One hundred and ninety years ago, in Estonia, Latvia and Lithuania, an unprecedented attempt to measure the Earth's size and the curvature of its surface was initiated. This was undertaken inan uncoordinated way and for unrelated motives by, on the one hand, scientists like Friedrich Georg Wilhelm Struve, Professor of mathematics and astronomy at Dorpat University in Tartu, Estonia along with two directors of the Vilnius Observatory, Jonas Sniadeckis and Petras Slavinskis and, on the other hand, military men like Carl Tenner, a man of Estonian descent serving at the time as an officer in the Tsarist army.

Work in the Vilnius district was initiated by Tenner in 1816 for the purpose of mapping, but that same year, Struve started working on a far more ambitious geodetic project in Livonia (an area covering the present territories of Estonia and Latvia). The entire project lasted from 1816 to 1855 and, making use of all the earlier work, extended over an unprecedented distance from north to south. It was dubbed the Struve Geodetic Arc in honour of the man who supervized the entire venture.

The Struve Geodetic Arc is no doubt one of the most unusual sites ever to be inscribed on the World Heritage List. The physical site is, in fact, composed of the small, durable traces of a chain of survey triangulations which, when completed, extended over 2,820 kilometres from Hammerfest in Norway to the Black Sea, crossing the territory of ten different countries: Norway, Sweden, Finland, Russian Federation, Estonia, Latvia, Lithuania, Republic of Moldova and Ukraine.

The original Arc consisted of 258 main triangles with 265 main station points - eight of which were determined in the Baltic countries.

The determination of the Earth's form and size has been one of the more vexing problems of Antiquity. Since around 500 BC it had been supposed that the Earth was not flat, but roughly spherical in shape and in the third century BC a surveying technique and theory for determining the size of the Earth was elaborated by Eratosthenes. This method remained in use until the era of satellite geodesy.

Eratosthenes' theory, using the measurement of length and angles based on observation of the stars, did allow scientists to determine the approximate size of the Earth, but the measurements themselves were still not accurate, mainly because of problems of method and equipment. For a more accurate determination of the Earth's radius, precise geodetic observations were needed and the method of triangulation elaborated by the Dutch geodesist V. Snellius in 1615 was subsequently applied, allowing

scientists to determine more exactly the shape of the Earth (but still not its size).

During the first half of the nineteenth century, however, instruments and methods for measuring improved considerably and new measurements of the Arc were undertaken. These measurements, which were extraordinarily complex and delicate to make, finally provided reliable information both on the Earth's ellipsoid form and on its size.

The Struve survey thus helped to determine the shape of the Earth and its size and played an important role in the development of accurate topographic mapping. The World Heritage nomination includes thirty-four of the original station points, with different markings - ranging from a hole drilled in rock, to an iron cross, cairns, or obelisks built of masonry.

The Struve Geodetic Arc was inscribed on the World Heritage List in 2005 as a transnational site. The ten countries which shared the task of preparing the nomination now also share the responsibility for its management.