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Deployment and calibration of reference reflectance tarps for use with airborne imaging sensors

Moran, M. Susan, Bryant, Ross B., Clarke, Thomas R., Qi, Jiaguo
Photogrammetric engineering and remote sensing 2001 v.67 no.3 pp. 273
cleaning, digital images, equations, heat, image analysis, reflectance, remote sensing, sensors, wind
Chemically treated canvas tarps of large dimension (8 by 8 m) can be deployed within the field of view of airborne digital sensors to provide a stable ground reference for converting image digital number (DN) 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 (θs and θv), 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 θs, θv, and for reasonable levels of tarp dirtiness. Results showed that, for operational values of θs and θv and for reasonable levels of tarp dirtiness, the variation of measured tarp p from the factory-designated p could easily be greater than 50 percent. On the other hand, we found that, if tarps were deployed correctly and kept clean through careful use and periodic cleaning, and if tarp p was determined through calibration equations that account for both θs and θv, 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. The major limitation of tarps as calibration sources was related to the difficulty associated with deploying heavy, cumbersome tarps under normal field conditions characterized by moderate wind, dust, heat, and possibly mud. This study should provide tarp users with the information necessary to properly deploy tarps and process results for accurate image interpretation.