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Normalhöhennull

Normalhöhennull

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Normalhöhennull or NHN is a standard reference level, the equivalent of sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

, used in Germany to measure height.

In geographical terms, NHN is the reference plane
Plane of reference
A term used in celestial mechanics, the plane of reference is the plane by means of which orbital elements are defined. The two main orbital elements that are measured with respect to the plane of reference are the inclination and the longitude of the ascending node.Depending on the type of body...

 for the normal height
Normal height
Normal heights are heights above sea level, one of several types of height which are all computed slightly differently. Alternatives are: orthometric heights and dynamic heights....

 of a topographical eminence above sea level used in the 1992 German Mean Height Reference System (Deutsches Haupthöhennetz). The plane is in the shape of a quasi-geoid
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...

. The reference height is a geodetic
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

, fixed point on the New Church of St. Alexander at Wallenhorst
Wallenhorst
Wallenhorst is a municipality in the district of Osnabrück, in Lower Saxony, Germany. It is situated in the Wiehengebirge, approx. 10 km north of Osnabrück....

 in the German state of Lower Saxony
Lower Saxony
Lower Saxony is a German state situated in north-western Germany and is second in area and fourth in population among the sixteen states of Germany...

. The geopotential height
Geopotential height
Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height"...

 of this point was calculated in 1986 as part of the United European Levelling Network (UELN), based on the Amsterdam Ordnance Datum also known as Normalnull ("Standard Zero") or NN, which represents the average level of the North Sea.

Definition



The NHN plane is a theoretical reference plane. It is derived by deducting normal height
Normal height
Normal heights are heights above sea level, one of several types of height which are all computed slightly differently. Alternatives are: orthometric heights and dynamic heights....

s from the normal plumb line. The difference between the resulting quasi-geoid and the reference ellipsoid
Reference ellipsoid
In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body....

 is called the height anomaly or quasi-geoid height.



Change-over from NN to NHN


Since 1 January 2000 the whole of Germany has changed its height system over to normal height
Normal height
Normal heights are heights above sea level, one of several types of height which are all computed slightly differently. Alternatives are: orthometric heights and dynamic heights....

s based on the datum of the Amsterdam 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 the Reseau Européen Unifié de Nivellement or REUN, which standardises the height systems of the European states. Heights in this system are given in metres above NHN or m (NHN).

The NHN was introduced because for heights above Normalnull the actual gravitational field of the earth
Earth's gravity
The gravity of Earth, denoted g, refers to the acceleration that the Earth imparts to objects on or near its surface. In SI units this acceleration is measured in metres per second per second or equivalently in newtons per kilogram...

 was not taken into account. As a result there were changes in both the old West German normal orthometric heights
Geodetic
Geodetic is an adjective meaning pertaining to geodesy, the science of measurement of the earth. See also:* Geodetic system* Geodetic airframe...

 (new methods of calculation) and the normal heights of East Germany (referred to the new Normaal Amsterdams Peil
Normaal Amsterdams Peil
Normaal Amsterdams Peil or Amsterdam Ordnance Datum is a vertical datum in use in large parts of Western Europe. Originally created for use in the Netherlands, it was adopted by Prussia in 1879 under the name Normalnull, and in 1955 by other European countries.Mayor Johannes Hudde of Amsterdam in...

 datum.

That said, current relief map
Cartographic relief depiction
Terrain or relief is an essential aspect of physical geography, and as such its portrayal presents a central problem in cartography, and more recently GIS and 3D Visualization....

s often show heights above the old reference planes.

Old East German height system


In the East Germany
German Democratic Republic
The German Democratic Republic , informally called East Germany by West Germany and other countries, was a socialist state established in 1949 in the Soviet zone of occupied Germany, including East Berlin of the Allied-occupied capital city...

 normal heights used to be referred to as heights above Höhennormal or HN. 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 the spirit level
Spirit level
A spirit level or bubble level is an instrument designed to indicate whether a surface ishorizontal or vertical . Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some...

points of first order are between 7 and 16 cm.

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