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the sampling
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Direct image
sary become
ner before
e images, in
of inherent
geometric nature in the instrument of
acquisition.
The unity of acquisition has resulted therefore
composed from a scanner DTP, from a grid for
the scanner calibration and from a software for
the determination of the calibration parameters
and for the following resembling of the
images.
For the evaluation of the characteristics of
geometric stability in the acquisition we have
used a grid reproduced on a proper support to
be acquired by the scanner (for instance on
crushproof polyester).
The sweaters of the used grid are characterised
from a regular step of 2 cm in both the
directions while the lines that materialise the
points have a thickness of 0.1 mm.
The software used for the correction of the
radiometric and geometric distortions induced
by the scanner, has been Stereoscan, a special
form of the Stereoview software.
The methodology of resolution of the
orientations has been the classical one and it
has allowed therefore a homogeneous
comparison with the residues obtained in the
analytical system.
Also methodology used for the construction
of graphic entities is absolutely similar
between the two systems, in this way, the
differences in the graphic product are
imputable entirely to the quality of the
stereoscopic coincidence and to the different
resolution of the images.
The analytical part of this test has been
conducted using an analytical plotting OMI
AP5, that has allowed, for its own
characteristics, an attainment of an accuracy of
2-3 microns.
The difference between the measures
furnished by Stereoview and those furnished
by the analytical plotting, expressed in terms
of coordinates of points has been used to
esteem the precision of the digital plotting.
For the photogrammetric plotting, either
analytical that digital, we have used ten points
for the relative orientation and six points for
the absolute orientation.
The coordinates of the support points have
been obtained through a GPS survey network.
The following table shows the values of the
residues of the absolute orientation in the case
of the analytical plotting and in that digital.
Point
Dx (m)
Dy (m)
Dz (m)
1
-0.034
0.036
-0.091
2
-0.078
-0.089
0.100
3
-0.019
0.021
0.012
4
0.083
-0.061
-0.052
5
-0.077
0.081
-0.044
6
-0.031
-0.164
0.075
Tab.l - Residual of absolute orientation; Analytical
plotting.
Point
Dx (m)
Dy(m)
Dz (m)
1
0.182
0.161
-0.121
2
-0.196
-0.269
0.051
3
-0.061
0.318
-0.043
4
-0.126
-0.083
-0.043
5
-0.226
-0.173
0.172
6
-0.139
-0.023
-0.101
Tab.2 - Residual of absolute orientation; Digital
plotting.
The plotting operations, both analytical and
digital, have been made by the same operator;
he has plotted some objects by analytical
instrument and by digital one, making two
DXF files; then we have superimposed the two
files so that to appraise possible removals. The
picture n. 1 shows this particular situation.
Fig. 1 : Superimposing of two DXF files.
After having selected some significant points,
we have calculated the comparison in terms of
measured coordinates of vertexes of buildings.
The next table n. 3 reassumes the obtained
values.
141
