Digital Orthoimages in Architectural Photogrammetry Using Digital Surface Models

The paper describes a new approach for the generation of architectural orthoimages using digital surface models, based on a CAD model of the object. The model is supposed to consist of surface elements, plane or curved. All surface elements of the CAD model are orthogonally projected onto the reference plane. For each grid element the elevation above or below the reference plane are calculated and stored in a corresponding digital surface model with the same grid distance as the required orthoimage. Higher elevations replace lower elevations. The result is a matrix with the maximum elevations over the reference plane. To calculate the grey value of each pixel in the orthoimage the corresponding position in the distorted image has to be determined. This can be achieved using the collinearity equations, the data of the interior and exterior orientation of the distorted image and the digital surface model. Occluded areas have to be excluded from the calculations.