Triangulation
Fixing the position of a distant point by measuring angles to it from the two ends of a baseline of known length.
Triangulation finds where something is by measuring angles. Stand at two points a known distance apart, the baseline, and measure the angle from each end to a distant target. You now know one side of a triangle and the two angles at its ends, and trigonometry fixes the third corner, the target, along with its distance.
Surveyors used it for centuries to map whole countries. They measured one baseline very carefully on the ground, then built a chain of triangles across the landscape from hilltop to hilltop, measuring only angles. Willebrord Snellius in the Netherlands did this between Alkmaar and Breda in 1615, about 116 kilometres, using 33 triangles; his result was 3.5 per cent short. The Cassini family triangulated the whole of France in the 18th century, and the Great Trigonometrical Survey of India, begun in 1801, mapped the Himalayan peaks, including Everest.
The weakness is that small errors in angles grow along long chains, and flat-ground formulas must be corrected for the curvature of the Earth. Surveyors limit the problem by laying out a network of large triangles first and filling in detail afterwards.
The same method works in the sky. Two cameras that film the same meteor from different places can triangulate its path through the atmosphere, its speed and where any fragments may have landed.
Triangulation uses angles. Trilateration, the basis of satellite navigation, uses distances instead.
Coming up in future stories
- The Meteorite That Hit a $300 Chevy 1992 · Peekskill, New York
- The Metre Is Wrong, and He Knew 1793 · Barcelona, Spain
- Was Everest Really Made Two Feet Taller? 1849–1856 · plains of northern India and the Survey office, Calcutta