Home > Uncategorized > Finding your place on the map

Finding your place on the map

If you are out in the field, how do you know where you are? If you have a map and compass, you have the tools, but you may not know how to use them together. We all know that compasses point north, and that maps show a section of land on paper. But to locate yourself on the map takes something called resection.

An orienteering compass has a rotating bearing that sits in a plate; its cousin, the lensatic compass, has a folding housing. Both types can be used for in the field, but the lensatic compass is recommended for this purpose, as it is much more accurate.

To find yourself on a map, pick two landmarks, preferably 90 degrees apart as seen from your position – the closer to 90 degrees, the better. Measure the bearing to one of the landmarks and write this number down. Add or subtract the magnetic offset for that region, then add 180 (if the number is between 0 and 180 degrees) or subtract 180 (if it is between 180 and 360) to find what is known as the back azimuth. This represents a line that can be drawn on a map, intersecting the landmark and the back azimuth bearing. Repeat the process for a second landmark. When the back azimuths are both drawn on a map, the intersection is where you are.

Example: the bearing to a water tower is 310 degrees, the bearing to a cabin is 230 degrees, and the local magnetic declination is 4 degrees West. This gives
310 + 4 -> 314 degrees – 180 = 134 degrees rev. az. 1
230 + 4 -> 234 degrees -180 = 54 degrees rev. az. 2
(If the magnetic declination is east instead of west, then subtract instead of add.) Place a protractor over the water tower on the map and mark 134 degrees relative to that point, then draw a line from the tower through the 134 degree mark. Repeat for the cabin. Where the two lines intersect is where you are.

This is basically what radio direction finding equipment does when it attempts to locate your position from your transmission, but in reverse. Where the rdf equipment uses two locations to make observations on a single point, you are using one location to make observations on two points. Either way, the math works out the same.

So this weekend, while you’re testing out your cold weather preps that Arctic Patriot has given an excellent set of lessons on, make sure you take a map and compass along and see if you can find your position with those mittens or gloves on.

Might as well practice two things at the same time, if you can.

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Categories: Uncategorized Tags: , ,
  1. 10 February 2011 at 00:56

    Intersections and resections are very valuable tools to have in your land nav tool box. In the days of cheap GPSs, these skills are fading.

    Thanks for the links!

    AP

  2. 11 February 2011 at 14:38

    Forgot to mention – always make your marks in pencil, lightly, and erase them after you no longer need them. You don’t want your enemy to know where you’ve been if you should be careless/unfortunate enough to lose the map.

  3. 11 February 2011 at 15:46

    One thing you didn’t hit on. That is what points or features are good to use for intersection and what are not. These make or break resection. A hilltop is good in an area where there are only 3 hills in a couple square miles but not so good in foothills where there are dozens of them in a square mile. Ditto for ponds or trails.

    Personally I am a big fan of tac points and hard backstops. A great example of a a viable tac point is where two distinctive trails or unimproved roads intersect or break into a Y. A hard backstop would be something that spans the area you are operating and are impossible to mess up. If there is 1 north south running paved road and you know you’re west of it that means something. If you are 90 degrees off the improved road and 170 degrees from the old quarry that means something. 90 degrees from a hill and 170 degrees from a creek probably doesn’t mean squat.

  4. 13 February 2011 at 20:44

    Excellent points, Ryan. The landmarks should be both distinct from other landmarks and relatively unique to the area, or you’re just not being precise enough.

  1. 29 April 2011 at 14:02

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