Triangulation surveying (17th-century France)

Timeline

1. 1671–1673Paris to Amiens (northern France) · FranceBetween 1669 and 1671 Jean Picard carried out continuous measurements across 13 triangulation stations along the Paris meridian, first using a telescope with crosshair reticle for precise sighting combined with a quadrant to read angles, measuring the length of one degree of meridian arc as 110.46 km, and in 1671 published Mesure de la Terre to announce the results to Europe. This data was later used by Newton to check calculations of the Earth's radius in the law of universal gravitation, a milestone in triangulation becoming an international standard.
2. 1672–1673Cayenne, French Guiana and Paris · South AmericaIn 1672 the French Royal Academy sent Jean Richer to Cayenne in French Guiana, where he and Cassini in Paris simultaneously observed the opposition of Mars, using triangular parallax to determine for the first time the solar parallax (about 9.5 arcseconds) and the Earth-Sun distance. Richer also found that pendulum clocks ran slow at Cayenne, implying weaker gravity at the equator, foreshadowing later debates over Earth's oblateness and demonstrating the scientific power of combining triangulation with astronomical observation.
3. 1676–1680France (multiple field survey stations) · FranceBuilding on the success of Picard's meridian measurement, Cassini I and Philippe de La Hire began around 1676 extending the triangulation network across all of France. Using the Paris meridian as a baseline axis, they expanded control triangles east and west, gradually covering France's principal provinces. This work laid the survey skeleton for the 18th-century Cassini map (Carte de Cassini, 1744-1793) and is the direct precursor of the world's first national topographic map based on triangulation.