normalnull

Normalnull (NN) is a historic vertical datum that was used in Germany and parts of Central Europe to express elevations above mean sea level. The term translates roughly as “normal zero” and served as the standard reference for height measurements in surveying, construction, and cartography from the 19th century into the late 20th century.

Definition and origins: NN was not defined by a single fixed point across all regions. Instead, the zero level depended on mean sea level as measured by tidal gauges along the North Sea and Baltic coastlines, with regional adjustments over time. The system developed within the broader effort to standardize surveying practices in the German states and successor countries.

Regional application: In practice, elevations on maps and in land records were given in NN, expressed in meters above this zero. Because the zero point could vary with historical tide gauge data and local corrections, older datasets may not be directly compatible with modern datums without transformation.

Transition to modern systems: Beginning in the late 20th century, German authorities adopted Normalhöhennull (NHN) as a unified vertical datum aligned with European standards and contemporary geodetic models. NHN provides a single zero level for the entire country, simplifying cross-border comparisons and digital elevation data. Consequently, NN is now obsolete for new measurements, though it remains visible in historical maps and documents.

Legacy: Today, NHN is used in official datasets, but NN can still be encountered in legacy records. Converting NN heights to NHN requires a transformation that accounts for the original anchor point and regional corrections applied at the time.