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.
Showing posts with label true north. Show all posts
Showing posts with label true north. Show all posts
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.
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.
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