Errors in the meridian arc of Delambre and Méchain determination, which affected the original definition of the metre, were long known to be mainly caused by an uncertain determination of Barcelona's latitude later explained by vertical deflection. When these errors where acknowledged in 1866, it became urgent to proceed to a new measurement of the French arc between Dunkirk and Perpignan. The operations concerning the revision of the French arc linked to Spanish triangulation were completed only in 1896. Meanwhile the French geodesists had accomplished in 1879 the junction of Algeria to Spain, with the help of the geodesists of the Madrid Institute headed by the late Carlos Ibañez Ibáñez de Ibero (1825-1891), who had been president of the International Geodetic Association (now called International Association of Geodesy), first president of the International Committee for Weights and Measures and one of the 81 initial members of the International Statistical Institute. Until Hayford ellipsoid was calculated in 1910, vertical deflections were considered as random errors. Plumb line deviations were identified by Jean Le Rond d'Alembert as an important source of error in geodetic surveys as early as 1756, a few years later, in 1828, Carl Friedrich Gauss proposed the concept of geoid. See also Deviation survey Gravity anomaly Vertical direction Zenith References External links The NGS website gives vertical deflection anywhere in the United States here and here. Geodesy Geophysics Gravity