In order to correctly interpret the surface information with remotely sensed data, the extinction effects of the atmosphere must be appropriately removed. The aerosol optical depth (AOD) serves as an important parameter in describing this effect. Several factors, such as the satellite observation geometry, terrain geometry, temporal variation of canopy, and so forth, produce errors in the AOD estimation. This study strives to avoid such errors by introducing an "optimal distance number" into the structure function method that has been developed previously in estimating the AOD. The results of our experiment measured by a ground-based sunphotometer showed that the average errors of the estimated AOD values was reduced from 40% to 11% after the "optimal distance number" was used, indicating a significant improvement.