Learn how to use the lensatic compass used by the US Military. Plotting a bearing on a map using a round lensatic compass. Sighting the bearing. To determine our location we are going to combine two pieces of information. Use a compass to explore, navigate, map terrain and travel around the Engineers, the engineer compass is also called a lensatic compass.

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To determine our location we are going to combine two pieces of information on our map. We know that we are somewhere along the shoreline of the lake. Using a compass we can sight a bearing to our cabin across the lake. When we plot the bearing on our map, our location will be where the line between our location and the cabin crosses the shoreline of the lake.

Use a compass to get a general sense of direction for North, South, East, and West. In our example at the lake, our cabin is NE of our current location. Refer back to this “reality check” occasionally during plotting to make sure what you are doing makes sense. Using our compass we have sighted a bearing to our cabin of mils from Magnetic North. The north reference is an integral part of any bearing. Make sure you include it when you say or write a bearing.

We can abbreviate to either mils Mag. We will be plotting our bearing relative to Magnetic North, so there is no need to adjust for a different north reference. Our bearing was taken from an unknown location, towards a known location. When we plot it on the map, we will start plotting at the known location, and extend the bearing line back towards the unknown location.

This is known as a back bearing. For more information on forward and back bearings, see our tutorial on forward and back bearings. We will be orienting our map with Magnetic North.


To do this we need to establish an accurate Magnetic North reference line on the map. You may be tempted to use the MN line on the declination diagram, but be careful. The declination diagram may not hkw drawn with an accurate angle, or the angle may not represent the current declination. The smaller the angle to Magnetic North, the more likely the diagram will show a larger angle.

The older the map, the more likely the declination shown in the diagram is no longer correct. Generally once you have drawn your own Magnetic North line, you will not need to do this again on this map sheet for many years. Using a protractor aligned with either True or Grid North depending on what the magnetic declination you are given is referenced tomake a mark on the map at the desired value.

Plotting a bearing on a map using a round lensatic compass

Then use a straight edge to draw and label the Magnetic North Reference line. A round lensatic compass has no straight edge to align. Instead you will use an imaginary line that runs down the center of the compass. Two parts of this line include the sighting wire, which you can compsas with things that are visible on the map, and the center of the rotating compass card. Some of these compasses have a notch at each end of the housing along this line.

If your compass does not have these notches, you should add them. Use a straight edge aligned with the sighting wire and central pivot point.

Mark the position of the notches to be with a pencil. Use a file to make a permanent notch at each end.

Or make a permanent scratch mark, paint line, or permanent marker line on each end. You will use these notches to align your compass with a line on the map or to mark the location of a bearing line your are about to draw. Note that we are using the magnetic dial in the compass.


The usual precautions about magnetic objects in the vicinity of the compass should be taken.

Plotting a bearing on a map using a round lensatic compass

Pay particular attention to the surface your are working on. Many wooden tables also have bolts, nails, screws, and other metal components. Don’t even consider using the hood or tailgate of a vehicle. Make a mark with your pencil at the notch on the end furthest from the sighting wire. Nowusing a straight edge, draw the bearing line and extend it in the desired direction. In our example will will extend the line as a back bearing.

The location we sighted the bearing to the cabin is where the back bearing intersects the lake shore. In order to detect any errors we may have made, it is recommended that you plot at least one additional bearing to a different known location. The need to carefully orient the map, and the precautions about metal in the vicinity of the compass makes this type of plotting more difficult uwe most other methods.

Perhaps this is why most military personnel are trained to plot bearings using a protractor on a Coordinate Scale and Protractor tool. Plotting a bearing on a map using a round lensatic compass Sighting tl bearing To determine our location we are going to combine two pieces of information on our map. Rough direction to the cabin.