The compass in Figure 1 below like a watch has a dial and a hand, or needle. The magnetic needle is inside a fluid filled vial. When the compass is held flat, one end of the needle will always point to magnetic North. Remember, however, that rifles, metal buckles or other metal objects may affect the compass reading.
The dial on a compass is called a card. The compass card has 360 points,
called degrees. The dial will help you to find East, West, South and all
the points in between. The numbers on the dial show the number of degrees
|Compass Direction||Degree Reading|
The compass needle is magnetized. That is why the needle always points to magnetic North. The needle is often color coded for you to find north.
The earth's magnetic pole is not located at the true or geographic pole. The magnetic needle in the compass always points to the earth's North magnetic pole.
The zero curve is an imaginary line where the needle points to true north, and where magnetic north and true north are in line. In the United States, this imaginary line runs from Mackinac Island in Lake Michigan to Savannah, Georgia. East of the zero curve, the compass needle points to the west of true north. West of the zero curve, the compass needle points to the east of true north. The angle that the magnetic needle points away from the true north direction is called variation (or sometimes declination). Variation becomes greater as you travel further away from the zero curve. Variation is also measured in degrees. For very accurate compass readings, it is necessary to adjust your compass to eliminate this error.
Compasses can be adjusted to compensate for differences between magnetic and true North. Follow these two steps:
1. Find the variation (declination) for your area by looking at Figure 2. For example, if you are in Seattle, you would have an East Variation (Declination) of 21 degrees. This means you change the declination setting on your compass by 21 degrees towards the East. If you were in central Texas, you would have an East Variation (Declination) of 8 degrees. Maps usually list the variation right on the map.
Variation (declination) can be either East or West, depending upon where you are. If the magnetic needle points East of true North, it is called East declination. If it points West, it is called West declination.
2. Squeeze the vial with the thumb and finger (See Figure 3 below). With the other hand, rotate the outer dial until the orienting arrow on the bottom of the vial points to the correct variation (declination) angle (21 degrees for Seattle; 8 degrees for central Texas). Figures 4A and 4B show examples of variation (declination) adjustment settings.
The compass now automatically reads true North.
Taking A Bearing
A bearing is the degree reading or direction from your position to another object. For example, if you are in the field and a mountain peak is directly East of your position, the bearing of the mountain would be 90 degrees. If the peak were directly South of your position, the bearing would be 180 degrees. If it were West of you it would be 270 degrees.
To take a bearing hold the compass level in front of you. The sighting line should be pointing toward the object on which you are taking a bearing (See Figure 5). Be sure the arrow and needle are lined up correctly and not backwards. The bearing to your object is now the degree reading on the sighting line (See Figure 5A)
Reading A Topographical Map
A topographic map shows hills and valleys by contour lines. Each contour line represents a constant elevation in feet or meters above sea level. The interval of height between lines is shown on the map. This interval of vertical distance is represented by the space between contour lines.
Figure 6 shows how terrain looks on a topographic map. The top part of the figure shows what it looks like on the map. The bottom part shows what you would see if you were actually standing there.
Note that where the lines are close together the elevation is changing rapidly. On this map you can see that the top of the peak is 9,022 feet above sea level. The bottom of the peak is located at 8,200 feet above sea level. Each line on this map shows a 100 foot change in elevation.
How To Obtain Your Bearings From A Map
Topographic maps have a diagram showing the magnetic variation (declination) angle. Check to see that your compass is adjusted for that angle. The variation (declination) section explains how to make the adjustment.
If you want to travel from your present location to another destination on the map:
A. Place your compass on the map so that the long edge of the compass base is on a line from your present location to the desired destination (See figure 7).
B. Hold the compass steady, rotate the dial so that the 'N' on the dial is pointed North on the map. (The lines on the bottom of the vial will not be lined up North to South because of the variation [declination] adjustment.) Your bearing now appears on the dial at the sighting line (See figure 8).
C. Pick the compass up and hold it in a horizontal position so that the magnetic needle is free to rotate. Pivot yourself until the orienting arrow is aligned with the magnetic needle. The compass now points in the direction you need to travel to reach your desired destination.
If you know the bearing in degrees to an object from your present location, it is a simple matter to find your line of travel to that object. First, rotate the compass dial until the known bearing is next to the sighting line.
Next, hold the compass in a level position, so that the magnetic needle is free to rotate. Pivot yourself until the orienting arrow is aligned with the magnetic needle. The sighting line of the compass now indicates the appropriate direction of travel to reach your objective.
Crazy Compass? Never lay your compass and map on the hood of a car or truck! Any metal object made of iron can make your compass point the wrong way. A radio, rifle, knife or power line can cause a false compass reading!
Don't get lost! Learn to use your compass and maps before you wander afield!
Until next week, Good Luck, Happy Hunting, and God Bless. . . . . . . . .Stu Keck
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