steel objects. Even metal on a tether used to attach the compass to your backpack or person, can affect the compass reading. Be sure to look out for—and stay away from—pocketknives, belt buckles, railroad tracks, trucks, electrical lines, and so forth when using a compass in the field.