DIGITAL ORTHOIMAGES IN ARCHITECTURAL PHOTOGRAMMETRY 
USING DIGITAL SURFACE MODELS 
Albert Wiedemann 
Technical University of Berlin, Department for Photogrammetry and Cartography 
Sekr. EB9, Straße des 17. Juni 135, D-10623 Berlin, Germany 
Tel: +49-30-314-21167, Fax: +49-30-314-21104, e-mail: albert@fpk.tu-berlin.de 
Commission V, Working Group 4 
KEY WORDS: Digital, Orthoimage, Architecture, Photogrammetry, Digital Surface Model 
ABSTRACT: 
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. 
KURZFASSUNG: 
Es wird ein neuer Ansatz zur Erstellung digitaler Orthophotos in der Architekturphotogrammetrie beschrieben. Er basiert 
auf einem Digitalen Oberfláchenmodell, welches aus einem CAD-Modell des Objekts hergeleitet wurde. Das CAD-Modell 
muß aus ebenen oder gekrümmten Fláchen bestehen. Das Digitale Oberflàáchenmodell ist ein Raster mit der Rasterweite 
des gewünschten Digitalen Orthophotos. Für jedes Rasterelement wird die senkrechte Hóhe über der Projektionsebene 
berechnet und registriert. Liegen über einem Rasterelement des Oberflàchenmodells mehrere Flächen, wird die höchste 
gewählt. Um den Grauwert jedes Pixels des Orthophotos zu ermitteln, muß die entsprechende Position im verzerrten Bild 
berechnet werden. Hierzu sind neben dem Digitalen Oberflächenmodell in den Kollinearitätsgleichungen die Daten der 
inneren und äußeren Orientierung des verzerrten Bildes erforderlich. Sichttote Räume müssen aus der Berechnung 
ausgeschlossen werden. 
1. INTRODUCTION Non-parametric approaches, like the projective rectifi- 
cation, deliver good results if the objects surface is com- 
The application of photogrammetry in civil and building Pletly plane. Fig. 1 shows a typical architectural image, 
engineering is not so much customary as experts on Fig. 2 the result of a projective rectification and the 
photogrammetry would like it. One of the reasons is the limitations of this approach. If the surface consists of 
time delay between the acquisition of images and the several planes the rectification may be carried out in 
delivery of the restitution results as well as the costs of — Separate steps and the results have to be combined after- 
the restitution process. Another reason is that the custo- ~~ wards (Marten et al. 1994). But the interactive partitioning 
mer receives the result of an interpretation process of the is a tedious task and is not suitable for irregularly curved 
photogrammetric operator. The purchaser has often pro- — Surfaces. Therefore a parametric approach using the ex- 
blems to describe exactly, which data he needs and in terior orientation of the image and a Digital Surface Model 
some cases the photogrammetric operator misunder- (DSM) is desirable. 
stands the order. 
In architectural photogrammetry the difference between 
In aerial photogrammetry orthoimages are cheap and fast the shortest and the longest distance to the object is 
available products with the geometric properties of maps, Much larger than in aerial photogrammetry. Therefore 
containing however the entire information of the image. errors in the digital surface models have an enormous 
The customer may extract the required data by himself ^ influence on the accuracy of the produced orthoimages. 
from the geometrically exact orthoimage. Errors in the planimetric position of discontinuities of the 
object's surface lead to unacceptable results. 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996