Resection has multiple meanings:
Resectioning involves enlarging the cross-section of a river channel by deepening or widening the river to increase its hydraulic efficiency. This allows a larger discharge to be contained within the channel. Dredging is used to remove surplus sediment from the river bed and vegetation clearance is used to remove it from the bed and banks.
Resection in surgery means the removal of part of an organ or structure.
The resection method enables you to find the position of an unknown target by measuring its position relative to three (or more) known stations. The geometry of the station positions you use will affect the precision and accuracy of your findings. The most common methods of computing the coordinates of a point by resection are Cassinis Method and the Tienstra formula. 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.