Abstract: The determination of precipitable water vapor (PWV) is one of the basic objectives of GPS meteorology. PWV calculated from radiosonde data is usually used as a reference to assess GPS derived PWV.The radiosonde-based PWV is derived by integrating the water vapor density along a balloon's uprising path which is a function of the observed temperature,relative humidity at different heights.Hence,the meteorological observations errors propagate to the calculated PWV.Moreover,these meteorological observations are discrete,and a numerical integration scheme is employed to calculate the PWV.Therefore,the density of observations also influences the accuracy of the PWV (approximation error).In this study,we analyze these two error sources in the determination of PWV using 50-day Hong Kong radiosonde data.We estimate an approximation error of 0.5 mm and observation error of 1.2 mm in calculating PWV.The total impact of the both error sources is about 1.3mm.