@EJFielding Thank you very much for your kindness and reply. I have Two GPS stations in my area where the horizontal and vertical displacement values are known due to the earthquake. I want to compare the displacement values that I obtained using DInSAR with the measured values for these two stations. . Should I assume a reference point(LOS displacement is zero) in the study area close to GPS stations . Subtract its LOS displacement value from the LOS displacement maps in the ascending and descending orbit, then apply the equations to obtain the absolute horizontal and vertical displacements? Or, after obtaining the horizontal and vertical maps, I can assume a point with zero displacement and subtract the value of its horizontal and vertical displacement from the horizontal and vertical displacement maps