Sampling of Bidirectional Reflectance from Multiangular High Resolution Airborne Imagery

The present investigation describes a method for sampling bidirectional reflectance information from multiangular airborne images. The method uses high resolution surface models in the determination of the location of the imaged point on the ground and in the image, and finally the orientation of the measured surface fragment. Two test sites were imaged with a wide range of azimuth angles at two different times. A high resolution HRSC-A stereo camera was used for image acquisition. Algorithms to reconstruct the image acquisition and retrieve the image samples from the HRSC-A image data, used with GPS and INS data and automatically derived high resolution digital terrain models, were implemented and used to determine the viewing and illumination geometry on the target surface. The image digital number of a sample point was recorded as an uncalibrated reflectance measure. A large number of directionally defined samples and a wide angular range of sample geometry were obtained by taking the surface orientation, extracted from the high resolution digital surface model, into account. The images were classified in order to aggregate the samples of a certain surface type acquired from the different images. The sampled reflectance data were tested and analysed by investigating the bidirectional reflectance of seven agricultural and forest targets. Angular uncertainty of the data, and other sources of error affecting the data quality were studied. Directionally defined reflectance data were acquired to assist in future the modelling and correction of bidirectional effects on airborne optical images used in mapping, urban modelling and for establishment of a bidirectional reflectance database. The multiangular image data, the developed sampling methods and the obtained bidirectional dataset proved to be feasible for investigations of the bidirectional effects of natural targets. Airborne imagery, including scanner and frame images, combined with digital surface models permits extensive investigation of the bidirectional reflectance of a wide range of natural objects and large habitats.