Detrended crustal deformation measurements from a global network of up to 200 continuous Global Positioning System (GPS) tracking sites are inverted for low degree and order (n, m ≤ 6) spherical harmonic series of surface mass variations. An independent geophysical model of the atmosphere, oceans, and water and snow on land is also used to guide the spherical harmonic truncation and to evaluate aliasing errors in the results. Inversion uncertainties decrease significantly as spatial coverage and data quality improve with time, especially in the last few years. Consequently, GPS inverted seasonal geocenter and low degree zonal harmonics converge to those determined by measurements from satellite laser ranging (SLR). Significant n > 1 and non-zonal variations have also been found.