Normalhöhennull (German pronunciation:[nɔʁmaːlˈhøːənˌnʊl], "standard elevation zero") orNHN is avertical datum used in Germany.
In geographical terms, NHN is thereference plane for thenormal height of a topographical eminenceheight above mean sea level used in the 1932 German Mean Height Reference System (Deutsches Haupthöhennetz).[1] The plane is in the shape of a quasi-geoid. The reference height is ageodetic, fixed point on the New Church of St. Alexander atWallenhorst in the German state ofLower Saxony. Thegeopotential height of this point was calculated in 1986 as part of the United European Levelling Network (UELN), based on theAmsterdam Ordnance Datum.
The NHN plane is a theoretical reference plane. It is derived by deductingnormal heights from the normalplumb line. The difference between the resulting quasi-geoid and thereference ellipsoid is called the height anomaly or quasi-geoid height.
Since 1 January 2000 the whole of Germany has changed its height system over tonormal heights based on thedatum of theAmsterdam Ordnance Datum, known as the German Mean Height Reference System, DHHN92. At the same time, the new NHN is the basis of the United European Levelling Net (UELN), formerly known as theReseau Européen Unifié de Nivellement orREUN, which standardises the height systems of the European countries. Heights in this system are given in meters above NHN orm (NHN).
The NHN was introduced because for heights aboveNormalnull the actualgravitational field of the Earth was not taken into account. As a result, there were changes in both the old West Germannormal orthometric heights (new methods of calculation) and the normal heights of East Germany (with respect to the Amsterdam Datum). The elevations differed — depending on location — by 0.06 to 0.16 m. As a result of new measurements as part of the changeover, however, variations of 0.59 m (Zugspitze) have surfaced.[2]
Olderrelief maps often show heights above the old reference planes. Current maps by the federal survey authorities are based on NHN.[3] At the beginning of 2013 most of the federal states (except Berlin, Thuringia and Saxony-Anhalt) had complete coverage by the new digital topographic mapping at 1:25,000 scale (DTK). Not all the maps have appeared in print yet.[4] On the DTK25 maps, NHN is used for elevations; however, on the DTK25-V scanned topographic mapsHöhennull (HN) andNormalnull (NN) are still being used.[5]
InEast Germany normal heights used to be referred to as heights aboveHöhennormal orHN. The 1958 Kronstadt Tide Gauge (Kronstädter Pegel) was used as the datum. The new NHN heights are typically 12–15 cm higher. The maximum deviations in thespirit level points of first order are between 7 and 16 cm.[6]