How to Use a Lensatic Compass, Part II

In the last article we talked about declination, and how magnetic north and true north will vary depending on location and surrounding features. Next we're going to talk about some of the uses of a lensatic compass, starting with the resection.

Resection with a lensatic compass is a process whereby you can determine your own location from that of two known points in the distance. Typically these features are prominent, such as a hilltop, a man-made structure, or an intersection, but they can be more nebulous if you're in a tight spot.

The key to the whole operation is having a map and being able to locate these features on that map. Then, from where you are, shoot an azimuth to the feature as accurately as possible, and calculate the back azimuth from that reading. The back azimuth is nothing more than the opposite direction of the reading you're taking. That is, take the azimuth you recorded and subtract 180 degrees from it, or 1600 mills if using a military lensatic compass.

Next, do the same for the second point.

Now, take your map and draw lines (at the back azimuth angle you calculated) on the map through the features that you're using as reference points. Those lines will intersect at some point if you're done your resection properly. The point where those lines cross is your location, and the accuracy of that location is only as high as the readings and calculations that you've made.

To check your process, and even add a little more accuracy to it, find another feature and perform the same steps. That line should intersect reasonable close to where the other lines crossed. The more back azimuths you can draw, the more accurate your location will be.

This, in a nutshell, is how to perform a resection with a lensatic compass.

What is a Lensatic Compass?

I get this question more often than you would think. The term "lensatic compass" is is used most frequently in the military, and only in reference to one specific model of compass. Most people assume that lensatic compass is the "proper" term for all hand-held compasses, but that it not the case.

There are several different types of hand held compasses that are designed for specific uses, and a lensatic compass is only one of them. An orienteering compass, for example, is designed for ease of use when taking a bearing from a map and heading off at full speed to the next point on a course. While speed is important in many tactical situations, a lensatic compass is not necessarily designed for speed. Rather, it is designed for accuracy and ruggedness.

Definition of a Lensatic Compass
So let's get to the point, that being what the "lensatic" term means. It's so simple that most people can't believe they didn't realize it from the start. It simply means that there's a lens on the rear sight used ofr magnifying the card dial and taking a more accurate reading. That's it. A lensatic compass is one with a magnifying lens on the sight.

Going back full circle to the specific purposes that compasses are built for, we realize that a military compass would be used fairly frequently to take sightings. Whether laying in artillery, taking a sighting to a target, or performing a resection to figure out how far you're off course, the lensatic compass excels at sightings and accuracy. An orienteer (a good one, anyway) doens't need to do this very often. Then look at their map, slap the compass down, twist the house to find the bearing to their next point, and take off running. No sighting necessary.

So from now on, when you hear the term lensatic compass, think lens, think accuracy, think ruggedness, and think sighting. It's that simple.