Historical Swiss reference systems
In the 19 th century – before the introduction of the reference system CH1903 for Swiss national surveying – other reference systems and map projections were used especially for producing the Dufour and Siegfried maps. These parameters are still useful today for analyzing historical data. Besides these two main map series, further special reference systems were developed in the second half of the 19 th century. These, however, will not be discussed here.
Reference system and map projection of the Dufour Map (CH1840)
Beginning in 1845, the Dufour Map series was published at the scale 1:100 000. The geodetic bases used for this map series were published by Johannes Eschmann in «Ergebnisse der trigometrischen Vermessungen in der Schweiz» in 1840. The reference ellipslid was the «Schmidt ellipsoid from 1828» (a = 6376804.37 m, f = 1/302.02, in the original a = 3271773.00 toises, b = 3260940.03 toises). In other sources this ellipsoid is called the «Schumacher» ellipsoid. Its fundamental point was the old observatory in Bern with coordinates from 1836:
Lat: 7° 26' 24.75" E (or in the original: 5.670000 gon east of Paris)
Lon: 46° 57' 06.03" N (52.168529 gon)
An equal area, untrue conical projection was chosen for the Dufour Map. It is also known as the Bonne projection or as modified Flamsteed projection. The coordinates of the fundamental point were assigned the values 0 | 0 as well as 600 000 | 200 000 («false easting | northing»).
- The untrue conical Bonne projection
The Repère Pierre du Niton was used as the fundamental point for height measurements. Its height was determined by Eschmann in 1840 with 376.2 m, which was derived from the height of the Chasseral at that time (1609.57 m).
Reference system and map projection of the Siegfried Map (from 1870)
The Siegfried Map is based on federal laws from December 18, 1868, governing the publication and the continuation of the topographic survey. The same parameters were used for the geodetic bases as published in 1840 in «Eschmann's Ergebnissen», i.e. the Schmidt ellipsoid and the Bonne map projection.
Only the height datum (Repère Pierre du Niton) was determined by Siegfried and changed to the value 376.86 m (so-called «old datum»). When this value was replaced by 376.6 m («new datum»), the heights as well as the contour lines of the Siegfried map remained in the old system.
Beginning in 1890, all of the older triangulation networks were transformed into the newly finished geographical network which was determined by the Swiss Geodetic Commission and replaced Eschmann's «Triangulation primordiale». This new network was based on the Bessel ellipsoid from 1841, the new geographic latitude 46°57'08.66" of Bern determined in 1869, the 0.021‰ longer prime baseline Chasseral-Rötifluh, and the 1.17" smaller prime azimuth between the observatory Bern and Chasseral. The longitudes were calculated only relative to the meridian in Bern. The map image was not influenced by all of these changes. However, all of the parallels at the scale 1:25 000 were shifted to the south by 3.2 to 3.6 mm, and the meridians were shifted by a maximum of 1mm outwards from the meridian in Bern. Such inhomogeneities which developed in time in the geodetic bases as well as in the maps themselves led to the replacement of the Siegfried Map Series in 1935 by the National Map Series.
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GeodesyLast updated: 16.08.2010
