Resection (orientation)

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Resection is a method of orientation (direction or position finding) using a compass and topographic map.

In surveying, the resection method makes it possible to find the position of an unknown target by measuring its position relative to three (or more) known stations. The geometric strength of the used station positions will affect the precision and accuracy of the outcome. The most common methods of computing the coordinates of a point by resection are Cassinis Method and the Tienstra formula, though the first known solution was given by Willebrord Snellius.

The unknown point is established by measuring the angles subtended by lines of sight from the new point to a minimum of three known (coordinated) points. In geodetic operations the observations are adjusted for spherical excess and projection variations.

Magnetic bearings are observed on the ground from the point under location to three or more features shown on a map of the area. Lines of reverse bearings are then drawn on the map from the features; two lines indicate the location of the observed point at the intersection, and three and more lines provide the resection point. The difference between the magnetic bearing observed and grid bearings of the map needs to be taken into account. This is a simple and quick method requiring an inexpensive magnetic compass, with application in navigation.

Precise angular measurements between lines from the point under location using theodolites provides more accurate results, with trig beacons erected on high points and hills to enable quick and unambiguous sights to known points.