Lensatic Compass: A Better Protractor

The normal method to transfer angles from your lensatic compass to your map and back is with a protractor and a pencil. From your known location on the map, you take your protractor, line it up to true north, find the angle to your next point, adjust for declination, and then shoot that angle with your compass. At that point you know what direction you need to head to get to your next stop, and if you pair that with an accurate distance you'll be on your way in no time.

The only problem is that this process takes time and tools. You need to write things down, draw lines, perform math, and make sure you do so with a reasonable degree of accuracy. There's a lot of room for error and mistakes, and it takes time, which is sometimes a very precious commodity.

There's a better way, I believe, from both a time and an accuracy standpoint.

Since the ultimate goal is to transfer the angle between the map and compass, why not do it directly? That way you skip the math, the protractor, the lines, and get it all done quicker.

So, take your map and lay it down on a flat surface (i.e. the ground). Take your lensatic compass and open it up all the way so it is one flat line. Lay the compass down on the map so its long side is parallel with the north/south axis of the map. I generally line it up with a north/south gridline.

Next, rotate the entire thing (map and compass) until your compass is telling you that it's pointing north. Now your map's true north is lined up to your local magnetic north, and you've just completely circumvented the whole problem of declination (also removing a potential source of error since declination changes with time).

From here it's simple. All you do is take you compass and line it up from your current point to your desired destination and look at the compass reading. That's the heading that you need to move along to reach your desired end (or way) point. Pick up your map and head out!

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.

How to Use a Lensatic Compass, Part I

Declination refers to the angle between magnetic north and true north. To make sure we're all on the same sheet of music, let's quickly define both of those terms as well.

Simply stated, Magnetic North is the direction that your compass will point at all times. As you rotate the compass (or yourself), the needle will always align with magnetic north, and thus you can see what direction you are actually facing by taking a reading from the compass. Magnetic north varies by location and time. That is, depending on where you are on the earth the direction that your compass indicates as north will be slightly different, potentially by as much as 30 degrees or more. Additionally, magnetic north is shown to change by several degrees in any given location over the span of 100 years. That means maps drawn up with readings from the 1800's will no longer be accurate. This is all due to changing fields around the earth, iron deposits, and local geography changing.

True north is defined as the direction of travel if you were to move directly towards the north pole. One ways of finding it without knowing magnetic north in the area is to look for Polaris in the sky, also known as the north celestial pole. This is as close to true north as you're likely to get.

Maps are drawn relative to true north, and compasses give you bearing relative to magnetic north. So if you've been following thus far, you can see the problem. If you have a map drawn to true north and a compass that is telling you north is 30 degrees to the east of that, you're actually travelling NNE relative to the map.

Cartographers reconcile this problem by giving the local declination angle when they produce maps. This means that you should look for it (called the GMA, or Grid Magnetic Angle) and adjust your compass accordingly. On a lensatic compass you can typically rotate and lock the dial such that your readings will automatically account for the declination angle, and this is the easiest and most fool proof way of addressing the issue.

Declination angles are expressed on maps in several different ways. For example, 10° W and -10° are the same way of expressing declination. Similarly, some maps will have an angle drawn on them with the declination angle indicated at the vertex of the drawing.

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.

Welcome To the Lensatic Compass Guide

Welcome to the Lensatic Compass Guide! The intent of this blog is to provide you with information on the lensatic compass, including where to buy them, how to use them, maintenance, and general interest information. If you have any questions about the lensatic compass that isn't covered, please feel free to leave a comment or send an email to lensatic_compass (at) gmail (dot) com.