Brdf Effects in Remotely Sensed High Resolution Images of Urban Areas

Satellite remotely sensed images with a resolution of 1 m are expected to be available in the near future. These images are well suited for the study of inhomogenous areas like cities. This paper presents research projects of our group concerning the bidirectional reeectance properties of man-made surfaces, such as roofs, streets, etc. The BRDF eeects (like e.g. specular reeection for forward scattering or Hot Spot for backward scattering) can lead to incorrect classiication results. The focus of this paper is the presentation of the measurement of the BRDF of a roof with corrugated tiles. The distance sensor-target was about 70 m. The sensor consists of two hyperspectral units, measuring from 610 to 1650 nm. The incoming irradiance was determined with a Spectralon panel. Measurements of the shaded roof were used to correct for skylight eeects. Particular attention was given to estimating the error, which turned out to be 10 to 12 % on average. The deviation of the measured BRDF values of the roof from Lambertian behavior was 25 % on average, with a maximum of 69 %. We tted an empiric, analytic function known as 'Walthall model' to the data. We used a version modiied to incorporate Helmholtz reciprocity and specular reeection. This function deviates 15 to 20 % on average from the measured roof data, with a maximum of 47 %. The failure of the model to pass a 2 test is due to the lack of rotational symmetry of the roof tiles. The specular peak shows a strong increase with wavelength. Two examples of BRDF eeects on asphalt in images acquired with an airborne Daedalus scanner are shown. In the principal plane Hot Spot and specular eeects can be seen, in the cross principal plane there is a small increase for large zenit angles.