Non-steady-state vertical velocities of up to 5 m y-1 exceed the vertical surface-parallel-flow components over much of the ablation area of Storstrømmen, a large outlet glacier from the East Greenland ice sheet. Neglecting a contribution to the vertical velocity of this magnitude, results in substantial errors (up to 20%) also on the south north component of horizontal velocities derived by satellite synthetic aperture radar interferometry (InSAR) measurements. In many glacier environments the steady-state vertical velocity component required to balance the annual ablation rate is 5 to 10 m y-1 or more. This indicates that the surface parallel flow assumption may be problematic also for glaciers in steady state. Here we derive the three-dimensional surface velocity distribution of Storstrømmen by using the principle of mass conservation to combine InSAR measurements from ascending and descending satellite tracks with airborne ice-sounding radar measurement of ice thickness. The results are compared to InSAR velocities previously derived by using the assumption of surface parallel flow, and to velocities obtained by Inin-situ GPS measurements. The velocities derived by using the principle of mass conservation are in better agreement with the GPS-velocities than the previously calculated velocities derived with the assumption of surface parallel flow.