The International Archives of the Photogrammetry. Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
254 
• They require volumetric delimitation of the objects that 
want to be rectified using stereoscopic restitution 
techniques, which means a product’s price increase. 
Therefore, a delay in orthophoto coverage generation, 
which is not assumable nowadays. 
• They do not do an analysis of multiple incidences that 
considers the best perspective solution to avoid 
stretching and occluded areas. 
• They are semi-automatic, it is necessary to decide which 
photographs are needed for the final completion of true 
ortho. 
• They do not resolve the appearance of dark areas due to 
shadows, attenuating or eliminating them. 
As a result, there is not a demand of this product, therefore, 
there are few new applications derived from an improvement of 
the study and interpretation of aerial imagery. It is essential to 
evolve in this sense and review the acquisition and production 
systems to lower prices and times of production. 
Motivated by the problems from actual orthoimagery, with the 
interest of improving their quality, to allow getting planimetric 
measurements directly from any element represented, and that 
the most of the surface is displayed with the highest radiometric 
quality, the DATOS project was proposed, funded by the 
Ministry of Industry, Tourism and Commerce within the 
PROFIT program. The main objective was to develop a 
prototype of True Ortho-Rectification Software to reduce to the 
maximum the unfavourable elements that increase the price in a 
disproportionate way, not allowing its use within the GIS and 
Cartography market. 
A derived objective of the True Orthophotos availability was 
the rapidity and simplicity of extracting information directly 
from the imagery in a precise way, with no need of extra field 
work. Therefore, this benefits the higher availability of 
geographic information not just for the different professional 
sectors but for the citizens. The orthoimage coverage is 
obtained rapidly, they are integrated easily in Cartographic and 
GIS servers, from which it would be possible to update special 
data with computer technology not very specialized and easy- 
use tools. 
2.1 Preliminary Studies 
Because of the complexity of the problem, it was raised the 
need to do a preliminary study that allowed to know in detail 
the actual problematic of True Orthophoto production within 
urban areas (Antequera, 2005) including a review of the actual 
methods and its procurement, to both commercial and 
investigation levels. 
The direct analysis of the real problems derived from the actual 
scheme of Ortho-Rectification in urban areas has allowed a 
detailed study of existent problems within large scale 
orthoimagery in urban areas. On the other hand, this stage has 
also been useful to review the actual solutions implemented in 
commercial software. 
One of the conclusions of this study was that the requirements 
of a GO should be normalized from a geometric point of view, 
not just because of the precision parameters established for an 
equivalent cartography, but also for the critical distance in 
which unaccepted occluded areas appear, such as the 
disappearance of very stretch streets due to leaning buildings. 
The preliminary study allows us to do an optimal planning of 
projects (depending on the buildings height and urban structure 
orientation, the flight axis is planned and the focal distance and 
overlapping percentages more adequate are chosen) minimizing 
the effects of occlusions appearing in GO, and also confirming 
the production problems from actual solutions invaliding this 
procedure. 
Once the principal problems was established, the actual 
methods of True Orthophoto generation have been revised. This 
task have allowed to put on scene the most advanced methods 
actually in research, to begin from the actual developments and 
try to improve the procedures and algorithms already 
experimented, and to consider the best solution to develop the 
LRTO application. 
2.2 State of the Art 
The actual methodology requires the volume definition of the 
objects (in vector format) that appears leaning in the aerial 
photography. This is a not automatic process, which needs 
specific human, software and hardware resources increasing the 
cost of the product and make complicate its introduction within 
the GIS and Mapping market. Moreover, its geometric quality is 
not enough precise as it is required according to the scale 
tolerances. Any error in the imagery orientation, like the error 
from the capture, makes the existence of a disorder between the 
vector and the image that does not allow the direct application 
in the calculation of the true ortho. This problem has been 
occurring historically in the production of linear maps, 
according with technology and needs of the moment, that have 
served for a determined application, but that have been replaced 
by newer coverts that require higher precision and a new 
structure of information adapted to new tools of exploitation. 
Actual technology allows precise and economic DSM 
production. A perfect knowledge of the digital elevation model 
allows a perfect correction of aerial photographs. LIDAR 
systems offer in an automatic way a great density of data in its 
first return where the real surface of terrain is shown. The 
knowledge of building contours and the definition of objects 
methodologies have been registered very important 
improvement related with the automatic (and semi-automatic) 
data extraction processes. Its implementation within 
orthorectification processes is immediate, nevertheless a review 
of the rectification algorithms is needed, that bear in mind this 
information. The differential methods developed and calculation 
strategies to simplify the orthorectification process, that allowed 
the procurement of digital orthoimages very efficiently, must be 
improved to rectify pixel by pixel methods, looking at all the 
information within the photograph and is corresponding 
elevation in the terrain. 
Most of existent techniques of true ortho generation are based 
in algorithms of Z-Buffer visibility (Catmull, 1974; Amhar et 
al., 1998; Rau et al., 2000; Rau et al., 2002; Sheng et al., 2003; 
Zhou, 2005), imposing to methods of occluding areas detection 
from digital models of buildings combined with digital models 
of terrain (Amhar et al., 1996; Kuzmin et al., 2004), that require 
the availability of high acquisition cost data. 
Methods based in Z-Buffer algorithms resolve the ambiguity of 
each orthophoto pixel considering the distance of the 
perspective centre to the point in question, considering visible 
the closest ones. Nevertheless, this technique presents certain 
problems that make these algorithms not to be implemented in 
market solutions: 
• Sensibility to digital terrain model resolution in relation 
to image resolution, which effect implies an imprecise 
detection of not visible zones. 
• The need to introduce additional points in building 
fronts that implies to come back to the use of digital 
models of buildings.