A Radiometric Aerial Triangulation for the Equalization of Digital Aerial Images and Orthoimages

Atmospheric haze variations, temporal differences, atmosphere's or ground surfaces' and Bidirectional Reflectance Distribution Function (BRDF), are well-known Sources of radiometric heterogeneities in aerial images. It is necessary to correct them for many applications, such as the generation of large seamless mosaics of orthoimages, or land-cover classifications. This contribution describes a method for equalizing digital aerial images based on a parametric, semi-empirical radiometric model. The model's parameters are computed through a global least-squares minimization process, using radiometric tie-points in overlapping areas between images. The method may be called a "radiometric aerial triangulation" since its principle is quite similar to a standard aerial triangulation. It leads to a relative equalization between images, keeping their original contrast and color information. The method has been successfully applied operationally to more than forty projects comprising three to four thousands digital images each, for the creation of orthoimages over France. The results are good as long as atmospheric conditions are favorable and stable.