The theodolite (Reichenbach & Ertel, München, 1821) allowed measuring both horizontal and vertical angles. Struve was the first who used this instrument more widely (TÜAM, photo T. Pung).
The vertical circle (Ertel, München, ca 1821-26). It was used to determine the vertical angles between the sky pole (a point in the sky near the North Star in the direction of the Earth’s axis) and the horizon (TÜAM, photo T. Pung).

The measurement of the Struve-Tenner meridian arc

The Great Russian Meridian Arc measurement in the direction of the Tartu meridian was carried out under the supervision of F. G. W. Struve in the years 1816-1852 and consisted of several stages.

The Napoleonic War accelerated the establishment of topogeodetic departments at the army headquarters in all the European countries. In 1812 also in Russia the military-topographical map depository was formed and in 1816 it was subordinated to the headquarters of the army. In 1822 the corps of military topographers was formed. Their first geodetic tasks were to establish triangulation networks in the provinces of Vilnius (Vilno) and Western Byelorussia (Grodno, Bielostok). In 1816–1828, under the supervision of the Estonian-born general Carl Friedrich Tenner (1783–1859), the main first class simple chain of this triangulation along the Vilnius meridian was built.

In 1819–1827 F. G. W. Struve, supported by the Russian Government, carried out the measurement of the degree along the Tartu Meridian between Suursaar and Jēkabpils (Jakobstadt). F. G. W. Struve and C. F. Tenner met, as proposed by Tenner, in Tartu in January 1828 and agreed to unite efforts in the measurement of the arc.

Tenner developed the triangulation network step by step in the southern direction, through Volynia and Podolsk provinces and Bessarabia reaching the estuary of the Danube in 1852. The measurements supervised by Struve were extended from Estonia to the territory of present-day Finland, continued in Sweden and finished in 1850 in Finmarken (today in Norway), at the Fuglenes cape in the town of Hammerfest at the Arctic Sea. In the north the work was supervised by Daniel Georg Lindhagen (1819–1906), a member of the Swedish Academy of Sciences, and Christopher Hansteen (1784–1873), the former director of the Observatory of Kristiania (Oslo).

Russian general of Estonian origin Carl Friedrich Tenner (1783-1859) (Tõnu Viik. Carl Friedrich Tenner – Vene geodeesia rajaja.

The whole triangulation chain consisted of 258 triangles to which were added the triangles which were measured for connecting the baselines with the starting sides of triangulation. The latitudes of the edge points on Struve’s meridian arc are: Fuglenes – 70º40’11’’; Staro-Nekrasovka – 45º20’03’’ which gives the length of the arc 25º20’08’’ or 2,820 kilometers.

The geodetic part of this immensely big work was not uniform. The baselines in one part of the arc were measured, as demanded by Struve, with the instrument constructed by the university mechanic Brücker and in the southern part with the instrument of Russian origin. The mean relative error in measuring the baselines with Struve’s instrument was about 1:1,000,000 but the mean error of baselines measured with Tenner’s instrument was 1:300,000.

The signs used by Struve in triangulation points (Г. Желнин. Основные астрономо-геодезические работы на территории Эстонской Советской Социалистической Республики с 1811 по 1940 год: диссертация ... кандидата физико-математических наук. Тартуский государственный университет, 1950).

It is necessary to stress that in considering the needs for the measurement of the degree the requirements for the accuracy of the geodetic work could be even lower. This is why these measurements could be fully used for calculating the dimensions of the ellipsoid of the Earth.

F. Krassovski, having taken only Struve’s result for determining the dimensions of the ellipsoid of the Earth, received the value of the larger half-axis

a = 6 378 445 m ± 169 m

and for the value of oblateness 1: 298,6 ± 11,5.

Leveling instruments were used for measuring horizontal angles. The „Ertel & Sohn“leveling instrument was bought at the time of L. Schwarzi in the last quarter of the 19th century (TÜAM, photo T. Pung).
The pantograph was used for enlarging and making smaller the drawings. Tartu University bought it from the family of Alexei Greigh’(1775-1845), the commander-in-chief of the Russian Black Sea Navy, ca 1850 (TÜAM, photo T. Pung)