The difficulties in validating aerosol optical depth from spaceborne lidars such as the Geoscience Laser Altimeter System (GLAS) on board IceSat and the lidar on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite with ground-based instruments are discussed. Because observations are often not collocated, matching errors, which increase with separation distance, confound the validation of instrumental errors. These matching errors can be assessed by comparing the aerosol total-column optical depth measured in a swath by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra satellite with point Aerosol Robotic Network (AERONET) Sun photometer measurements at different separation distances. In particular, the relationship of the correlation of the two sensors' aerosol total-column optical depth observations with increasing spatial separation is determined. The use of back trajectories to reduce the loss in correlation with increasing spatial separation is then evaluated. Matching errors are found to increase faster over land than over water sites, with the correlation dropping by 20% in 200 km over the land sites and 500 km over the ocean sites. Constraining the area over which the MODIS aerosol optical depth is calculated to within 30¿ azimuth of the average back trajectory only improved the correlation for a site where long-range transport of aerosols often occurs.