Submitted to: Photogrammetric Engineering and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 1999
Publication Date: N/A
Interpretive Summary: With recent advances in digital technology, it is now possible to obtain high-resolution images of agricultural regions from aircraft-based cameras. When using such images to monitor crop and soil conditions for farm management, it is necessary to standardize the image data to avoid image variations due to changes in atmospheric conditions and sensor calibration. This standardization can be accomplished by deploying a large chemically treated canvas tarp of known reflectance within the flight line of the airborne camera. In this study, we investigated the sources of error in tarp deployment for image standardization. We found that tarp reflectance was affected by solar angle, viewing angle, dirtiness, and size. We derived tarp calibration equations that could be used to determine tarp reflectance at any time of day or year, and we determined the magnitude of tarp degradation under sunny, windy, and dusty conditions. This calibration will improve reflectance accuracies by 100% in extreme cases, and allow much better sensitivity of airborne images to changes in soil and crop conditions. This will allow aircraft-based images to be used for seasonal monitoring and managing of crops and grasslands to improve utilization of scarce and expensive resources.
Chemically treated canvas tarps of large dimension (8 x 8 m) can be deployed within the field of view of airborne digital cameras to provide a stable ground reference for converting image digital number to surface reflectance factor (p). However, the accuracy of such tarp-based conversion is dependent upon a good knowledge of tarp p at a variety of solar and view angles, and upon good care and proper deployment of tarps. In this study, a set of tarps of p ranging from 0,04 to 0.64 were evaluated to determine the magnitude of error in measured tarp p associated with variations in solar angles, view angles, and tarp dirtiness. Results showed that for operational values of solar and view angles and for reasonable levels of tarp dirtiness, the uncertainty of tarp p can easily be greater than 50% of the true tarp p. On the other hand, we found that if tarps were deployed correctly and kept clean through careful use and periodic cleaning, and if tarp reflectance was determined through calibration equations that account for both solar and view angles, the greatest sources of error were minimized. General calibration equations were derived and provided here; these will be useful for applications with tarps of the same factory-designated p values as those used in this study. Furthermore, equations were provided to allow calibration coefficients to be determined from the value of factory-designated p for the visible and near-infrared spectral bands. Finally, based on tarp deployments in field studies, we determined that the minimum ratio of tarp width relative to camera spatial resolution was 8:1. The major limitation of tarps as calibration sources was relative to the difficulty associated with