How did surveyors measure Everest without climbing it?

Surveyors measured Everest by triangulation: they never needed to climb it. Triangulation fixes a point by measuring only angles to it from the two ends of a fixed baseline, so the point becomes the third corner of a triangle with one known side and two known angles. The Great Trigonometrical Survey of India, begun in 1802 by William Lambton under the East India Company, chained these triangles across the subcontinent.

The method rewards care at the start. The initial baseline was measured with great care, since the accuracy of the whole survey depended on it. Error is minimised if a mesh of large triangles is established first, and points inside are located from it. Surveyors built a gridiron of triangulation chains running north to south and east to west, rather than triangulating all of India. The Company thought the project would take about five years, but it took nearly 70.

The Himalayas were the hard part. Surveyors were forced to work from the Terai, a region south of Nepal, where torrential rains and malaria killed three survey officers and forced two others to retire. From 1847 they made detailed observations of the peaks from stations up to 240 km away, with weather limiting work to the last three months of the year.

The answer came from a desk, not a summit. In 1852 Radhanath Sikdar, an Indian mathematician and surveyor from Bengal, was the first to identify Everest as the world's highest peak, using trigonometric calculations based on measurements by the surveyor Nicolson. The figure was announced in 1856, and an Indian survey recalculated it as 29,029 ft (8,848 m) in 1955.