The Norwegian Height System NN1954 Revisited
Keywords:NN1954, normal height, spheroidal-orthometric height, orthometric height, postglacial rebound
AbstractVertical reference systems based on national precise levelling networks have been realised in Europe since the 1860’s. The heights are related to gravity, which by convention may be measured values or amodel. The Norwegian Height System was adjusted in 1956 when the southern part of the country had been levelled once. Because of the lack of measured gravity, Clairaut’s formula for gravity was used in the orthometric correction. The intention was to establish an orthometric height system, but this was never tested or confirmed. In the literature the system is often referred to as orthometric, but some times as normal orthometric. This study shows that the Norwegian Height System was realised by a strongly deformed net due to lack of land uplift corrections. The derived heights are neither normal nor orthometric. When the land uplift is handled in a proper way, the heights are close to normal heights. The remaining small differences are shown to depend on the free air anomaly.
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