Compass Use
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Compass Navigation
Although most navigation can be done with a map only, being able to use a compass is useful when visibility is limited or terrain features subtle. Compass navigation skills should be seen as a complement to sound map-reading skills. Students MUST master map reading FIRST!
Educational Goals
Students should know how to orient their map using a compass, be able to take follow a compass bearing, and understand how to triangulate their position.
General Concepts A compass provides us with a consistent reference point and allows us to navigate with out landmarks. The Compass points to the earth’s magnetic force which emanates from the magnetic north pole around Hudson Bay. These lines of force are caused by the flow of the earth’s molten metal core. True north is the top of the plane towards the center of spin. Declination is the angular difference between true north and magnetic north at your location.
Basic Compass Parts Base plate: plastic base containing the direction-of-travel arrow. Bezel or Housing: the movable plastic circle with both the degrees and a north indicating arrow printed on it. North seeking needle: This floats on a pivot inside the housing. The red end of this needle points to magnetic north (“Santa is red and lives up north”).
Boxing the Needle Boxing the needle is the first step in compass navigation. To do this, hold the compass flat in your hand at mid-chest level. Rotate the housing until the direction of travel arrow lines up with north on the Bezel. Now rotate the compass by moving your palm until the north-seeking arrow lines up with-or is “boxed in” –the north-indicating arrow printed on the housing. The needle is now boxed and pointing toward magnetic north.
Orienting the Map A compass can be helpful in orientating a map when, due to low clouds, thick vegetation, darkness, or featureless terrain, there are no real landmarks to use. The technique for orienting your map using a compass follows these steps:
• Lay the map on a level surface.
• Locate the declination diagram in the lower left corner of the map.
• Lay the compass on top of the diagram and “box the needle”
• Without disturbing the compass, rotate the map until the MN vector (the line that points to magnetic north in the diagram) lines up along the edge of the compass (the edge parallel to the red needle). You have now aligned the magnetic north line on the map with the magnetic needle and your map is oriented to true north.
Taking and Following a Bearing To take a bearing, orient your map and then draw a straight line from your present location to your destination. Without disturbing the map, lay the compass edge along the intended line of travel. Rotate the housing or bezel to box the needle. The direction of travel arrow is now pointing in the direction you want to travel. The number on the housing dial is your bearing.
Now keeping the needle boxed, lift the compass up to eye level. Sight along the direction of travel arrow and pick out a landmark which lines up with it. To follow the bearing, walk to your landmark and repeat the process of sighting along the direction of travel arrow with needle boxed. Keep doing this until you reach your destination. Fewer sightings result in fewer errors, so it helps to choose distant landmarks.
To take a field bearing with out the map, choose a landmark and point the direction of travel arrow at it. Rotate the housing to box the needle and follow the bearing in the manner described above.
Triangulation Triangulation is used to determine one’s position using two or more known points. Again, you start by orienting your map. Then choose two known and visible landmarks. Take a bearing on one of the landmarks. Place the edge of the compass (keep the needle boxed) on the map, so that it runs through the center of the landmark you just sighted. Double check to see that the needle remains boxed and that the map’s orientation has not changed.
Draw a line along the edge of the compass through the landmark. Extend the line in each direction. You are on that line somewhere. You can even draw this line without a compass if you are careful. Now repeat this procedure on a second and for more precision, a third, landmark. It is helpful to pick landmarks that are at least 60 degrees apart in order to improve the accuracy of the triangulation.
The intersection of all three lines is your position. Given human error, however, all three lines rarely intersect. Often they form a triangle, but your location should be somewhere in that triangle. Notice that triangulation with a compass is just a more refined way of orienting yourself using terrain association.
Triangulation is easier when you are traveling along a defined linear feature (ridge, trail, drainage, or river). Once again start by orienting your map. Then trace a line along the linear feature you are following. Take a bearing on a landmark and transfer the line to the map. The intersection of the bearing line and your travel feature is your position.
Teaching Considerations
Introduce this instruction once the students are comfortable with their map reading skills, but do not wait until it is too late in the course for them to practice with your help. Compass use can be a distraction which impedes students’ map-reading development, and the topic can be confusing if the instructor is unable to teach compass use in a simple manner. As with any topic, avoid teaching it if you have a weak personal experience base.
The “key points” mentioned above avoid adding and subtracting declination. This method is suggested because it is simple and easy to teach. Defer questions about declination calculations until after the class. These often confuse less experienced students, disrupt the flow of instruction, and cloud a simple method with useless complications. Unfortunately, most compass books are riddled with this unnecessary math.
Compass use should be taught in a step-by-step progression that builds upon previous skills. For ease of understanding teach it in at least three to four installments. Here’s a sample progression 1) general concepts boxing the needle, orienting the map; 2) Measure a bearing off the map and follow it; 3) Triangulating a position; 4) triangulating with range lines.
Students either seem to understand compass navigation easily or they struggle with it. Tailor your instruction to bring people along as they are ready. Organize a hiking group with students ready for more instruction and make that a focus of the day’s trail education. Compass instruction is most effective in a setting that has clear, easy to identify landmarks. A simple orienteering exercise can be a fun way for students to practice their skills. Make sure the distance of the legs are long enough to require accurate techniques. Instructor accuracy in setting the points is often the crux of a successful orienteering game. In many cases this instruction is not vital for students to complete independent travel, but it can be useful.
Leadership Opportunities
Some students come with a background in compass navigation and are comfortable with their use. Let these students help their less-experienced colleagues learn the skill. The trick is to get them practicing appropriate leadership. They should understand that their goal is to help the others, not to tell people everything they know about compasses.
Resources: Kals W.S. Land Navigation Handbook, 1983, pp.81-148, 202-222 The NOLS Wilderness Guide
