Using GOCE to straighten and sew European local geoids: preliminary study and first results

National geoids of neighbouring countries usually do not fit to each other along the borders. This problem is mainly due to inconsistencies between the national reference frames used to estimate local geoids: it is well known that a change in the height datum and in the reference ellipsoid causes systematic effects in terms of geoid undulation. Therefore, before merging national geoids, these effects should be estimated and removed. The availability of a global geoid coming from satellite data, such as one of the GOCE models or a GOCE-GRACE combined model, can be of great importance to solve this problem. In fact, these models are not affected by local biases since they do not make use of any ground gravity data and they are referred to a global geocentric ellipsoid. In the present work a merging strategy based on two steps is proposed. Firstly, the bias and the systematic effect due to an ellipsoidal centre variation are estimated for each national geoid by exploiting GOCE data; particular attention is dedicated to the covariancemodelling of all involved quantities. Secondly, these straightened geoids are “sewn” to each other by applying a standard collocation procedure along the borders. In this way the final result is an unbiased geoid, joining in a optimal way the national models. The merging strategy is here applied to create a new geoid model for Spain, Portugal, France, Italy, Switzerland and part of the Mediterranean Sea.