Using Map and Compass Together
As
mentioned earlier, map and compass are a team. Each can work
independently, but they function best together. Before you use
map and compass together, however, it is necessary to understand
some of the limitations of each.
A compass needle points toward magnetic north. The top of the topographic map indicates true north, which differs from magnetic north.
True north is the actual north polar axis of the earth. Magnetic north is an area near Hudson Bay-over 1,000 miles from the true North Pole. It is difficult to give an exact location for the magnetic north pole because it moves. Reasons for this movement are complex and changes occur irregularly over hundreds of years. The difference between true north and magnetic north can vary greatly in different parts of the U.S. The only place in our hemisphere where true north and magnetic north coincide is on a line (called the agonic line) that runs from the west coast of Florida through Lake Michigan to the magnetic north pole. At any point between this zero line and the Atlantic Ocean, the compass needle points west of true north. At any point between the zero line and the Pacific Ocean, the floating needle points east of true north.
This discrepancy between map "language" and compass "language" is extremely important to anyone using map and compass together. For example, consider a hiker or hunter in Wyoming where the compass needle points 15 degrees east of true north. If the sportsman takes a bearing from his map and sets it on the compass without correcting for the 15-degree error and then starts following the bearing, he will be one-fourth mile off course after only one mile of travel. For every four miles he walks, he will move a full mile farther off course. In Missouri, declination varies 5 1/2 to 6 degrees so it is not such a significant difference. Fortunately, there is a simple procedure for making map and compass speak the same language.
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The
angular variation between true north and magnetic north is called
declination. A knowledge of declination is used two ways:
- to adjust a compass (magnetic) reading to a map (true reading) and
- to convert a true reading (taken from a map) to a magnetic reading that can be followed with a compass.
A symbol for declination, accompanied by a number, appears in the lower margin of all USGS topographic maps. The number is the declination for the area covered by the map. The symbol indicates whether the declination is east or west.
Two kinds of bearings are used with map and compass. A bearing taken from an object on the landscape is a compass bearing. It must be converted to a true bearing to be transferred to the map. When transferring compass bearings to a map, you must add easterly declination and subtract westerly declination, which is exactly the opposite of the adjustment you make when working from map to compass. A bearing taken from the map is called a map bearing or true bearing and must be converted to a compass bearing before use in the field. To follow a bearing taken from a map with a compass, you must subtract easterly declination and add westerly declination.
For example, consider again the sportsman in Wyoming. As explained, failure to correct for the 15 degree easterly Steps in following a map course with a compass are easy. declination could put the person seriously off course. TO prevent this, the sportsman should reset his compass. Because the compass needle points 15 degrees east of true north, 15 degrees should be subtracted from the map bearing (true bearing) to be followed. Subtracting compensates for the easterly error. Anyone following a map course in the eastern U.S., where the declination is west would correct the bearing by adding the appropriate number.
A
simple device for remembering whether to add or subtract declination
is the rhyme: "Declination east-compass least; declination
west-compass best." So, for an easterly declination, the
compass bearing should be less than the true bearing (compass
least-subtract). For a westerly declination, the compass bearing
should be greater than the true bearing (compass best-add). This
procedure applies whenever a compass is set to a true (map) bearing
(converting true bearing to magnetic bearing).
When you are using maps in the field, you can use a simpler method to correct the declination.
- With a pencil and straightedge, extend the magnetic north arrow in the bottom margin of the map across the face of the map.
- Draw a series of lines parallel to the first line about 2 inches apart. Space them evenly over the entire surface of the map.
- Now, to take a map bearing, use the same procedure explained in the map section with one important exception: instead of rotating the compass housing until the orienting arrow is pointing to the top of the map, or true north, rotate the housing until the orienting arrow is aligned with the magnetic north lines you have penciled in. Any bearing taken from the map using this procedure can be followed directly in the field-without other compensation for declination.
This
method avoids adding and subtracting because instead of aligning
the compass orienting arrow with the top of the map, you align
it with magnetic north. Thus all bearings are automatically converted
to map "language" or true bearings. Similarly, any
bearing taken from an object in the field can be plotted directly
on the map, using the magnetic north lines, without further compensation
for declination. By drawing those lines lightly on the map, all
the adding and subtracting is unnecessary.
Steps
in following a map course with a compass are easy:
- First, on the map, align the direction-of-travel arrow on the compass with the desired route. Disregard the floating needle.
- Now set the compass housing so that the etched orienting arrow in the base of the compass housing is aligned with the magnetic north lines you have drawn on the map.
- If you have not added magnetic north lines, rotate the compass housing until the orienting arrow points to the top of the map (north) and then add or subtract the proper declination to the bearing shown.
- Make sure the compass base plate stays aligned with the travel route shown on the map until the bearing is read. Now take the compass off the map and hold it level at waist height with the direction-of-travel arrow pointing ahead.
- Slowly turn your body until the floating needle is directly over the etched orienting arrow. The direction-of travel arrow on the compass base plate now points toward the destination shown on the map.
When you are on a compass course, it is best to follow visible landmarks along the route. Take short sections of a long route and walk from landmark to landmark rechecking the bearing periodically. This is safer and faster than charging over the landscape for long distances with your head down, watching the compass needle.
When you work with map and compass in the field, position the map so that its direction corresponds to directions in the field. To orient a map, simply set the compass to zero bearing and align the base plate with one of the magnetic north lines. Turn the map and compass together until the floating needle is aligned with the orienting arrow. The map is now oriented to the landscape so objects in the field will appear in the same direction as on the map.
The compass is used most often to follow a route laid out on a topographic map. Sometimes, however, you may want to take compass bearings from objects in the field and plot their location on a map. Outdoor enthusiasts can use this procedure, called resectioning, to identify a favorite spot for fishing, hunting, or other activity. Resectioning has only one prerequisite-there must be at least two, but preferably three, identifiable points on the landscape that also appear on the map.
Resectioning without a compass
To
resection without a compass:
- Orient the map by inspection-that is, align the map on the ground so that the two or more landmarks selected to serve as references appear on the map in the same orientation that they appear on the landscape.
- With a straight stick, align the distant landmark with its symbol on the map.
- Draw a line across the map so that when you sight over the line, the distant landmark appears.
- Repeat this procedure for each landmark without moving the map.
Resectioning with a compass
To resection with a compass:
- Select three landmarks to serve as reference points.
- Take a compass bearing from your location to the first landmark.
Transfer
the bearing to the map by laying the compass on the map so that
the edge of the base plate touches the landmark symbol. (See
A on diagram.)
- Without changing the bearing, swing the entire compass (keeping the edge of the base plate on the landmark) until the orienting arrow becomes aligned with the magnetic north lines drawn on the map. Make sure your orienting arrow points in the proper direction. If you have not drawn magnetic north lines, you will need to correct for declination.
- Draw a line passing through the landmark symbol using the compass base plate as a straightedge.
- Take a compass bearing from the other landmarks and transfer to the map as before. The point or area at which the lines intersect is your location.
For best results when resectioning, use landmarks that are
at about 45-to 90-degree angles from each other. Landmarks within
45 degrees of each other are subject to greater error. Often
you have small errors in your readings. Thus, if you use three
points when you resection, you create a "triangle of error.
" Your location is within this triangle of error. If this
triangle is extremely large, recheck your bearings. One of them
may be miscalculated.
