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The Working Group on “Regional Dense Velocity Fields” (see http://epncb.oma.be/IAG) of the International Association of Geodesy (IAG) aims at densifying the International Terrestrial Reference Frame and creating a dense velocity field based on regional and global GNSS networks. With the goal to generate a high-quality solution for a core network, several newly reprocessed global and regional cumulative position and velocity solutions were submitted to the Working Group. In order to find a consensus on discontinuity epochs for stations common to several networks (an issue which was problematic in previous submissions), the new submissions were restricted to contain only the core networks over which the analyst has full control so that ITRF2008 discontinuities could be applied. The 3D-RMS of the agreement of the new solutions with the ITRF2008 (after outlier rejection) varies between 0.6 and 1.1mm/year; it is extremely good for some solutions, while others still require more iteration to reach the required level of agreement. A part of these disagreements has been identified and often originates in the use of different data time spans within the ITRF2008 and submitted solution.

In a collaborative effort with the regional sub-commissions within IAG sub-commission 1.3 “Regional Reference Frames”, the IAG Working Group (WG) on “Regional Dense Velocity Fields” (see http://epncb.oma.be/IAG) has made a first attempt to create a dense global velocity field. GNSS-based velocity solutions for more than 6,000 continuous and episodic GNSS tracking stations, were proposed to the WG in reply to the first call for participation issued in November 2008. The combination of a part of these solutions was done in a two-step approach: first at the regional level, and secondly at the global level. Comparisons between different velocity solutions show an RMS agreement between 0.3 and 0.5 mm/year resp. for the horizontal and vertical velocities. In some cases, significant disagreements between the velocities of some of the networks are seen, but these are primarily caused by the inconsistent handling of discontinuity epochs and solution numbers. In the future, the WG will re-visit the procedures in order to develop a combination process that is efficient, automated, transparent, and not more complex than it needs to be.