The longest geodetic arc ever mesured by classical goedesy is long of 25° from the Baltic to the Black Sea through the Dorprat meridian. This arc is based upon measurements made, from 1816 to 1855, by a russian general, a norvegian, a Swedish and F.G.W. Struve, the founder of the Pulkovo Observatory; its lenght is 2° more than the arc Delisle, then at the Petersburg Observatory, intended to determine along the meridian of this observatory in the year 1737. Russia, at that time, became part of the european triangulation, a prelude to circumterrestrial modern campaigns.