Satellite UV mapping is usually based on the independent pixel approximation (IPA) which neglects horizontal photon transport between adjacent columns. Horizontal inhomogeneity of cloud fields therefore causes uncertainties in the derived UV radiation fields. While these effects are small for large pixel satellites, the broken-cloud errors increase as the pixel size decreases. By comparing results of 1-d and 3-d UV radiative transfer calculations for three selected cloud scenes that cover a rather broad range of cloud inhomogeneity the main 3-d cloud effects on the atmospheric UV transmission are identified and quantified in their order of magnitude. With respect to the different spatial resolutions of satellite instruments it is further shown how 3-d cloud effects average out with increasing spatial scale. It turns out that locally the IPA cause maximum uncertainties up to ¿100% for a spatial resolution of about 1¿1 km2 (e.g., AVHRR), they are reduced to ¿10% for a resolution of about 15¿15 km2 and below 5% for a resolution greater than 30 km (e.g., TOMS). ¿ 2001 American Geophysical Union