Nesaules Hill and point “Nessaule-kalns” of the Struve Geodetic Arc

How to reach: Look for a tourist sign in Kārzdaba, Cesvaine rural territory, on road V840. Stay on the road for another 3 km until the next sign. Then leave your car on the roadside and go on a hike along a forest road. After 600 m look on your left for the next sign, this is where you will have to go about 500 m uphill along a forest trail until the top of Nesaules Hill.

What to see: protected area and ancient forest + point “Nessaule-kalns” of the Struve Geodetic Arc + survey markers from various times + forest trail.

Nesaules Hill. Height: 284.2 m above sea. The eighth highest hill in Latvia; only Gaiziņkalns (311.94 m above sea), Sirdskalns, Abrienas Hill, Ķelēnu Hill, Lielais Liepukalns, Āriņu Hill, and Dzierkaļu Hill are higher.

On the top of the hill there are 5 survey markers from various times:
* a historical evidence of triangulation performed on Nesaules Hill by military surveyors of the Russian Empire in 1904 (the round stone in the image), and an evidence of an auxiliary point for astro-geodetic observations (the rectangular concrete block in the image);

* survey post of the 1st class GPS point “Nesaule” of the national geodetic network (the longer post in the image) and a concrete post above the triangulation station “Nesaule” erected in 1956 (the oldest post in the image).

* point “Nessaule-kalns” of the Struve Geodetic Arc, restored in 2019.

Struve Geodetic Arc
* The Struve Geodetic Arc (SGA) is a unique 19th century geodetic survey system for determining the size and shape of the Earth. It was carried out by the professor of astronomy Friedrich Georg Wilhelm von Struve and the military topographer Colonel Carl Friedrich Tenner between 1816 and 1855.

* The Arc extends approximately along the 26° meridian from Hammerfest on the shores of the Arctic Ocean in Norway to the town of Izmail near the mouth of the Danube River at the Black Sea, crossing the territory of 10 countries: Norway, Sweden, Finland, Russia, Estonia, Latvia, Lithuania, Belarus, Ukraine and Moldova. The total length of the Arc is 2822 km. The measured meridian circle includes 25º 20´ of the circumference of the Globe.

* The Geodetic Arc of Struve consists of 258 triangulation triangles. 265 main points and 60 auxiliary points were established, as well as coordinates were determined for 13 points. The distance between the points is 30 - 40 km. The Arc surveying started from the Tartu  Observatory. The relative accuracy of the measurements was very high -– only 4 mm error per kilometre.

* The scientific purpose of SGA was to clarify the flattening of the Earth in the direction of the poles according to Newton's theory. The practical purpose of SGA: to perform measurements and to obtain data in order to do mapping of Tsarist Russia and the Kingdom of Sweden. The political purpose of SGA: triangulation was the "great science" of the 19th century – country participation in the survey raised its political and military prestige.

* The observations of Struve's Geodetic Arc were led by two outstanding personalities of that time:

* Friedrich Georg Wilhelm von Struve (1793 - 1864) was an astronomer of German origin, a professor at the University of Tartu and the director of the Tartu and Pulkovo observatories. In 1831, he was appointed as a royal adviser to the Russian Empire, as well as a nobleman, and since then, his surname has been "von Struve".

* Karl Friedrich Tenner (1783 - 1859) was a military surveyor of German origin. From 1809 to 1811, he participated in the formation of the first triangulation networks in the Russian Empire. During the period from 1816 to 1822, he was assigned to triangulate the provinces of Vilnius, Courland, Grodno and Minsk. In 1832, he was appointed as an honorary member of the St. Petersburg Academy of Sciences.

* Latvia was the only territory where both surveyors – Struve and Tenner – conducted surveys. Struve installed 11 points in the Northern part of Latvia – from the point Oppekaln to the point Jacobstadt. Tenner created 5 points in the Southern part of Latvia – from the point Pilkaln to the point Bristen. Two of the 16 points in the territory of Latvia are shared geodetic-astronomical points – Jacobstadt and Bristen, where observations have been made and the coordinates of the points have been determined. The performed measurements were one of the first trigonometric measurements in the territory of Latvia, serving as the basis of which the map of Vidzeme was compiled.

* In 2005, the Struve Geodetic Arc was included in the UNESCO World Heritage List as a unique, the longest and most accurate system of geodetic surveys of its time and as an outstanding testimony to the scientific, historical and technical development of its time. The UNESCO World Heritage List includes a total of 34 points in the Struve Geodetic Arc. From the points of the SGA located in the territory of Latvia, the points located in Jēkabpils and Ziestukalns are included in the UNESCO World Heritage List.

* In Latvia, the Struve Geodetic Arc Preservation and Development Council has been established, which consists of representatives of the Latvian Geospatial Information Agency, the Latvian National Commission for UNESCO, the National Cultural Heritage Board and local governments in whose territories the SGA points are located. 

Lat, lon: 56. 9602356, 26.1848377, Nesaules kalns, Aronas pagasts, Madonas novads

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