The Remote Sensing Group at the University of Arizona has been using reflectance-based vicarious calibration of earth-observing satellites since the 1980s. Among the sensors characterized by the group are the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the MODerate Resolution Imaging Spectroradiometer (MODIS) that are both on NASA's Terra platform. The spatial resolution of MODIS requires that the group use a large-sized site such as Railroad Valley Playa, Nevada as a test site. In addition, the large footprint size of MODIS forced a modification to the ground-sampling scheme for the surface reflectance retrieval. This work examines the impact of the new sampling scheme through coincident ASTER and MODIS imagery making use of the higher resolution spatial resolution of ASTER. ASTER and MODIS imagery were obtained for dates on which both sensors imaged the Railroad Valley test site and ground-based data were collected at the site. The results of the comparison between the sensors shows differences in the radiometric calibration that exceed the accuracy requirements of the sensors, but that the sampling strategy for large-footprint sensors produces reflectance-based results at the same 3% level of accuracy as that for small-footprint sensors.