Compass Knowledge Base
Determining your Position with a Dial Pocket Compass and a Map
by Eric Newman
So you've used a standard
pocket compass with a compass needle to take a sight and it was a bit of a pain.
Now you are wondering if there is an easier way.
Yes, a pocket compass with a floating compass dial makes sighting much easier.
To take a bearing you need a protractor, which is a circle divided 0 - 360 degrees, and you have to reference the protractor to north.
Luckily, a protractor is actually built into the compass rose.
When using a compass with a compass needle, you need to orient the compass rose to north, which you do by rotating the compass rose until the
north end of the compass is exactly over north on the compass rose.
That is a bit of a pain.
In this tutorial we will use an open face
compass with a floating compass dial marked 0 - 360 degrees.
See Figure 1.
The beauty of a compass with a floating compass dial is that the compass magnets are built into the floating compass dial, so it automatically
aligns to north, and you do not have to align it manually.
To locate your position, first locate two or three landmarks that you can also locate on your map.
A landmark can be a water tank, church spire, radio antenna, or a distant hilltop.
Ideally, you want two landmarks about 90 degrees apart, or three landmarks about 120 degrees apart.
You do not want any two landmarks to be in a nearly identical heading, or 180 degrees apart.
It is also critical that you can clearly see the landmarks and locate them on your map.
Now find the magnetic bearing for the first landmark.
Face the direction of the landmark and hold your floating compass dial pocket compass just below eye level.
Make sure you hold the compass steady and level.
Sight the landmark across the center of the compass bearing and read the degree marking on the compass dial directly away from you.
Note this heading because you will need it to find your location on your map.
In Figure 2, we are sighting a bearing of 222 degrees.
Repeat this procedure with the next landmark and note this second angle.
For our example, the second landmark is in the direction of 186 degrees.
If you can identify a third landmark and determine its magnetic bearing, that is even better.
Click for larger image
Click for larger image
Now transfer your landmark headings directly onto your map.
Lay your map out flat and locate the compass rose on the map.
Sometimes there are two concentric compass roses, with one in a slightly different orientation from the other.
If this is the case, use the one marked "magnetic."
You can either transfer a parallel line between your landmark and the map's compass rose,
or you can place your pocket compass directly onto the map and transfer the landmark heading to the map's compass rose by moving the compass.
With the compass aligned to the map, you can use its compass rose to draw a
line through the first landmark using the angle of the first landmark's bearing.
Repeat this with your other landmarks, drawing a line for each in the direction of its direction.
If you do not have a compass rose on your map, then north is up, east to the right is 90 degrees, south is 180, and west is 270 degrees.
The problem is that these are probably referenced to true north, and your pocket watch style compass reads magnetic north.
The difference between true north and magnetic north is the local magnetic declination, which depends on your general area.
You need to determine the declination in your area.
The magnetic declination in Los Angeles is 13 degrees east.
New York City is 13 degrees west.
If you are lucky enough to be lost just west of the Mississippi River on the agonic line where the declination is zero,
then magnetic north and true north are the same, so your compass reads true north.
If you are not so lucky, simply adjust your compass readings for your declination.
For New York where the declination is 13 degrees west, just subtract 13 degrees to your compass reading to get a heading referenced to true north.
In the Los Angeles area with 13 degrees east declination, add 13 degrees to your compass reading.
And get a better map.
Now that you have your location, verify that it is reasonable.
Using the example in Figure 3, you should be near a reservoir and near the top of a ridge.
The ridge near you should be running roughly east-west and the closest shore of the reservoir should be southwest of you.
Refer to the topographic map in Figure 3 and note the 222 and 186 degree lines.
Where the two lines intersect is your location.
If you have three landmarks, consider your position to be in the triangle formed by the three lines.
The triangle should be fairly small.
If your triangle is large something is wrong.
You probably incorrectly identified a landmark, misread the heading, or did not draw the lines correctly.
This is a good reason to use three landmarks, as two lines from two landmarks will not show a problem.
The more landmarks you use, the more confidence you will have in your position.
Click for larger image
Copyright © 2010 Eric Newman * All rights reserved