Investigation of accuracy of GNSS velocities obtained by interpolation method

Turkey's national datum was defined for the first time in 2005 as a 4-dimensional Turkey National Reference Frame (TNRF). The TNRF is in the 2005 epoch and compatible with the ITRF-96 datum. The data obtained from cadastral studies in Turkey should be transformed to ITRF96 datum (2005.0 epoch) to archive. The velocities required for the transformation calculate by interpolation, except for the TUTGA and Tusaga-Active sites. There are many factors that affect GNSS velocities. Some of these can be expressed as changes caused by tectonic movements and insufficient data in the number of GNSS measurements.There are changes in the coordinates and velocities of the sites with the effect of the earthquakes in our country, which is located on active tectonic belts. The post-seismic period in which the earthquake effect continues can be between 1-2 months and 5-15 years, depending on the magnitude of the earthquake. During this time, the velocities and coordinates of the sites must to be updated. In this research, the velocities of C1 and C2 sites in Afyonkarahisar and its vicinity, which were calculated by the interpolation method in the previous years, were provided from the official institutions. Then, the current velocities of these sites were calculated by interpolation using the current velocity field published by the General Directorate of Maps (HGM). The velocity differences at each sites were calculated using these two velocity fields and difference maps were created for each coordinate component. According to the results, the maximum differences were found to be 17.14 mm/yr for the X component, 13.89 mm/yr for the Y component and 13.95 mm/yr for the Z component.