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

Chemically treated canvas tarps of large dimension (8 by 8 m) can be deploy ed within the field of view of airborne digital sensors to provide a stable ground reference for converting image digital number (DN) to surface refle ctance factor (rho). However, the accuracy of such tarp-based conversion is dependent upon a good knowledge of tarp rho st a variety of solar and view angles (theta (s) and theta (v)), and upon good care and proper deployment of tarps. In this study a set of turps of rho ranging from 0.04 to 0.64 we re evaluated to determine the magnitude of error in measured tarp rho assoc iated with variations in theta (s), theta (v) and for reasonable levels of tarp dirtiness. Results showed that for operational values of theta (s) and theta (v) and for reasonable levels of tarp dirtiness, the variation of me asured tarp rho from the factory-designated rho could easily be greater tha n 50 percent. On the other hand, we found that, if tarps were deployed corr ectly and kept clean through careful use and periodic cleaning and if tarp rho was determined through calibration equations that account for both thet a (s) and theta (v) the greatest sources of err or were minimized. General calibration equations were derived and provided here; these will be useful for applications with tarps of the same factory-designated rho values as th ose used in this study. Furthermore, equations were provided to allow calib ration coefficients to be determined front the value of factory-designated rho for the visible and near-infrared spectral bands. The major limitation of tarps as calibration sources was related to the difficulty associated wi th deploying heavy, cumbersome tarps under normal field conditions characte rized by moderate wind, dust, heat, and possibly mud. This study should pro vide tarp users with the information necessary to properly deploy tarps and process results for accurate image interpretation.