Land Navigation Practical Exercise – Resection

In some of our previous articles, we’ve tackled the issues of preparing yourself to pass the Land Navigation course which is always a very tough one. Today we’ll do a practical exercise on finding your location if you are unsure of your exact map placement.

The official term is Resection 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.

For the purposes of our practical application, we used only two but obviously, three or more is ideal. There are three methods of using resection, using the map and compass method, the straightedge method and the modified method. In this exercise, we used the map and compass method, but we’ll go over the second two as well.

Map and Compass Method

Orient the map using the compass and terrain association
Identify two or three known distant locations on the ground and mark them on the map.
Measure the magnetic azimuth to one of the known positions from your location using a compass.
Convert the magnetic azimuth to a grid azimuth.
Convert the grid azimuth to a back azimuth. If the magnetic azimuth is less than 180 degrees add 180. If the azimuth is more than 180 degrees subtract 180.
Using a protractor, draw a line for the back azimuth on the map from the known position back toward your unknown position.
Repeat the same steps for a second position and a third position, if desired.

The intersection of the lines is your location. Determine the grid coordinates to the desired accuracy, an eight-digit grid being the preferred accuracy for our purposes.

Straightedge Method:

To accurately find your position, the successful navigator must:

Orient his map using his compass and/or terrain association
Locate at least two known distant locations or prominent features on the ground and mark them on the map.
Lay a straightedge on the map, in this case using the compass and a known position as a pivot point.
Rotate the straightedge until the known position on the map is aligned with the known position on the ground.
Draw a line along the straightedge away from the known position on the ground toward your position.
Repeat the same process using a second known position that you can see from your location.

Where the intersection of the lines meet on the map is your location. Determine the grid coordinates to the desired accuracy. You then should be able to get an eight-digit grid coordinate and find your way to your point.

Modified Method:

The modified method is the least preferred and the least accurate 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.

Check out the attached video, we’re still trying to iron out some issues with these, it is process. But it does show you the practical exercise to get the correct information.