CORRECTION OF ATMOSPHERIC AND BIDIRECTIONAL EFFECTS IN MULTISPECTRAL ADS40 IMAGES FOR MAPPING PURPOSES

The radiometry of photogrammetric images is influenced by various effects from outside the camera. One prominent effect is the additional path radiance from atmospherically scattered sun light. This occurs especially at short wavelengths and long atmospherical path lengths, which gives rise to an increasing blueshift towards the borders of the images. Another effect is the bidirectional reflectance from the ground surface (BRDF). This effect depends on the illumination and the viewing geometry as well as on the wavelength and is caused by a varying amount of subpixel shadows on the ground. At high solar elevation, frame sensors encounter a bright area within the image, the so-called hot spot; line scanners like the ADS40 show an across-track brightness gradient. This prevents precise intraand intercomparison of images, affects spectral ratios and is adverse to proper mosaicking. In order to correct the blueshift in the images a dark pixel subtraction algorithm is applied to the data, which accounts for the largest effects of the atmosphere. The algorithm takes into account the view angle dependence of the path radiance by calculating column statistics. For the bidirectional effect an automated semi-empirical algorithm is presented to correct a set of line scanner images simultaneously to a defined viewing and solar geometry. Statistics of the image brightness are calculated and the model fitted to the averages. These statistics can be calculated either from all pixels or can be class specific. The method is applied to ADS40 data after system and atmospheric correction in order to produce a well-defined input for the orthorectification and mosaicking. As an example, images from a flight campaign are processed to orthophotos and mosaicked without further dodging or feathering. * Corresponding author.