Measurement of Reflectance Properties of Asphalt Surfaces and Their Usability as Reference Targets for Aerial Photos

Reference targets with known reflectance properties are needed in remote-sensing in-flight calibration. The spectral and directional reflection properties of nine asphalt surfaces and concrete, and sand were measured. Corresponding polarization properties were also measured for five asphalts and for both sand and concrete. Measurements were obtained using the Finnish Geodetic Institute Field Goniospectrometer. The newly constructed smooth asphalt surfaces had lowest reflectances, and they produced strong forward scatter. The aged and deteriorated surfaces produced more isotropic scatter. The overall reflectance of the aged surfaces was higher than that of the newly constructed surfaces, and they were darkest when viewed close to nadir. Near nadir reflectance of all asphalt surfaces had low angular dependence. Light reflected from the newly constructed asphalt surfaces was found to have a large polarization ratio in the forward direction, as the aged asphalt surfaces were found to be less polarizing. All measured asphalt surfaces were spectrally flat, without dominating features. The measurements showed clearly that asphalt surfaces cannot be used as stable reflection targets without additional knowledge of the asphalt surface. Asphalt can serve as medium-accuracy white balancing media, but more quantitative use for calibration purposes requires the reflection properties to be either individually measured at each location or the properties of the asphalt to be known. The latter is a possible practical element in digital aerial image calibration, but it requires further studies.