A NEW PROCEDURE FOR THE AUTOMATIC PRODUCTION 
OF TRUE ORTHOPHOTOS 
Andrea Biasion (*), Sergio Dequal (*),Andrea Lingua (*) 
(*) Politecnico di Torino — Dipartimento di Georisorse e Territorio 
Corso Duca degli Abruzzi, 24 — 10129 Torino - Italy 
Email: andrea.biasion@polito.it, andrea.lingua@polito.it, sergio.dequal@polito.it 
KEYWORDS: Photogrammetry, Cartography, Digital Orthorectification, Urban Orthoimage, DEM/DTM, City, Automation 
ABSTRACT: 
The digital orthophoto is a cheap and efficient product that is used to represent the correct shape of any 3D object, in photographic 
form, by means of its orthogonal projection onto a predefined plane. In mapping applications, if the zone to be represented contains 
urban areas, the problem becomes more complex because there are many discontinuity lines (breaklines) and hidden areas: this 
requires a more sophisticated orthoprojection procedure. The lack of information corresponding to the hidden areas is avoided if use 
is made of all the images (“multi-image” procedure) available in the photogrammetric block. More rigorous interpolation methods 
should be used to take into account the problem of discontinuities. Stereoplotting of the many break-lines (a difficult and expensive 
procedure) or, even, a DTM formed by a large amount of points (the so-called “dense DTM”, or DDTM) gives the necessary data. 
For this purpose, the authors have recently conceived and implemented an original software composed of the following modules: 
—  AccOrtho (=ACCurate ORTHOphoto), to produce rigorous digital orthophotos starting from multiple images and a DDTM; 
— GeneDDTM (GENEration of a DDTM), which is able to build a dense DTM (suitable for AccOrtho) from a 3D digital map, 
using sophisticated interpolation techniques. 
This paper describes the two software structures in detail and a practical application of both the programmes for a relevant mapping 
project, which is still in progress: a true orthophoto of the whole town of Turin, in 1:2.000 scale. 
1. INTRODUCTION correctly by Q but, if a traditional ortoprojection is used, point 
P is lost (hidden by point Q due to the geometry of the 
An orthophoto is a metrically correct photographic ^ projection) and is substituted by the visible point Q which is 
representation of the territory, with the same accuracy as a doubled (in Py) on the resulting orthóphoto. 
traditional topographic map. K also contains a great deal of 
information that is not present in a topographic map, such as the 
details that are present in aerial photos. The natural or 
antropized territory is represented as it really appears, without 
the introduction of codes or symbology as used in digital map 
production. An unskilled user can understand and correctly read 
an orthophoto, while the correct interpretation of a digital map 
requires a specific technical background. 
The geometry of this particular photographic cartography is 
obtained by orthogonal projection of each pixel of an aerial 
image onto a cartographic plane. In this way, the original 
perspective representation, as an aerial photo, is transformed 
into an equivalent metrically correct image: it is possible to Figure 1: Orthoprojection with an erroneous description 
measure angles and distances (in a known scale factor) and read of the shape 
coordinates on the orthophoto, exactly like on a traditional map. 
An orthophoto can be employed in several land. planning m # ES 
applications (infrastructural planning, land use monitoring, i.e.). ; C i y 
In an urban context, where the land surfaces are characterized 
by many discontinuities and hidden areas, there can be some lo 
problems with commercial software. 
It is necessary to use a specific procedure for the automatic 
production of a true and rigoruous orthophoto [Dequal et al., | 
  
  
  
  
  
  
  
  
  
2002]. p DTM 
2. TRUE ORTHOPHOTO OF A DISCONTINUOUS am 
SURFACE 4 ... OTTOPROD 
Po Qo 
The case of a DTM (see figure 1) that, only correctly describes 
the ground, and not the existing buildings, is considered. The 
projection of point Q originated an uncorrect location of point Figure 2: Orthoprojection with hidden areas 
Q»'; instead of the right position of Qy that also coincides with 
orthoprojection Po of point P. Figure shows this effect: the The image of the hidden area is not available on the considered 
base of the building 1s represented in the right position, while photo, and should be extracted from other photos (when 
the roof is moved to an incorrect position. m available). The doubled image however leads to confusion, and 
Let us now consider the case in which the building is also is obviously undesiderable. If these two effects are considered, 
described by the DTM (figure 2). Point Q is represented 
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