We present a direct comparison of nonisotropic, integrated water vapor measurements between a ground-based Global Positioning System (GPS) receiver and a water vapor radiometer (WVR). These line-of-sight water vapor observations are made in the straight line path between a ground station and a GPS satellite. GPS double-difference observations are processed, and the residual line-of-sight water vapor delays are extracted from the double-difference residuals. These water vapor delays contain the nonis¿tropic component of the integrated water vapor signal. The isotropic component is represented by the zenith precipitable water vapor measurement and can be scaled to a specific elevation angle based on a mapping function. The GPS observations are corrected for station-dependent errors using site-specific multipath maps. The resulting measurements are validated using a WVR which pointed in the direction of the observed satellites. The double-difference technique used to make these water vapor observations does not depend on accurate satellite clock estimates. Therefore it is especially well suited for near-real-time application in weather prediction and allows for sensing atmospheric structure that is below the noise level of current satellite and receiver clock errors. This paper describes the analysis technique and provides precision estimates for the GPS-measured nonisotropic water vapor as a function of elevation angle for use with data assimilation systems. ¿ 2001 American Geophysical Union