We analyze 580 continuous GPS stations in California from 5 analysis centers to quantify the uncertainty in published velocities and develop a composite velocity for each station. The horizontal positions are similar but the reported velocity varies by time series algorithm. Vertical rates for individual stations differ up to 5 mm/yr, with systematic differences in some areas. The published uncertainties show variability between analysis centers and are underreported, suggesting these formal errors do not reflect the true velocity uncertainties. Differences by a factor of 4 are found in the vertical and is comparable to deformation rates. An interpolated ensemble vertical velocity field is developed and regions with the highest rates of uplift or subsidence correspond to the largest variance in velocities between analysis centers, but high station density can reduce these uncertainties. Applications that rely on sub-centimeter GPS accuracy should consider the inherent uncertainty in published vertical velocity rate estimates.