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The reverse of the ''intersection'' technique is appropriately termed ''resection''. Resection simply reverses the intersection process by using ''crossed back bearings'', where the navigator's position is the unknown.<ref>Mooers, p. 132–133</ref> Two or more bearings to mapped, known points are taken; their resultant lines of position drawn from those points to where they intersect will reveal the navigator's ___location.<ref>Mooers, p. 132–133</ref>
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When resecting or fixing a position, the geometric strength (angular disparity) of the mapped points affects the precision and accuracy of the outcome. Accuracy increases as the angle between the two position lines approaches 90 degrees.<ref>Seidman, David, and Cleveland, Paul, ''The Essential Wilderness Navigator'', Ragged Mountain Press (2001), ISBN 0-07-136110-3, p. 100</ref> Magnetic bearings are observed on the ground from the point under ___location to two or more features shown on a map of the area.<ref>Mooers, pp. 129–134</ref><ref>Kals, pp. 43–49</ref> Lines of reverse bearings, or ''lines of position'', are then drawn on the map from the known features; two and more lines provide the resection point (the navigator's ___location).<ref>Mooers, pp. 129–134</ref> When three or more lines of position are utilized, the method is often popularly (though erroneously) referred to as [[triangulation]] (in precise terms, using three or more lines of position is still correctly called ''resection'', as angular [[law of tangents]] ([[cotangent|cot]]) calculations are not performed).<ref>Touche, Fred, ''Wilderness Navigation Handbook'', Fred Touche (2004), ISBN 978-0-9732527-0-5, ISBN 0-9732527-0-7, pp. 60–67</ref> When using a map and compass to perform resection, it is important to allow for the difference between the magnetic bearings observed and grid north (or true north) bearings ([[magnetic north|magnetic declination]]) of the map or chart.<ref>Mooers, p. 133</ref>
Resection continues to be employed in land and inshore navigation today, as it is a simple and quick method requiring only an inexpensive magnetic compass and map/chart.<ref>Mooers, pp. 129–134</ref><ref>Kals, pp. 43–49</ref><ref>Touche, pp. 60–67</ref>
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{{main|Resection (surveying)}}
In surveying work, the most common methods of computing the [[coordinate]]s of a point by resection are [[Giovanni Domenico Cassini|Cassini's]] Method and the [[Tienstra formula]], though the first known solution was given by [[Willebrord Snellius]] (see [[Snellius–Pothenot problem]]). For the type of precision work involved in surveying, the unmapped point is located by measuring the angles subtended by lines of sight from it to a minimum of three mapped (coordinated) points. In [[geodesy|geodetic]] operations the observations are adjusted for [[spherical excess]] and [[projection variation]]s. Precise angular measurements between lines from the point under ___location using [[theodolite]]s provides more accurate results, with trig beacons erected on high points and hills to enable quick and unambiguous sights to known points.
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