involved in the image coordinates
measurements, we can deduce that the accuracy
of the restitution depends only on the quality of
the input data.
In other words, the digital stereoplotter does
not bring any kind of errors in the
stereorestitution process.
Digital images used for stereorestitution can
have two different origin:
e images coming from digital cameras
e images coming from scanning devices
In the first case, the good quality of the photo
camera either from mechanical or optical point of
view, guaranties the goodness of generated
images; in the second case the image created on
the traditional film, will be transformed in
numerical data by an optical-mechanical device
that can introduce errors in the geometry of the
image: the problem of accuracy change from the
stereoplotter to the scanner device.
3.1 Images from digital cameras
In the recent past have been marketed world-
wide many models of cameras able to output
directly a digital image in the most used
formats.
Generally these cameras (e.g. Nikon E2, Kodak
DCS 420 or 460) are used by photo-reporters
and therefore they are producing images not
suitable for photogrammetric purposes, where
high precision are commonly required.
However, considering that the image coming
from a digital camera are usually affected only
by deformations due to optical aberrations of
geometrical type (are absent, for instance, the
deformation due to the not perfect flattening of
the film), we realized an auto-calibration
procedure of the camera based on a shot of a
tridimensional target and an automatic
processing of the image.
The procedure, based on auto-calibration
techniques, is easily done and can be repeated
on images taken at different distances from the
target giving information on the distortion and
principal point position in all conditions of
focusing of the lens.
44
3.2 Images from scanners
To use pictures on film in a digital system it is
necessary to transform them in numerical
images by means of scanners.
The accuracy of the data collected by a digital
system depends mainly on the precision of the
scanner that should be realized with the same
quality of an analytical stereoplotter.
We can classify the scanners in two categories:
e High precision photogrammetric devices
e Commercial scanners for publishing
Photogrammetric scanners are characterised by
high geometric accuracy, high resolution (over
2500 dpi) and the possibility to choose the
scanning direction to align it with the one
identified by the fiducial marks. These devices
are very expensive and also the working costs
are elevated.
The costs of the scanners can reduce the
diffusion of the digital systems, therefore we
elaborate a calibration procedure so that even
the image scanned with a publishing devices
can be used for photogrammetric application
with good accuracy results.
For the calibration we use a high precision grid,
the same used to calibrate an analytical
stereoplotter. Such a system, similar to that
used on the reseau cameras (those without the
film flatness devices), increases considerably
the accuracy.
The procedure performs the following steps:
1. Scanning of the grid superimposed on the
film.
2. Automatic search of the grid nodes and
calculation of the homographic correction
parameters.
3. Interior orientation of the photo.
4. Digital image resampling for scanner
distortion and lenses aberration correction.
4. Image correlation
The well known procedure can perform the
collimation automatically demanding to a
processing system the identification of the
homologous points.
It is easy implemented in the digital systems.
Besides the help to the operator in the
measuring job it can increase the precision of
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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