Satellite altimeters measure variability of geostrophic currents but not the time-mean geostrophic velocity (and therefore not the total absolute geostrophic flow). This poses challenges in the Southern Ocean, because the Antarctic Circumpolar Current is a top-to-bottom feature, with non-zero bottom velocities, and hydrographic estimates of absolute dynamic topography are unable to capture the effects of the bottom velocities. Since the advent of exact repeat altimetry, a variety of different methods have been tested. Methods based on independent gravity estimates are successful at capturing large-scale features but do not appear to represent small-scale variations in mean dynamic topography. The choice of mean has implications for interpretation of the large-scale flow.