In our previous Land Navigation article, we talked about Resection, which is invaluable at finding your exact location if you’re unsure of where you are but can see and identify points in the distance.
As you’ll remember Resection is the method for finding your exact location on a map by determining the grid azimuth to at two or more well-defined locations that can be pinpointed on the map. For greater accuracy, the desired method of resection would be to use three or more well-defined locations.
We spoke about the three methods but I received an email from a prospective Special Operations candidate named Jim who was confused about what constituted a Modified Resection problem and how one would use this method effectively.
Good question Jim, here is a scenario that can and does frequently happen. You’re moving along on your azimuth through wooded but fairly flat terrain with few terrain features but enough foliage to keep you from seeing very far in the distance.
You cross a large linear terrain feature, a road, a creek bed or in my case for the purpose of our practical exercise today a railroad line. Now if you are close to your point, you could be to the left or the right of where you need to go. If you can see a point in the far distance, you can shoot an azimuth to it and pinpoint your exact location.
Now we’ll revisit the exact sequence for pinpointing your location using the modified resection method we spoke about the other day.
Modified Resection Method:
The modified method is the least preferred method but is useful in certain situations. If you’re not certain of your exact location but are located on a linear terrain feature such as a road, creek or similar body of water, this can pinpoint your position.
- Orient the map using a compass and/or by terrain association.
- Locate a point in the distance that can be easily identified on the ground and on the map.
- Determine the magnetic azimuth from your location to the distant known point.
- Convert the magnetic azimuth to a grid azimuth.
- Convert the grid azimuth to a back azimuth, using the add or subtract 180 accordingly depending on the degrees.
- Using a protractor, draw a line for the back azimuth on the map from the known position back toward your unknown position.
Your location is where the line on the map crosses the linear feature. Determine the grid coordinates to the desired accuracy, for our purposes an eight-digit grid coordinate.
For our video demonstration, practical exercise, we were moving along our azimuth and came across a railroad track. Just barely visible in the far distance is a wind turbine that sits high upon a hilltop. Once we are able to locate the wind turbine on the map, we take an azimuth, using our compass, and determine the grid azimuth.
Then we draw a back azimuth using the “more than 180 degrees subtract 180 or less than 180 degrees add 180” rule and draw a line on our map. Where the line intersects the railroad line is our exact location.
If this were close to our point during a Land Navigation course, this would be a prime attack point as you’d have a very good place to search for the point. However, if you’re still far from your point, it is a great place to check your exact location and ensure that you’re heading on the correct azimuth.
Photo courtesy of DOD