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with Normalnull (abbreviation: N.N. or NN), also zero level, is designated a reference value for elevator data on the earth's surface (z. B. the height of mountains or the depth of sea ditches).

In aviation the heights of the air spaces are usually indicated over NN. Also aviation maps like the I.C.A.O. map possess data concerning NN.

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the indication of the height and/or. Deep one takes place thereby usually in one of the following forms: „Meters over NN “, „m and. NN “, „m NN “, „m and. D. M. “or „m u. M. “(colloquially and in Switzerland officially „meters over () sea (esspiegel) “, correctly „meters over the middle sea level “). Sometimes in aviation and also in the navigation one speaks of the Mean Sea level (MSL), the middle sea level and gives the barometrically measuredHeight in foot on.

If one determined a reference, one can be passed by leveling the height into the interior. Biased errors can lead with inappropriate leveling to measuring errors within the range of several meters:

  • On long distances those add upMeasuring error of the individual measurings up. (Computation of errors)
  • the deviation of the earth from the Kugelgestalt leads to further errors. Thus it came with the elevator data of the city Saarbruecken to large differences in height, according to whether one the measurement of the North Sea or at the Atlantic began.
  • Bridge construction with stage: With the building Swiss part of the city running castle an embarrassing breakdown happened to the new Rhine bridge between the German and that: Between both sides a difference in height of 54 centimeters opens itself. While those Switzerland the level of the Mediterranean to reason puts, orients themselves Germany at the North Sea: „The difference of 27 centimeters is naturally well-known, and on the paper everything was clear “, described Beat of Arx, department manager in the board of works Swiss Canton Aargau. In practice it gehapert then however: „On Swiss side would have had to be raised the level around 27 centimeters. Instead it was lowered however by 27 centimeters. “


the reference value is for Germany since 1879 defines by the middle level of Amsterdam. In the GDR the designation was elevatornormally HN and as date applied from 1958 to the introduction of the all-German elevator system NHN that approximately 14 centimeters lower Kronstädter level,i.e. H (NN) =H (HN) +14CM. Die HN-Höhen unterschieden sich nicht nur in der Pegelhöhe, sondern auch vom Höhensystem her, da HN-Höhen genauso wie NHN-Höhen sogenannte Normalhöhen sind. In east Berlin, with the elevator system of the German National Railroad and at the binnenwasserstrassen became also in thatGDR invariably NN-heights continues to use.

meter over standard height zero (NHN)

the newest development is that 1992 imported all-German official leveling net 1. Order, the DHHN 92. The new heights are called heights over standard height zero (NHN). Forthe standard heights the more exact elevator system was selected according to the theory by Molodenskij (system HN), reference levels is the zero point of the level of Amsterdam (system NN). The NHN was introduced, since with the NN the gravity field of the earth was not considered. The gravity fieldfor the earth forms no ball separate resembles more a potato. This „potato “describes only an earth covered with water as Geoid, which only the centrifugal energy and the gravitation affect. The determination of the gravity field Geoids succeeds on sea level well, can howeverin case of a mountain like the Mont by two meters differ to blank. Beside the German main leveling net (DHHN92) also the United European Levelling Net ( UELN) takes these „to potato “as reference system

thereby changes both the normal orthometrischen West German NN-heights (new calculation method),and the standard heights of East Germany (purchase to a new level).

<math> \ begin {matrix} h_ {\ rm Germany} &=& \ h_ {\ rm Switzerland} - 8~ {\ rm cm} \ \ \ &=& h_ {\ rm \ ddot Austria} - 27~ {\ rm cm} \ end {to matrix}< /math>


for Switzerland applies the Repère Pierreyou Niton with 373,6 m over middle sea level of Marseille as starting point of the measurement ( the Pierre you Niton is a large Findling in the Geneva lake with Geneva). The Swiss elevator data deviate over + 0.08 m from the Germans. Thoseofficial designation reads:m and. M. („Meters over sea “).

<math> \ begin {matrix} h_ {\ rm Switzerland} &=& \ h_ {\ rm Germany} + 8~ {\ rm cm} \ \ \ &=& h_ {\ rm \ ddot O sterreich} + 19~ {\ rm cm} \ ends {to matrix}< /math>


for Austria applies the middle level of Triest to the Adria (meters over Adria) as zero point. The Austrian elevator data deviate over + 0.27 m from the Germans.

<math> \ begin {matrix} h_ {\ rm \ ddot O sterreich} &=& \ h_ {\ rm Germany} + 27~ {\ rm cm} \ \ \ &=& h_ {\ rm Switzerland} -19~ {\ rm cm} \ end {to matrix}< /math>


NN is only one of many possible elevator systems.

Besides one can use the Geoid in such a way specified , which one determines by the measurement of satellite orbits as reference value. The surface marks itself,would form, if the whole earth were covered by sea.Physically it concerns thereby a Äquipotentialfläche, a surface, those everywhere perpendicularly to the direction of the force of gravity stands oneself and on that the weight potential (the potential energy) everywhere the same valuehas. The earth has now no more spherical shape, it resembles also not more a flattened ball (ellipsoid of revolution), but - exaggerated said - rather a potato. However the deviations are so small that them from the universe seen afteras looks before like a ball.

In the English linguistic area the designation Sea level (short SL) for NN is used.

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