resection

Resection is a surveying method for determining your own position on the ground when you have a map and can see at least two (preferably three or more) distant points whose map locations are already known. You take compass bearings or angles to those visible features, then plot those sight lines back onto your map; where they intersect is your position. It is the inverse of radiation, where you know your position and measure outward to unknown points.

The method works because bearing lines from different observation points will converge at only one location. With two known points, you get two sight lines that cross at a unique spot; a third point confirms the answer and catches mistakes. The known points are typically landscape features like church spires, hilltops, water towers, or marked survey stations visible from your location. On the map, you draw a line at the reciprocal bearing from each known point (the bearing you would see if standing there looking back at you), and these lines meet where you stand.

In practice, resection is faster and requires less equipment than other methods of finding your position in unfamiliar terrain. A compass and a clear map are enough. Accuracy depends on the angle between your sight lines: angles close to 90 degrees give tight intersections, while shallow angles create large uncertainty zones. Resection becomes unreliable if all visible known points lie in a narrow arc around you, because small bearing errors then produce large position errors.

When and why resection matters

Field workers use resection to relocate themselves after becoming disoriented, to verify a computed position, or to establish a checkpoint without carrying survey instruments to the site. It is common in wilderness navigation, military mapping, and preliminary site surveys. The method also helps when you need to identify your exact location for safety reasons, such as alerting rescue services or confirming property boundaries in rough country where direct measurement is impractical.

The term comes from the geometry: you are re-sectioning the map by creating new lines that cut through known positions. Historic surveyors performed resection with a three-arm protractor called a trisector, which mechanically solved the angles. Modern practice uses a compass, straightedge, and the map itself. The method has been standard in land surveying and military reconnaissance for centuries.