I recently picked up a book on map and compass navigation at
a local library book sale. In
reading though it, I was particularly interested in a topic they called
“Resection.” If you’re in the
medical profession, you’re familiar with that term, as it means to surgically
remove part of an organ. In
navigation, though, it has a different meaning. According to the US military, it is defined as “the method
of locating one's position on a map by determining the grid azimuth to at least
two well-defined locations that can be pinpointed on the map.”
In my book, Outdoor Navigation with GPS, I used the somewhat more familiar term,
triangulation. Basically, it is a
way to find your position on a map by measuring the bearing to at least two
known objects—things like mountain peaks, radio towers, or any other objects
you can reliably identify on the map.
You measure the bearings to the objects with your compass, then draw
lines through each of the objects on the map at the bearings you measured. Your position is where all the lines
intersect.
With the exception of Outdoor Navigation with GPS, every book I have ever read on the subject stops
there. It’s as if you can find
your position to pinpoint accuracy just by following those simple
instructions. But there’s more to
it than that. Would you know the answers to these questions?
- How accurately do you need to measure the bearings to each object?
- Will your measurements be more accurate if the objects you measure are far away or nearby?
- Will your measurements be more accurate if the objects are close to each other or far away from each other?
- Under what conditions would your triangulated position be in error by more than a mile, even if you have accurately measured and plotted your bearings?
Here are some quick answers. For more details, refer to Outdoor Navigation with GPS:
- Try to measure bearings as accurately as possible. With care, you should be able to measure a bearing to an accuracy of about 2 degrees.
- The accuracy to which you can find your position will be better if the objects are nearby. If the object is a mile away and you measure its bearing to an accuracy of 2 degrees, the uncertainty in your location is less than 200 feet. If the object is 10 miles away, your uncertainty grows to nearly 2000 feet.
- Try to measure objects that are at right angles to you. Avoid objects that are in line or 180 degrees apart.
- The error in your position can be more than a mile if the objects you are measuring are far away (more than 10 miles), the accuracy of your measurement is no better than 5 degrees, and the objects are not at right angles to you.
Here’s a diagram from my book that illustrates the
concepts. Notice how your position
accuracy is better at Position 1, where the objects are nearby, than at
Position 2, where they are farther away:
