projection. One must however still consider the typical
limits of architectural photogrammetry, in which sudden
variations in the object’s depth are the cause of the fact
that small variations in camera position can substantially
change the extent of the object’s surface, which is visible in
the photograph.
The photographs of the interior have a mean scale of
about 1:150, because they had to allow for plotting at 1:25.
One of the two photographs has been digitizedby a
scanner of 2000 dpi (dots per inch) and with a range of
256 grey values, using a CCD camera mounted on the
Digicart 40. The size of the corresponding resulting .TIF
file resulted to be about 6 Mbyte.
In order to carry out a qualitative comparison of the
results, the other photograph has been read at 256 grey
level resolution with a commercial scanner of 300 dpi,
generating a .TIF file of about 1 Mb.
The two photographs used for the realisation of the
rectifications have first been printed in black and white on
a 18x18 cm^ format. Successively, the obtained print has
been digitized by a scanner of resolution 300 dpi and 256
grey levels, generating two .TIF files of about 4 Mb each.
The scanning requires a few dozens of seconds for the 300
dpi resolution. All things considered, the data transfer and
change of format prior to the actual processing require a
much longer time. Already in this situation, it becomes
evident that one of the practical problems of such a
procedure are the hardware conditions and the
characteristics of the network.
It is also worth noting the characteristics of a thus
generated images. The representation of both on a
videoscreen with a Matrox board driven by a Compaq PC
486 of 50 MHz, does not show large qualitative differences
between the various images. When the analysis is repeated
after having carried out a series of zooms, considerable
differences between the 300 dpi and 2000 dpi resolution
become obviously evident.
Galileo Siscam is already developing software which
allows for the use of the characteristics of the grid of 11 x
11 reseau crosses, which are present on every photograph
taken with the Rollei 6006, in order to allow for the
correction of deformations introduced during a possible
printing, those of the paper support and those introduced
by the scanner, as if the total effect were generated by a
normal distortion.
2.4 Processing by Orthomap
The processing by the scanner is called by the
orthoprojection program with the D.T.M. and the
corresponding breaklines, after having identified the
characteristics of the camera and having identified the
control points with the mouse.
A large question mark arises in this phase: while
breaklines in aerial photogrammetry are easily identifiable
elements, in the architectural field they become a partly
anomalous element. They can not always be plotted
because often they are not visible on one of the
photographs, or on both. For example, a window still must
be represented by two three dimensional elements, more
or less coinciding in the view, indicating respectively the
beginning and the end of the mentioned still. From
experience one can say that normally only one of the lines
can be plotted. It is like as if in aerial photogrammetry one
would have to plot both the eaves and the ground line of
buildings, without considering that the relative depth
variations in architectural photogrammetry are usually
much larger.
The white areas of the plot are necessarily also present in
the D.T.M.; there will be parts of the image without
corresponding 3-D objects, which have known coordinates.
For these areas, the orthoprojection will be erroneous, or,
at best, ill determinated, if one does not edit the file of the
plotted breaklines generating the corresponding lines.
Apart from this, the program runs sufficiently fast on a
hardware platforms as that, which was used.
Figure 2 shows the orthoprojection of the image digitized
at 300 dpi, processed without using the breaklines, with
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