ordnance datum

In Germany, the reference surface for levelling is the mean sea level of the North Sea. Altitudes are given in Meter über Normalnull (m ü.NN), Meter über Normalhöhennull (m ü.NHN) or Meter über Höhennull (m ü.HN), i.e. meters above mean sealevel, based on the mean sea level of Amsterdam.

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