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Tuesday, May 25, 2010

Urban Survival Skills – Land Navigation, Resection Using a Compass

Our previous map reading posts were meant as an orientation to the individual new to Urban Survival Preparation and we will continue to drip land navigation tips and skills. The intent is for the fledgling Urban Survivor to practice these techniques and tips, first in a controlled environment building to a rehearsal like practice either on similar terrain to, or the actual terrain he/they may be moving through if withdrawing from their urban location to their prepared safe location.

The technique in this post is called resecting your position from a known point. The military calls this intersection and resection. It is basically locating two (or more) terrain features on the map then finding them visually. These terrain features should be easy to pick out and will most probably be hills, mountain peaks or can even be man made structures that are annotated on your map.

Again, resection is used to determine your unknown position by determining lines of bearing or azimuths from known terrain features.

The steps are:

From your unknown position you determine two or three terrain features, ideally 120 degrees apart, that you can locate on the map.

You shoot a compass azimuth from your unknown position to each terrain feature, then compute the back azimuth. The back azimuth is simply your azimuth plus 180 degrees or minus 180 degrees. If you are at an unknown point and can identify a mountain both visually and on the map and you shoot a compass azimuth from your unknown point to the Mountain, lets say the magnetic compass azimuth is 170 degrees, you can determine that from you to the Mountain the direction is 170 degrees magnetic and from the Mountain to you the magnetic azimuth is 350 degrees magnetic (170 degrees plus 180 degrees).

You will have to convert the magnetic azimuth to a grid azimuth (see previous post), then plot (on your map) a line coming from the mountain at 350 degrees. Your unknown position is somewhere on that 350 degree line of bearing that you plotted.

If you find another terrain feature and do the same, you will have two lines of bearing (or back azimuths) that cross. Where they cross is your unknown position.

Again, from each known terrain feature plot the back azimuth on the map, after converting for G-M angle. Where the lines intersect is your unknown, now known, position.

1 comment:

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