Repeating circle

In the mid-1780s Charles de Borda made several improvements to geodetic instruments for measuring the distance between two points. He devised a repeating circle based on the principle of measurement by triangulation, enabling the determination of a distance no longer by measuring the terrain, but by considering the angles formed by elevated reference points (church towers, castles, trees). When two angles and one side of a triangle are known, the length of its other two sides can be deduced. The margin of error is reduced by taking more measurements. In 1792 Jean-Baptiste Delambre and Pierre Méchain used the repeating circle to measure the distance of the Dunkerque–Barcelona arc as part of determining the length of the metre.