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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004 
  
In the first case of single rectification, a mean plane of 
projection is used for the rectification process. This is an easy 
and well-known method. Features that are from both sides of 
the mean plane are actually distorted. Thus, the orthoimage 
does not reflect the strict reality and the accuracy depends of 
the position of the points in regard to the plane. An example of 
this case is given in Figure 1b. 
  
(c) 
Figure 1. (a) The original image. (b) An example of single 
rectification where feature distortion is obvious. (c) Interest 
point for DTM generation. 
In conventional orthoimage production, a DTM that may have 
been extracted through an image matching process is used for 
the rectification. Such a DTM cannot illustrate the exact model 
and structure of the object unless all the breaklines of the 
objects are precisely described. In this case, the single plane is 
replaced with a surface. Even if the surface does not accurately 
express the whole structure, the result of this process is better 
than the previous one. 
Figure lc gives a picture on how the interest points, used for 
DTM generation, are distributed throughout the whole object 
area. The points’ irregularity is not enough to reflect the exact 
model of the object. 
The third case, the one that is of principal interest in this paper 
Is been elaborated in the next sections. 
3. THE PROPOSAL FOR GENERATING A TRUE 
ORTHOIMAGE 
The need for generating true orthoimage came up as a need in 
order to outline in a better and accurate way the product of the 
rectification process. 
In the following a few reasons for the need of true orthoimage 
generation are presented: 
1. The more precise representation of the object under 
rectification. 
2. The conditions for object restitution are better, 
especially in the areas where the surface continuity 
changes. 
3. The documentation restrictions are controlled better 
with a true orthoimage. 
4. GIS applications suited under more well-defined 
specifications. 
Focusing in close-range photogrammetric applications the 
motivation is the accurate documentation of architectural, 
archaeological or industrial objects. The orthoimage product 
can be used both for the accurate raster documentation and the 
extraction of vector data. 
3.1 The use of a 3D model 
According to the presented approach, the only structural 
background needed for the orthoimage generation is a 3D 
model describing the surfaces of the object. 
A descriptive 3D model for the object may have a structure like 
the following, where each surface is described by the 
coordinates of its nodes. 
begin 
X1> Y1> Z1 
Xo. V2. 23 
X 
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te 
Uu 
N 
2 
3» 
X4, Va, Z4 
0,0,0 
X5, Y5, Z5 
Xo. V6. Z6 
X7, V7, 27 
0,0,0