Global Navigation Satellite Systems (GNSS) can provide a temporally dense set of geodetic coordinate observations in three dimensions at a limited number of discrete measurement points on the ground. Compared to this, the Interferometric Synthetic Aperture Radar (InSAR) technique gives a spatially dense set of geodetic observations of ground surface movement in the viewing geometry of the satellite platform, but with a temporal sampling limited to the orbital revisit of the satellite. Using both of these methods together can leverage the advantages of each to derive more accurate, validated surface displacement estimates with both high temporal and spatial resolution. In this paper, we discuss the properties of both techniques with a view to combined usage for improving future national datums. We apply differential GNSS processing to data observed at a local geodetic network in the Sydney region as well as time series InSAR analysis of Radarsat-2 data. We compare and validate surface displacements resulting from the two techniques at 21 geodetic monitoring sites equipped with GNSS and radar corner reflectors (CRs). The resulting GNSS/InSAR displacement time series agree at the level of 5 to 10 mm. ... mehrThis case study shows that co-located GNSS/CR sites are well-suited to compare and combine GNSS and InSAR measurements. An investigation of potential multipath effects introduced by the CRs attached directly to GNSS monumentation found that daily site coordinates are affected at a level below 0.1 mm. The GNSS/CR sites may hence serve as a local tie for future incorporation of InSAR into national datums. This will allow frequent updates of national geodetic networks and corresponding datums by using the large-scale and spatially dense information on surface displacements resulting from InSAR analyses.