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.


Marcus said...

Hey there!
Are prismatic compasses generally better than Lensatic compasses? Or would it be rhetorical to compare since they serve different purposes.

Ranger Rick said...

Great question. Prismatic compasses are generally considered to be a specialty item, and accurate to fractions of a degree when hand-held. When tripod-mounted they are even more accurate.

That being said, lensatic compasses are divided into 6400 mills, so if you could get 1 mil accuracy you'd be down to about 1/17 of a degree. Practically speaking, though, I don't think 1 mil accuracy is realistic, especially hand-held. Even in the military when they want to lay in artillery batteries from a survey point they use much more accurate, tripod-mounted, survey equipment.

So long answer to a short question - from my experience the prismatic hand held compass is slightly more accurate, but not by any useful margin in real conditions.

Marcus said...

Thanks for the indepth reply :)
Just last week i was playing around with an army issued prismatic compass and hours later i found a lensatic compass at a shop. Both seemed to have similar "parts" (in lay man terms, i have no idea what they are really called), rotating bezel , pop up cover with vertical line , foldable thingy to view the degrees closely.

HikerPilot said...

It will be helpful to folks who are really new/trying to grasp the basics, to add additional instruction on how to actually do what the 5th paragraph says, ("Now, take your map and draw lines...")

Perhaps a simple way to introduce the two methods for drawing the lines would be to add the following after the last sentence, "This, in a nutshell. . . ":
"To draw the back azimuth lines, which are relative to Magnetic North, you may chose between two methods: 1) Apply the declination (previously explained) to convert from Magnetic Back Azimuth to True Back Azimuth, then lay a protractor on the nearest line on the map running True North or True East-West (you may have to connect points at top and bottom, or sides, of the map to have any such True North or True East lines to measure from). Or, 2) without using math and a protractor, you may align your map to the real world using the Lensatic compass and then set your Magnetic Back Azimuth on the face of the compass and directly draw the back azimuth using the edge of the compass when properly set up. This will be explained in more detail in the next article, "The Lensatic Compass: A Better Protractor".

taj k said...

Ranger Rick,
I'm confused; for finding a back azimuth with a military lensatic compass, you said to subtract 1600 mils. My question is, doesn't it need to be 3200? Was that a typo or am I missing something?

Ranger Rick said...

@HikerPilot - very good suggestion, thank you.

@taj k - you're correct! Typo! Good catch.