Normal height zero
The normal height zero (NHN) is the designation of the reference area for the specification of heights above sea level in Germany, the level of which is derived from the Amsterdam level (Normaal Amsterdams Peil, NAP for short). Heights above sea level are normal altitudes .
In practice, the reference area is achieved by defining the height of one or more fixed height points . For example, the point at the New St. Alexander Church in Wallenhorst ( Osnabrück district , Lower Saxony ) served as a reference point for the DHHN85 and DHHN92.
The NHN area corresponds to a quasigeoid that runs through the zero point of the Normaal Amsterdams Peil. Both heights in the German main height network 1992 (DHHN92) and the currently valid heights in the German main height network 2016 (DHHN2016) are labeled "heights above sea level". The NHN therefore only designates the height reference system; for a clear description of a height, the reference frame (the realization of the height reference system through specific measurements in a specific measurement period) must also be specified, e.g. B. for the currently valid heights: "Height above sea level in DHHN2016".
The NHN heights replace the NN heights and the HN heights in Germany. Only heights in Germany are referred to as NHN heights. Differences to heights in other countries are described in the article Height above sea level .
Changeover to normal height zero
The designation normal zero (NN) was introduced in 1879 when altitude reference levels such as "Hamburger Null", "Höhe above Swinoujscie", "Level zu Neufahrwasser", which had previously been used side by side, were replaced by a uniform altitude reference first for Prussia and soon afterwards for all of Germany. The sea level heights were derived from the level of the Amsterdam water level. The calculation of the first sea level heights (heights in the "old system") was carried out without any seriousness correction. From around 1912 heights were calculated in the so-called "new system" (System 1912), with the actual gravity being approximated by a model, the normal gravity, for the correction due to the earth's gravity field. Such heights are called normal orthometric heights . The heights in the German Main Height Network 1985 (DHHN85) , which was calculated from the remeasurement of heights in the Federal Republic of Germany in the 1980s, were also referred to as heights above sea level. NN heights can therefore refer to different height reference frames, between which a difference of several centimeters must be expected.
The heights in the former GDR were normal heights, based on the Kronstadt level . The heights of the State Leveling Network 1956 (SNN56) were designated as heights above HN56, the heights of SNN76 as heights above HN76. Occasionally, however, only the designation HN (height zero) was used without specifying the trade fair epoch.
NHN heights in the DHHN92
After the reunification of Germany, its leveling networks were reunified relatively quickly. For this purpose, 16 connecting lines between the two power supply units were measured and then all measurements were evaluated together. As a result, the German Main Elevation Network was created in 1992 (DHHN92). Measured gravity values were used to correct the measured values. The surveying authorities of the German federal states decided that the newly calculated heights, like the old NN heights, should refer to the Amsterdam gauge. This means that the German heights have the same reference level as the heights in the United European Leveling Network . For the height definition, the more exact normal heights were selected instead of normal orthometric heights. In order to make the changes visible to the user and to avoid confusion, the designation NHN was introduced, which virtually merges the designations NN and HN.
Differences new and old federal states
As a result of the conversion, both the West German and the East German heights changed; the former due to the change to normal altitudes, the latter due to the change from the Kronstadt level to the Amsterdam level. The deviation of the sea level heights from the sea level heights in the new federal states is between +12 cm and +15 cm (the sea level heights are greater) and is thus much greater than the deviation of the sea level heights to the sea level in the old federal states, which is usually only a few millimeters in the flatlands. Greater differences of −5 cm to + 4 cm occur at the borders with the new federal states, which were caused by the influence of the newly added measurements. Points in the high mountains are an exception. Differences in the decimeter range can result from the different calculation of the gravity correction (normal orthometric heights or normal heights), for example for the Zugspitze the sea level is 0.59 m higher than the sea level.
In Germany, the federal states are responsible for official land surveying . As a result, the new height information is recorded and published inconsistently. In the case of DTK25 maps, normal height zero (NHN) is specified as the height reference, in DTK25-V maps, the scanned topographic maps, height zero (HN) or sea level (NN) is used.
NHN heights in DHHN2016
The German Main Elevation Network 2016 (DHHN2016) was decided by the AdV on September 21, 2016 and introduced in all federal states by June 30, 2017. It is part of the Integrated Spatial Reference 2016 . The underlying new measurement of the leveling network of the 1st order between 2006 and 2012 led to deviations from the DHHN92 in the range of a few centimeters. Larger differences in the decimeter range can occur in mining areas. To distinguish between heights of the previous DHHN versions, DHHN2016 heights are referred to as heights above normal height zero (NHN) in DHHN2016 .
- Manfred Spata: Historical water levels and reference heights in Europe . In: German Shipping Archive . No. 21 , 1998, pp. 379–392 ( dsm.museum [PDF]).
- Information from the LVA NRW (PDF; 303 kB)
- DIN series of standards 18709: Terms, abbreviations and symbols in geodesy - Part 6: Geodetic reference systems and reference surfaces Edition 2016-04. Beuth-Verlag Berlin 2016
- Lang, H. and Steinberg, J .: On the development of the height networks on the territory of the new federal states . In: Allgemeine Vermessungsnachrichten (AVN) . tape 8-9 , no. 100 , p. 295-309 .
- Working group of the surveying administrations of the federal states of the Federal Republic of Germany (AdV): German Main Height Network 1992 (DHHN92)
- Weber, D .: The German Main Elevation Network 1992 . In: AdV (Ed.): German Main Height Network 1992 (DHHN92) . Bavarian Land Surveying Office, Munich, p. 7-16 .
- "The German Height Reference System" website Federal Agency for Cartography and Geodesy 2020. Accessed on March 26, 2020
- Customer information 3/2009. (PDF; 1.1 MB) In: vermessung.bayern.de. State Office for Surveying and Geoinformation Bavaria , September 2009, p. 2 , accessed on April 25, 2013 .
- "Documentation DTK25" website Federal Agency for Cartography and Geodesy 2019. Accessed on March 27, 2020
- "adv-online.de" as of December 31, 2018. Accessed on March 27, 2020