True Orthophoto for Architectural Surveys 
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generated by a laser scanner device can be considered the optimal solution for a correct and complete 3D description of the shape of 
a complex object, both from the technical and economic points of view. 
The fourth possibility for the generation of a DDTM foresees the use of refined interpolation techniques applied to a three- 
dimensional digital map of the object. A digital map obtained through photogrammetric techniques in fact constitutes a three- 
dimensional description that is made up of simple geometric elements (points, open lines and closed lines). 
These elements, though simplified, permit one to reconstruct a DDTM that correctly describes the discontinuities that are present on 
the architectural object. An original software (in Visual Fortran) has been developed for this purpose and is able to automatically 
generate a DDTM with a redefined grid dimension, starting from a photogrammetric restitution. This software recognises the surface 
entity on the basis of the code and uses it in the correct way for interpolation. 
4. TRUE ORTHOPHOTO GENERATION 
In 1996 Amhar and Ecker proposed an original solution for the Qp Qr Qr Qs 
generation of true orthophoto. The procedure, devoted to the ■■■■....HB..... ■>»•••• 
production of orthophotos in urban areas, used a DSM managed by 
means of a relational database. All images are classified in terrain and 
building surfaces and the orthopoto is generated in separate phases: 
first the terrain then the roofs. The results of these treatments are then 
merged in a single digital orthophoto. Hidden areas are eliminated 
through superimposition of the orthophoto generated from other 
images. 
The solution proposed in this paper tries to simplify this approach. The 
input data for the generation of a true orthophoto are: a DTM and a 
series of oriented images containing the radiometric description of all 
the points to be orthoprojected. 
The aims of the procedure are: to maintain complete automation so as 
to guarantee the same performances of traditional orthoprojection 
software and to avoid the previously highlighted problems (see par. 2). 
Let us consider the object in figure 3. In perspective images, higher 
points hide a lower point, therefore the procedure must run from the 
highest to the lowest point. 
The procedure starts from point R. The best recording of the grey value of this point can be found in the image which has the 
projection centre nearest to the point itself (image II). In order to avoid the duplication of the images (see fig. 2), this pixel should be 
inhibited: for this reason a “flag image” is created where each pixel records the height used for the orthoprojection of the 
correspondent pixel on the original image. Point R has also been recorded in 12 and, for the same reason, the pixel representing point 
R on 12 should also be inhibited. 
The procedure orthoprojects point S with the same criteria (point S will only be recorded in II). When the procedure orthoprojects 
point P, it finds the pixel on II that was used before for point R. The flag image inhibits a second use of this pixel, because the 
height recorded on it is higher than the height of point P. Then the procedure looks for the grey (or colour) value in 12. The pixel is 
not inhibited and the orthoprojetion of point P is possible. After this, the procedure orthoprojects point Q. The first attempt is to use 
the corresponding pixel on II, but this pixel has been used for point S and the “flag image” then inhibits the radiometric value 
reading. The second attempt is to use the corresponding pixel on 12, but this pixel has been inhibited because it contains the grey (or 
colour) value of point R. In this case no more images are available and the orthoprojection of point Q cannot be defined. This simple 
example describes all the possible cases in projecting a true orthophoto.. 
5. THE SOFTWARE ACCORTHO 
The procedure that is described in the previous section was implemented in a specific software called ACCORTHO (ACCurate 
ORTHOprojection). 
The input data consists of a regular DDTM, that can be generated: 
1. starting from an irregular DTM acquired using laser scanner, or 
2. from pre-existing three-dimensional digital map; 
and a set of oriented digital images. 
The software works in two separate steps. In the first, it selects and prepares the data. In particular it: 
• calculates the heights of each pixel of the output image (a true orthophoto) and orders the pixels according to decreasing 
heights; 
• extracts the portions of the digital images involved in the orthoprojection; 
• prepares an index of the images in order to find the radiometric value to use whenever possible. The images are ordered on 
the basis of the distance between the projection centre and the considered pixel; 
• generates an empty flag image for each input image. 
The second step of the procedure puts the process described in the previous section into practise. 
Fig- 4 shows the flow chart of the basic functions.