The Art of Survey of the Earth from Finnmark Expeditions in Finnmark for «Struve's Geodetic Arc» 1845-1850

Gerd Johanne Valen 2016 The Art of Survey of the Earth from Finnmark Expeditions in Finnmark for «Struve's geodetic arc» 1845-1850. Written by Gerd Johanne Valen 2016. Struve's geodetic arc was added to Unesco’s list of the world’s natural and cultural heritage in 2005. The geodetic arc was measured from 1816 to 1852. It was the first accurate measurement of a long geodetic arc which made it possible to calculate the earth's size and shape. It was also the longest and northernmost geodetic arc in the 1800s. The co-operation between prominent surveyors, geodesists and monarchs characterized the story of this world heritage. Less well known is an account of the surveyors' seasonal fieldwork in West Finnmark from 1845 to 1850. What were the major challenges? How did they use the landscape of West Finnmark to measure a chain of triangles and a series of measurements from Hammerfest to the Black Sea? This article will mainly look closer to the story we can read from the fieldworks logbooks. The World Heritage Struve's geodetic arc was named after Fredrik Georg Wilhelm Struve (17931864), who was a professor of mathematics and astronomy in today`s Tartu in Estonia. He was responsible for the triangulation in Livland, which was a part of Russia. In 1820 Struve put forward a suggestion to the authorities to carry out a chain of survey triangulation in the Baltic region. He argued that this work would have great scientific value and would also be important for mapping the country. The military surveyor Carl Tenner was engaged in surveying work in Lithuania. He managed to combine the need for mapping and triangulation. Together he and Struve published the results of their investigations in 1832. At the same time the idea of an extended geodetic arc from the Arctic Ocean and down to the Black Sea was beginning to take shape. Now it was a question of also persuading the authorities in Norway and Sweden. In 1844 the Academy of Science in Stockholm received a request to join a Swedish-Norwegian project. Nils Haqvin Selander, who was the director of Stockholm Observatory, wrote a letter of recommendation to the authorities, as did Christopher Hansteen, director of the Observatory in Christiania. Nordkapp (the North Cape of Norway) was pointed out to be the northernmost endpoint, and among the considerations discussed was where the greatest challenges would be, and which route should be chosen from Nordkapp to the Finnish border. After studying the maps of the area, Selander concluded that there were two possible routes, one via Porsangerfjord and one via Altafjord. He thought that the greatest challenges might the Swedish surveyors face in Muonio. Another problem would be the transport routes via the Alta River because many large areas of inner Finnmark were uninhabited with few people who could assist in transporting the expedition. In Norway they were more concerned by convincing the authorities that it was necessary to participate, and felt the challenges could be resolved by sending two competent people to do a reconnaissance of the terrain. On July 13th, 1845 it was decided by a joint Swedish-Norwegian parliamentary committee that work on surveying the arc should begin in both countries. Thanks to this quick decision, less than a month later the expedition team was in Hammerfest. The measuring of Struve's geodetic arc in Finnmark was Norway's first participation in an international scientific collaboration for a scientific cause. Selander and Hansteen had the Gerd Johanne Valen 2016 responsibility for the surveys in Sweden and Norway in 1846. Hansteen himself did not take part in the fieldwork. That was left to two army officers, Fredrik Klouman and Christopher A.B. Lundh. Both had specialized in surveying at the Military High School. Klouman learnt about Finnmark when he spent two seasons doing fieldwork in 1840 and 1841 for NGO, Norway's Geographic Survey. Their job was to reconnoiter the triangle points, select station points for surveying the arc and locate suitable sites for base line measuring (initial measurement) and astronomic observations. Then they should come up with a recommendation of which station points could be used for the Norwegian part of the measured chain. D.G. Lindhagen was a Swedish astronomer working in Struve's team in Poulkova. He participated in the summer of 1850, carrying out base line measuring and astronomic observations in Alta and in Hammerfest. His reports from 1850 in Norway and 1851 in Lapland are included as appendices in Struve's publications. His associate, Lysander, assisted him with the fieldwork that season. Although the sources do not name those who assisted the expeditions, the four seasons spent in an area with poor transport roads could not been carried out without considerable effort by local craftsmen, carriers, boat masters, cairn builders, and local people with an extensive knowledge of the area. The following presentation will concentrate on the first reconnaissance trip in 1845 and the survey work in 1846 and 1847, along with Klouman's and Lindhagen's measuring of the base line in Alta and their astronomic observations in Hammerfest in 1850. The content of the source is partly difficult, because diaries and reports concentrate largely on measurement results. The purpose of the presentation is to reconstruct the course of events and raise some questions. How did they organize the work? Which travel routes and methods were chosen? And what were their greatest challenges? But before we look in more depth into the expeditions in Finnmark, we need to say something about their method and what kind of climatic challenges they faced. Method The method is based on the simple principle that if you know the length of one side and two angles of a triangle, you can work out the lengths of the other two sides. Then a series of triangles can be established. The chain in Norway consisted of 15 station points on the tops of mountains. Signals were built and established on mountaintops from Tyven to the Finnish border. Signals are physical constructions that can be circular or square cairns, or wooden towers. In Finnmark they built stone cairns on mountaintops which functioned as trigonometric points from which a survey of triangulation could be taken. The plan was to build cairns from loose rocks, being the only material readily available. The cairns were to be hollow, circular and two meters tall, which was the standard dimension at the time. In the hollow center, a wooden marker would be carefully placed and adjusted. It consisted of a cylindrical center log 10 cm in diameter and 3.5 meters tall, supported by three smaller logs and the cairn. Two half barrels with a diameter of roughly 92 cm (3ft) and a height of 61 cm (2ft) were mounted on top of the center log. The half-barrels were partly covered with white paint to make the markers visible from far away. But not all the markers built from the available rocks in the area were equally solid. Only five years after the markers were built, it was reported that those on Haldde and Luvddiidčohkka had been badly damaged. Gerd Johanne Valen 2016 In 1845 and 1846 Lundh was busy measuring barometric altitudes to determine height above sea level. Since air pressure decreases with height above sea level, by using a special formula it is possible to calculate how high up you are. For exact measurements this figure must be corrected for air temperature and humidity. Several measurements were taken at two to three heights in the terrain at specific times of the day - early morning or afternoon. The figures for Alta were compared with the figures for Hammerfest on the same day, and all the measurements from inland stations were correspondingly compared with measurements taken at the Kåfjord observatory the same day. By the comparations the measurement was corrected for variations in air pressure. When the station points were established, the zenith measurements could start. This is a vertical angle measuring. The zenith is the point directly above the observer, i.e. +90-degree position, and a zenith distance is the vertical angle between the direction to the zenith and the direction to a point aimed at. By measuring the zenith distance (vertical angle) between two end-points you can determine the difference in altitude between two points; this is called trigonometric altitude measuring. The instrument used in 1846 and illustrated below was a Repsold universal from the Royal Observatory in Christiania. The instrument could measure angles both horizontally and vertically, and carry out measurements in the opposite direction of the rotation directions. When the triangulation measurements of the 15 station points which made up the principal network of triangles were completed in 1847, the network's scale needed to be determined. For this purpose, a baseline was established in Alta. The easternmost endpoint A was located at Elvebakken and the westernmost point B at Bukta. Both points were marked with granite blocks dug into the ground and protruding cm above the surface. Iron bolts encased in lead were fixed to each block. The baseline equipment was constructed by Professor Struve himself. The equipment consisted of 4 steel rods 2 toisers, or about 4 meters long. In all, the base line was 577 rod lengths or 2251 meters (1154 toisers). To transfer the measured length from the baseline to the principle arc network itself, they established a so-called extended triangulation network. The network consisted of 10 points which formed triangles of increasing sizes. The baseline constituted one of the sides in the smallest triangle, while one of the sides in the largest triangle was identical with one side in the principle geodetic arc network.

Load more