Atmospheric effects on interferometric synthetic aperture radar(InSAR) measurements are quantitatively studied based on a tandem pair of SAR data and a month-long continuous GPS tracking data obtained at six stations. Differential atmospheric signals extracted from the SAR data for two selected areas show apparent power law characteristics. The RMS values of the signals are 2.04 and 3.66 rad respectively for the two areas. These differential delays can potentially cause in the two areas peak-to-peak deformation errors of 3.64 and 6.52 cm, respectively, at the 95% confidence level and Gaussian distribution. The respective potential peak-to-peak DEM errors are 123 and 221 m. The GPS tropospheric total zenith delays estimate indicates that a peak-to-peak error of about 7.8 cm can potentially be caused in a SAR interferogram with only 1 d interval at the 95% confidence level. The error increases to about 9.6 cm for 10 d interval. The potential peak-to-peak DEM and deformation errors estimated from GPS total zenith delay measurements are however quite similar to those estimated from InSAR data. This provides us with a useful tool to pre-estimate the potential atmospheric effects in a SAR interferogram before we order the SAR images. Nevertheless, the results reveal that even in a small area the atmospheric delays can obscure centimetre level ground displacements and introduce a few hundred meters of errors to derived DEM.