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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
made to transfer the procedures to a production environment.
For each pixel
(following the ordered list)
Y
Projection onto all images
For each image
(following the ordered list)
Y
Catch íts grey value g
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already used? * (Ores do in)
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images? i
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: Record the height in the
g = empty flag image
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orthophoto
Figure 8: Flow-chart of the basic ACCORTHO functions
The main changes are:
* the final orthophoto image is divided into small windows,
to minimize the computer power that is needed in the case
of large format orthophoto production. It should be
considered that the colour orthophoto of a map in 1:2000
scale (standard format) with a pixel size of 20 cm (-300
dpi) is composed of about 24 Mbytes (2300x3400 pixels).
Figure 9: Window construction and image selection
* The ortoprojection procedure extracts the portion it
considers useful for the orthophoto, from the available
images, in each window (see fig. 9): in this way 'the
loading operations of the starting digital images are
limited, considering that, in the best case, there will be a
maximum of 6 images.
* The orthophoto box can be defined on the basis of standard
map sizes, the user just has to choose the reference scale
and the sheet, and the software calculates the boundaries of
the orthophoto that are necessary during the
orthoprojection phase (circumscribed rectangle). White
colour (255, 255, 255) is assigned outside the cartographic
size in the final orthophoto.
e The software generates a georeferenced digital image, that
is ready to use in a GIS.
The user interface has been completely re-developed, using the
possibilities offered by Visual Fortran language in Windows
OS. The main window (a), two loading data windows (b and c),
541
the window used to define the cartographic size (d), and the
elaboration window (e) are shown in fig. 10. Italian and English
versions are avaible.
Figure 10: Dialogue windows of the ACCORTO software
6. TRUE ORTHOPHOTO OF THE CITY OF TURIN
The Turin municipality decided to develop a local GIS, in
collaboration with the Politecnico di Torino, to have an
orthophoto coverage of the whole urban area (12.000 ha). This
allowed the authors to test the implemented software. The map
division of the area is shown in fig. 11, in 1:2000 maps.
The highlighted part involves the historical centre of the city,
which contains several architectonical and historical buildings
of great valuc.
23) 24: DEN 2
a5 34
Figure 11: Maps involved in a first orthophoto production phase
6.1 Primary data
The initial data consists of:
1) aerial photos taken on a recent acquisition flight (2002),
acquired with a scanner at 600 dpi resolution, in JPEG
format, oriented by aerial triangulation;
2) a digital map at a scale 1:1000 containing all the data
necessary to produce a DDTM, in DXF format.
The great automation of the software means that the initial data
have to be arranged in a rigorous way.
Each aerial image requires a file that contains the internal and
external orientation parameters. A specific software was
developed for the external parameters to extract the coordinates
of the projection centre and the angular parameters for each
photo through an aerial triangulation procedure. All these
parameters are recorded in a file which has the same name as
the image and a . ori extension.
The digital map is reconstructed in a single file from which the
