Normalhöhennull
Mean sea level ( MSL ) is the reference surface for heights above sea level in the German main level network in 1992. The area forms a Quasigeoid. The reference height is fixed to a height fixed point on the Church Wall Horst. The geopotential of the point was determined in 1986 in the framework of the European Nivellementnetzes ( UELN ), starting from Amsterdam level.
Definition
The mean sea level surface is a theoretical reference surface. It is caused by removal of normal heights along the normal plumb line down. The deviation of the resulting quasigeoid from the reference ellipsoid is called the height anomaly or quasigeoid.
Conversion of NN on NHN
With a resolution of the Surveying Authorities of the German states began in 1993, the conversion of the reference altitude on normal heights of the German main level network DHHN92. The new mean sea level is also the basis for the United European Levelling Net ( UELN ), formerly known as Réseau Européen de unifié leveling ( REUN ), under which the height systems of the European countries are unified. The DHHN92 Heights are given in meters above mean sea level (m above sea level. NHN ).
The NHN was introduced because of the heights above mean sea level, the actual gravity field of the earth was not considered. This means that both the far West German normal - orthometric heights ( new calculation method ) and the normal heights of East Germany ( with respect to the new level Amsterdam) change. The heights vary - depending on the location - 0.06 to 0.16 meters. Through new calibration under conversion but deviations of 0.59 meters ( Zugspitze ) are also already occurred.
Older cards often contain levels above the old reference surfaces. Current topographic maps of the surveying authorities of the federation is based on NHN. Early 2013 report, most states ( except Berlin, Thuringia and Saxony- Anhalt) complete recording of the new digital topographic map 1:25,000 ( DTK). But it's not yet appeared in print all the cards. In DTK25 card is specified as height reference mean sea level, at DTK25 - T maps (scanned topographic maps ) but is still zero height (HN ) or sea level (NN ) are used.