Resection (orientation)

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

In surveying, the resection method is used to map the position of an unmapped feature or point by measuring its position relative to three (or more) mapped points. The geometric strength of the mapped points that are used 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 unmapped point is mapped by measuring the angles subtended by lines of sight from it to a minimum of three mapped (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 only 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.

ee also

*Triangulation
*Trilateration
*Real time locating
*Orienteering

External links

* [http://www.map-reading.com/resect.php Map-reading.com guide to resection]
* [http://www.survivaliq.com/navigation/direction_par8.htm SurvivalIQ guide to resection]