Over the last two decades, repeat pass interferometric synthetic aperture radar (InSAR) has been widely used as a geodetic technique for measuring the Earth's surface, including topography and deformation. Like other astronomical and space geodetic techniques, repeat pass InSAR is limited by the variable spatial and temporal distribution of atmospheric water vapor. In this paper, a topography-dependent turbulence model (GTTM for short) has been developed using GPS data only to produce zenith path delay difference maps for InSAR atmospheric correction. Application of the GTTM model to ERS Tandem data over the Los Angeles Southern California Integrated GPS Network area has shown that use of the GTTM can reduce water vapor effects on interferograms from ~10 mm down to ~5 mm, which is of great interest to a wide community of geophysicists. The principal finding of this paper is that interpolation methods should be applied to zenith total delay (ZTD) differences from different times instead of ZTD values themselves for the purpose of InSAR atmospheric correction. This is crucial to reduce (if not completely remove) topographic effects on ZTD values.