Orthometric correction and normal heights for Italian levelling network: a case study

Levelling increments must be corrected for gravity in order to get proper dynamic or orthometric heights. In Italy, most of the levelling lines have no associated gravity observations. Thus, only levelling increments are available, and corrections were computed only on a subset of the existing lines. However, gravity is densely observed, and thus, predictions of gravity along the levelling lines can be computed. This, in principle, would allow the estimation of corrections along the whole Italian levelling network. Furthermore, global model-derived gravity could be used for the same task (e.g. one could use the EGM2008 global geopotential model to get gravity estimates). To check for the reliability of these procedures, a test has been performed along levelling lines in the western Alps area. Both dynamic and orthometric corrections have been computed in order to finally get geopotential numbers, normal heights and orthometric heights from raw levelling increments. The results proved that reliable results can be obtained using the Italian gravity database, while predictions based on the EGM2008 model led to poorer estimates.