RJRP+4M Sidrabini, Sausnēja parish, Latvia
The geodetic measurement of the Struve arc at the Sestukalns point was carried out in 1824, with the work personally supervised by Struve.
The measurement point was restored in 1904 by Russian military surveyors using new triangulation methods. The point is located on the P79 highway Koknese – Ergli, off to the right at the 31st kilometer, about 100 meters to the right of the sign "Struve Meridian Arc Point Sestukalns," along a forest road about 300 meters, on the top of a hill. The geodetic point Sestukalns of the Struve meridian is situated on the hill Ziestu (216.5 m above sea level) in the municipality of Sausnēja, Madona district.
Latvia is the only country where Friedrich Georg Wilhelm von Struve, as well as Karl Tenner, conducted measurements and established base points. Wilhelm von Struve surveyed the northern part of Latvia up to Cape Jakobstadt, while Karl Tenner surveyed points in the southern part of Latvia – from Bristene to Pilkalns. There are a total of 16 points, of which 11 were organized by Struve and 5 by Tenner.
The Struve Arc, once known as the "Russian" and later the "Russo-Scandinavian Meridian Arc," is one of UNESCO's World Heritage monuments. The arc consists of 265 triangulation points, where measurements were conducted from 1816 to 1852, allowing for the precise determination of the Earth's size and shape.
The reference points of this triangulation network were marked in various ways on the ground: hollows carved into rocks, iron crosses, stone pyramids, or specially erected obelisks. Often they were marked with sandstone bricks laid at the bottom of a pit; sometimes it was a granite cube with a cavity filled with lead, placed in a pit with cobblestones.
During the project to include the Struve Arc in the UNESCO list, which lasted 8 years, special search and geodetic work was undertaken in each country to locate the original points. All information from all the Struve Arc countries was collected, structured, and standardized.
Not all of the original points were found during the special search and geodetic work carried out in recent years with active cooperation from scientists of the interested countries, and many of them were found to be heavily damaged. Therefore, only the best-preserved points – a total of 34 – were included in the World Heritage site.
Sources:
http://www.gototrip.com/publications/geodezicheskaya-duga-struve
https://www.ergli.lv/upload/File/Struves_celvedis.pdf
