20010909.0650 Sunday , Sep 9, 2001, 6:50AM


Basic Compass Primer

Some people are accused of liking to "hear themselves talk." Apparently, those who write about using the compass like to "see themselves write." After reading many websites and book sections, and one book entirely on the subject, let's see if I can simplify how we use a compass.

In school we learned that a circle is divided into 360 degrees. The same is true for the face on the compass. North is the top of the circle, and both the ending and starting point. That is set at 0 degrees, and 360 degrees, if you want to look at it that way. East is 90 degrees, South 180 degrees, and West is 270 degrees.

Every direction you can walk, from where you are right now, has some relation to where it might fall on an imaginary circle drawn around you. The advantage of a compass is that it always points North. If you line up your compass with, say a distant high tree, you can read the angle you must travel to get there on the compass face. If you run into an obstacle while walking in the woods, you can sight an object across from it along the angle line you wish to travel. On the other side of the obstacle you can position yourself such that the object sighted is again reading the same angle on your compass. You are now back on a straight line toward your objective.

A circle is, well, a circle. If you headed on an angle of 90 degrees, and looked back 180 degrees behind you, where you came from would be in view. If that "where you came from" were marked by another tall tree, or landmark, that could be used to help keep you on line. If, looking back, you could get yourself to a point that the compass showed 180 degrees, then you would be heading along the same straight line.

If there were no landmarks to sight by, you could keep going at a specific angle no matter what. You might wind up above or below, (North or South) of where you wanted to be exactly, but you would NOT have gone in a circle.

The Earth's magnetic pole is what your compass lines up with. The magnetic pole keeps moving a little each year, and is not at the same place the map makers North Pole is. Therefore, maps are marked with the difference between the map makers North Pole and the Earth's magnetic pole, called Magnetic Declination. Magnetic Declination only matters if you are using a map. Local navigation is not effected by it. If you want to head on an angle, according to a map, you must adjust your compass to compensate.

If you are on the right side of where Magnetic North and True North is on a Declination Map, you must turn your compass back that number of degrees. In our area there is a 5 degree West declination. True North, what they call the map makers North, is at the 355 degree mark on our compasses. Any angle we might want to go must be corrected by that five degrees West declination. Again, only important if you are using a map. Maps commonly have the declination number on them.

If you have a map, and can sight two or more landmarks, you can spot where you are. Sighting your compass you can read the angle that they are from you. Transferring to the map a line at the same angle from each landmark will give you an intersection where you are on the map.

If you don't have a protractor you can use your compass. Once the sighting is made lay the compass on where magnetic north lies on the map. Now turn the compass body to the same degree you just sighted. A straight line along the edge of the compass will now give you your bearing line on the map. The above works with either a plate compass, or the folding, lensatic ones. A lensatic compass will fold out flat, to be used on a map.

Let us drop the discussion here. More information might confuse you, now. I just talked about sighting an azimuth, a back azimuth, adjusting for magnetic declination, plotting your location, and a little about using any type of compass. Easy when you don't just "run on." Let me know if this helps.

Paul Phillips