Compass Page 4
As stated earlier, the floating dial of the compass is graduated in degrees. There are 360 degrees in a circle. Looking at the dial, 0 degrees is North. The numbers increase in a clockwise direction, and when you reach east, you are at 90 degrees. Continuing in a clockwise direction to south, you are at 180 degrees. West is at 270 degrees, and when you reach 360 degrees, you are back at north. North is both 360 degrees and 0 degrees.
Remember, the compass points at MAGNETIC North, not TRUE North. The term for this difference is the DECLINATION. The declination between true and magnetic north will depend on where you are on the earth. It may be as little at 1/2 degree or as much as 10 or 15 degrees or more. To know exactly, you will need to look at the topographic map for where you are (or will be working in). We will talk more on this later.
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The reading you get from the compass is called an AZIMUTH. The azimuth is read directly off the compass's floating dial and is expressed as DEGREES MAGNETIC. As an example, a reading of 92 on the compass is "92 degrees magnetic." You put the "magnetic" on the azimuth to separate it from true north.
An azimuth is the direction forward from the POINT OF ORIGIN. The point of origin may be where your standing now. There is another azimuth called a BACK AZIMUTH. A back azimuth is a direction 180 degrees opposite of the azimuth. Using our example of 92 degrees magnetic, we can determine the BACK AZIMUTH of this direction by adding 180 to the azimuth.
92
272
Therefore, the back azimuth of 92 degrees magnetic is 272 degrees magnetic. Why is this useful?
Let's say you leave your car and move in a direction of 92 degrees magnetic for 2 miles. Now, your ready to go back to the car. Instead of trying to hold the compass "backwards," just convert the azimuth to a back azimuth (272 degrees magnetic) and follow it back to the car.
Adding 180 degrees to the azimuth is the formula for finding the back azimuth, when the original azimuth is LESS THAN 180. If the original azimuth is MORE THAN 180, you SUBTRACT 180.
272
92
What would happen if you added 180 to 272? You would get a back azimuth of 452 degrees magnetic which does not exist on the compass.
What if the azimuth is exactly 0 or 360? You are heading north, and just need to head south (or 180). If your azimuth is 180, your heading south. So just head north (360 or 0).
Remember, all azimuths have a back azimuth. The back azimuth can be determined by adding 180 to any azimuth less than 180, or subtracting 180 to any azimuth that is more than 180. If the azimuth is exactly 0, then add 180. If the azimuth is exactly 360, subtract 180. If the azimuth is exactly 180, add or subtract 180 to get 0 or 360 (north).
This is a lot of information to absorb at one time. Be sure you understand this before continuing.
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