Military considerations in the early 1770s declared the need for a systematic mapping of the eastern regions of Norway along the border to Sweden. After a failed attempt of direct map sketching in the field, the geographical circle was introduced in 1779 to establish a triangular network as a backbone for further positioning of natural and man-made features. The resulting maps were used in preparation of fortresses and planning of defensive field operations. The scale of the triangular network was established by an astronomical baseline supported by linear baselines measured on frozen lakes during winter time. Many stations had latitude determinations from circum-meridian observations of the sun and stars to control the precision of the geodetic triangulation. When discrepancies became too large, a new baseline and a new reference point was selected. The original reference point was the flagpole of the fortress at Kongsvinger, which served as the zero-meridian for mapping in Norway until 1850. Other reference sites, for which accurate latitude and longitude were determined from several years of astronomical observations, were established in Trondheim, Bergen, and Kristiansand as the original triangular arc was expanded around the entire coast of southern Norway to close at Kongsvinger after 3 decades of observations. This allowed astronomical control of the geodetic results.
A map of the district surrounding the largest lake in Norway was published in 1796 by the Topographical Society of Norway. Their members contributed private funds for this unique map and for the publication of the Topographical Journal, containing multidisciplinary descriptions, including environmental, of many regions of the country. The map was produced by lieutenant Niels S Darre, a military surveyor and cartographer, as a private undertaking in his own home district. The geodetic reference point of the map is the church at Vang, the position of which had been determined by repeated astronomical observations by its vicar, Abraham Pihl. He had been trained as an astronomical observer by professor Thomas Bugge at the Round Tower Observatory in Copenhagen. A supporting latitude determination in Lillehammer is also noted on the map, obtained by Maximilian Hell in 1769 on his return from Vardø in Northern Norway to Vienna after a successful observation of the transit of Venus. The observed magnetic deviation from astronomically determined north is given. The map also identifies a marker of the great flood of the river Glomma in 1789, causing environmental and social devastation. We have digitized the map and compared it to a 67 km × 82 km section of a modern, digitized topographical map of Norway, by transforming the UTM projection to a meridian through the church at Vang. While most features coregister acceptably by visual inspections of the old and new maps, we note geometrical deviations of up to 1 km in the locations of western shorelines.