Georeferencing the first bathymetric maps of Lake Balaton, Hungary

摘要

Lake Balaton is located in the Pannonian Basin, Hungary (46°50′ N, 17°50′ E), and is characterized by its large area (594 km ~2) and very shallow water depth (avg. 3.5 meters). The main tributary is the Zala River, which enters the western bay, and the only outlet is the Sió River in the East. Sámuel Krieger conducted the first known survey focusing on Lake Balaton in 1776. The original purpose of Sámuel Krieger's work was to illustrate his plans of draining and canalizing Lake Balaton. This map indicates several proposed canals and bathymetric contour lines according to a water level drop of 1, 2, or 3.33 Viennese fathoms (1 Viennese fathom = 1.89 meters). The map also shows settlements, land use and relief. Krieger measured water input from tributaries, documented the water level fluctuations of the lake, and summed his results in the "Descriptio", a document with several tables of data and a written description of Lake Balaton, the Sió River, and the possible benefits of his plan of draining the lake. Almost 90 years later, the water level was lowered by approximately 1 meter in 1863, cutting off large marsh areas from the water system of the lake. The first bathymetric map was surveyed in 1895 after the lake was partially drained. The bathymetric survey was carried out with the purpose of estimating the water volume held by the lake. Understanding water balance was important for flood control after the Sió Canal and lock was built in 1863. Water depth was measured in 2884 points, along sections near the shore, and scattered points in areas of low relief. Depth was measured with a sounding line or pole. Horizontal positions were measured optically from military triangulation points, and elevations were leveled from a network of benchmarks placed for this survey. Distances were measured in fathoms but elevations were measured in meters for better accuracy. Reprojection of the scanned map was possible, but we had to correct minor errors by triangulation. Surviving benchmarks, depicted buildings and railway bridges were used as control points. The resulting map was used to create a Digital Elevation Model of the lake floor for investigating sedimentation processes.