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.
