The International Archives of the Photoerammetrv. Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
In 2007, two new methods have been presented (Habib et al„ 
2007) to safe limitations and requisites of high cost of 
methodologies based in Z-Buffer existent algorithms, based in 
the analysis of angles along radial directions from nadiral point 
to detect occlusions: 
• Radial circular sweep method. Determines a map with 
occlusions doing a radial sweep of the DSM increasing 
the angular value of visibility a for each azimuth angle 0 
(Figure 1). 
Perspective center 
/i \ 
► o, *» ao 
Figure 1. Radial circular sweep method (Habib et al., 2007) 
• Spiral sweep method. Introduces the variant of the spiral 
sweep of DSM starting in the nadiral point checking 
directly angles of visibility in radial direction. 
These experimentally tested methods, present some problems 
for the generation of individual orthophoto that require the 
fusion of common areas to complete zones detected as 
invisibles, and with no mosaiking, for which no continuous 
coverts of true orthophotos are generated, having to introduce 
seamlines needed to mosaic orthophotos from the 
neighbourhood in a posterior stage. 
3. DEVELOPMENT OF THE ALTAIS-LRTO SYSTEM 
The Altais LRTO system was raised by modules, starting with 
the development of a basic orthorectification method, which 
could integrate progressively algorithms that resolve the 
problems that actual orthophotographs have, until the most 
rigorous solution was reached of True Orthophoto. 
3.1 Ground Ortho Module: GO+. 
Orthorectification Module to ground level based in a simple 
mathematic model of collinearity equations and additional 
parameters, which uses calibration data from the camera, and 
inner and exterior orientation data from imagery. 
The raised solution is rigorous through an orthorectification 
method pixel to pixel from the most nadiral image, different 
from the differential methods used in most of the existent 
programs that were simplified solutions adapted to computer 
technology of that moment. 
3.2 Right Ground Ortho Module: RGO+. 
It is an orthorectification module to ground level, which 
includes an algorithm that searches imagery depending on the 
incidence angle of the perspective ray. This module supposes 
the introduction of a new method of Orthorectification and 
Mosaiking based in an algorithm that searches optimal 
photographs, not just the minimal distance with respect to the 
nadiral point considered (known as MOST NADIRAL), but 
also the optimal incidence angle to avoid presence of 
“stretching areas” within the final digital orthoimage, method 
that is called MOST RIGHT. 
Figure 2. Most Right Method 
Usually flights are planned to a scale or resolution that 
differences of local scale of photography are absorbed. For this 
height differences within the terrain are taken into account, so 
that the final scale allows uniform GSD greater than the input 
GSD, within an admissibility percentage. 
However, the slope of the terrain is not taken into account in 
these calculations, which influences directly in the scale and 
therefore in the resolution of the imagery. 
In figure 2, it can be seen how the solution Most Nadiral offers 
less resolution for segment R 2 from the image with centre of 
projection in O 1 that from the image with centre of projection in 
O. 
As the incidence angle i over segment R 2 is greater than angle 
i 1 , we have r‘ 2 < r 2 and therefore: 
E Vr =-£-> —= E Vr . 
Vr 2 R' 2 R' 2 Vr 2 
where E Vr i^ t ^ ie flight local scale to which the segment R, 
is represented in photograph i. 
If the sensor resolution is “s”, and “g” and “g 1 “ are the ground 
resolutions respectively obtained from the imagery with centres 
in O and O 1 , we have: 
s s 1 
= g 
'Vr 2 
This is why a priori the improvement of resolution is clear. 
If g° is the nominal output resolution, it will exist a limited 
incidence angle i°, which is a function of the aperture angle of 
perspective ray and the slope of terrain, just as g'> g°, from 
which there will be a stretch in the imagery for view angles 
smaller, i.e. where the next relation is not true: 
GSD input / GSD output <l. 
The optimal incidence angle is obviously the one that is closest 
to 90°, however it will have to be valuated the advantages and 
disadvantages of using this algorithm without restrictions.