Differences between long term precipitable water (PW) time series derived from radiosondes, microwave water vapor radiometers, and GPS stations reveal offsets that are often as much as 1-2 mm PW. All three techniques are thought to suffer from systematic errors of order 1 mm PW. Standard GPS processing algorithms are known to be sensitive to the choice of elevation cutoff angle at this level. We present a processing protocol that is shown to reduce elevation angle dependence in geodetic estimates of zenith delay and, hence, reduce the systematic errors in derived precipitable water. This protocol includes use of the Niell zenith delay mapping functions and International GPS Service phase center models.