Meridian Arc Measurement
The aim of meridian arc measurements is to determine a length corresponding to one degree of meridian arc in order to calculate the size and shape of the Earth. In the course of measurements the astronomical coordinates of end points as well as the length of arc are determined.
Erastothenes (c.276-194 BC) used arc measurements for the calculation of the circumference of the Earth. It was presumed that the length of 1 degree of latitude should have been the same at the pole and at the equator. Isaac Newton’s (1643-1727) law of universal gravitation helped to prove that the earth was somewhat flattened in the direction of its axis of rotation and thus the length of 1 degree of latitude was different at the pole and at the equator.
The determination of the shape and size of the Earth served as inspiration to the astronomer F.G.W. Struve » in the planning and performing of the meridian arc measurements.
Struve Geodetic Arc
The Struve Geodetic Arc » is the first accurate measurement of a long segment of a meridian, which helped to establish the exact size and shape of the planet and was carried out between 1816 and 1855 under the guidance of the professor of astronomy and geodesy of the Tartu University Friedrich Georg Wilhelm Struve. These were the most extensive measurements in the 19th century resulting in a meridian arc with the length of 2822 km corresponding to 25°20’, the accuracy of measurements being ± 12 m.
The Struve Geodetic Arc » is a chain of survey triangulations stretching down the 26°E line of longitude from Hammerfest on the Arctic Ocean to Ismail on the Black Sea. The original arc consisted of 258 main triangles with 265 main station points.
Instrument for baseline measurement constructed by Struve
The measurement results of the Struve Arc were used by the German scientists F.W. Bessel for the determination of new ellipsoid parameters and by C.F. Gauss for the specification of cartographic projections.
Today the points of Struve Arc are situated on the territories of 10 countries. In Estonia, F.G.W. Struve measured altogether 22 points of which 3 are still preserved.
Inscription of the Struve Geodetic Arc on the World Heritage List
The Struve Geodetic Arc was included in UNESCO’s World Heritage List » in July 2005 as an outstanding example of a scientific and technical achievement. The listed site includes 34 of the original station points: 4 in Norway, 4 in Sweden, 6 in Finland, 2 in Russia, 3 in Estonia, 2 in Latvia, 3 in Lithuania, 5 in Belarus, 1 in Moldova, 4 in Ukraine.
In Estonia there are three points of the Struve Geodetic Arc included in the World Heritage List: Tartu Observatory and the endpoints of Simuna-Võivere baseline.
The Tartu Observatory » was founded in 1810, and this point served as the origin for Struve’s all observations relating to the Arc. Struve used a point under the centre of the observatory’s cupola, which had not been preserved. During the restoration of the Tartu Observatory also the point was restored and monumented in 2002 with a 12 mm bronze marker and surrounding inscription bedded in the floor. Struve’s sketches and observation data were used for that purpose.
Tartu Observatory (Photo A. Tennus)
Endpoints of Simuna - Võivere baseline
The baseline Simuna-Võivere was situated in the fields of Avanduse and Võivere estates. The height difference between the endpoints of the baseline was 6.3 m and the length of baseline 4.5 km.
Võivere endpoint of Simuna-Võivere baseline
The baseline’s endpoint in Simuna is marked with a 1.90 m high granite monument in which the year of establishment 1849 is engraved.
The baseline’s endpoint in Võivere was considered destroyed, but in 2001 it was found during the GPS measurements. The monument consists of a limestone foundation of 204×204 cm on which there is a large round granite stone with a drill hole for the marker. In 2011 the Võivere endpoint was marked with a glass pyramid.
The Land Board is the national representative of the Republic of Estonia in the Struve Geodetic Arc Coordinating Committee.