Follow the Lines

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Published on: Jun. 2, 1997

Last revision: Oct. 26, 2010

you can direct someone else to a target. If they have the proper starting point and the bearing, they will head in the right direction.

In order to find your way back, all you have to do is shift the index mark 180°, line up the north pointing needle and orienting arrow again and travel in the direction indicated by the compass base.

Now that we know how to travel over the landscape by compass, we can apply these same techniques to working with a topo map.

It's a great exercise and good fun to use a compass and map to plot a course at home, then take the compass out into the woods and try to follow the course precisely.

Since maps represent the land, you plot a course on them the same way. Here however, you don't have to use the magnetic needle at all, since on most maps, the top of the paper indicates north. Simply align the compass base with the direction you wish to travel, turn the compass housing so that the orienting arrow points toward the top of the map and read the bearing at the index mark.

In a perfectly machined world, we could follow the bearing we took on our map and reach our destination in the field. Our globe, however, possesses a seeming idiosyncrasy: the compass needle doesn't point to true north. Instead, it points toward an area near Hudson Bay, over 1,000 miles away from the actual north pole, to which your map is oriented.

What this means is that unless you're traveling somewhere near a line that runs through Florida and Lake Michigan, your compass and map will not agree. As you move away from this line, called the agonic line, the declination or angular difference between true north and magnetic north increases.

It only takes a bit of adjustment to accurately compensate for declination, however. USGS maps and many others include a declination symbol and the declination amount for the center of the map. If the agonic line is east of where you are, the declination is said to be easterly.

In Missouri, the declination varies from 5.5 to 6 degrees. This is important to know so that you can plot courses from topo maps, such as those on the back of conservation area brochures, that don't show declination.

When you make adjustments from a map to the actual route in the woods or fields, you have

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