Taking bearings

The rapid development in the 19th century led to the construction of railways, canals and roads for the transport of raw materials and finished products. A good knowledge of geography and logistics was necessary for the planning of these cross-border traffic routes. The maps of that time contained details of the countryside, but there was no information on the heights of the various mountains, valleys, roads or lakes. There was no consistent and comprehensive system of height measurements. Every major city had its own local network of height points with a specified reference point. In Prussia it was decided in 1875 that all height measurements and points would be based on a standard zero point.

Some levels in the North Sea and the Baltic were available which had been used as a reference point in the last decade for differential levelling in Prussia. In 1879, a point on the old Berlin Observatory was specified as standard reference height and called the normal height point. Its height above the zero marks of the various levels was about 37 metres above sea level. Thus, the reference surface, which lies exactly 37.000 m below the normal height point, was called the normal zero. It is a devised level which agrees with the average height, of the water level of the North Sea and Baltic. The normal height point was the reference point for the already existing bench mark for the entire German Reich. Between the years 1912 and 1954, the German national height system was divided into different sections (geometric levelling). In the Federal Republic of Germany the normal zero height reference system (NN) of 1912 was still valid.

The height reference system introduced in 1953 zero height (HN), with the starting point of the Kronstädt level near St. Petersburg, was used until the end in the former GDR. The different elevation data between the FRG and the GDR showed differences of 12 cm to 16 cm. The survey offices of the old Federal countries decided in 1993, to replace the different height systems with a standard height reference system for the whole of Germany, called the German main height network (DHHN). On 1st January 2002, the new height reference system (DHHN) came into force in Germany.

From a known measurement point (either from a levelling point or a point already associated with a levelling point) the difference in height to the new point is calculated and determined by forward and backwards sighting. This uses a horizontal measuring instrument, for example, a theodolite, which is set up between the known point (looking backwards) and the new point to be measured (looking forwards). A levelling staff is set up at each of the two points.  

The height of the horizontal plane is read by means of the measuring device on both measuring staffs. The height difference between the two points can be obtained from the reading of both bars. The scale on the levelling staff can be read on the crosshairs of the theodolite. The scale shows centimetre fields. The staff can be read to within a millimetre at distances up to 25 m. The decimetre sections (0.1 - 4.0) are quantified. There are also measuring staffs with a length of 5 m.