The use of GNSS/levelling and gravity data for the Spanish height system unification

From 2001 to 2008, the National Geographic Institute of Spain (IGN) carried out the REDNAP project to the establishment of a National High Precision Levelling Network in the whole Spanish territory. Within REDNAP, spirit levelling and gravity observations were complemented by GNSS data. The levelling network of the continental area and those of the main islands were referred to different tide gauges thus producing different height systems. In the paper, the GOCO-05S model from the GRACE and GOCE gravity missions is used to unify such systems. More precisely, it is used to estimate the normal heights of the mean sea level at the reference tide gauges, i.e. the height system biases. In the proposed solution such biases are determined through the least squares adjustment of the differences between height anomalies derived from GNSS/levelling and height anomalies derived from a proper combination of global gravity models. An accurate modelling of the observation error covariances is taken into account as well. The estimated accuracies of the resulting biases are in the order of few centimetres apart from that of continental Spain which is an order of magnitude better.