The importance of the orthometric correction in the North of Italy considering a global model of geoid: a real case study

Today one of the open problems from the geodetic point of view is the determination of a geometric (and not physical) reference system that allow the harmonisation of the altimetric reference systems at continental scale. Trying to solve this problem at European level, one of the goal of the IAG Inter-Commission Project 1.2 is to provide the fundamentals for the installation of a unified european vertical reference frame. The EVRS (European Vertical Reference System) is a zero-tidal gravity-related height reference system, based on normal heights. Today all people use and know the orthometric heights, not the normal ones. So the goal is to move from the first one to the second one. In order to have correct normal heights the first step is to have correct orthometric heights: that does not always happen in the sense that if the orthometric corrections are not applied to the heights determined by high-precision leveling, it is not possible a correct unification of the reference systems. Within the HELIDEM (HELvetia-Italy Digital Elevation Model) project, among whose aims is to unify the reference systems between Italy and Switzerland with regard to regions involved in the Project, it was decided to analyze whether the orthometric corrections obtained for this area, considering a global model of geoid undulation (EGM2008), were sufficient to correct the elevations determined by static GNSS surveys carried out by Politecnico di Torino and Istituto Geografico Militare (IGM). In order to achieve the objectives of the project mentioned above, it was necessary to perform GNSS static campaigns on benchmarks of the Italian geodetic reference network that were subsequently leveled with high-precision leveling. The considered benchmarks are settled in the Piedmont area and were chosen in the way that the height difference between two adjacent vertices was limited mainly to reduce elevation waste errors resulting from static GNSS measurements. Both dynamic and orthometric corrections have been computed in order to finally get geopotential numbers, normal heights and orthometric heights from raw levelling increments. In order to complete and make the analysis more significant, some measured gravity values were also considered for the estimation of the orthometric corrections, in order to compare these results with those previously described. This analysis showed that the measured gravity values well agree with those obtained from the EGM2008 up to 800-1000 meters of altitude, since the values of orthometric corrections obtained in both cases are quite agree (the order of magnitude of the differences are centimeters).