e Laplace’s filter,
e the fast method - "colour in the neighbourhood".
Possibility of obtaining the image in the form of
distinguished object contours (quasi binary images) is
important because of its further applications to
engineering projects elaboration supported by the CAD
systems. Digital image containing recorded, contoured
object (or possibly converted to binary form through
use of the computed threshold) can be processed
photogrammetricaly and exported further to CAD
environment.
3.5 Bundle adjustment
Our own software for photogrammetric 3D positioning
in off-line mode of operation was implemented. The
software was written in FORTRAN-77 programming
language. Special algorithm for general sparse matrices
was used (George, 1981).This program is oriented to
application in close range and it is based on the method
of bundle adjustment. The program allows
conventionally to determine exterior orientation
parameters, object points coordinates (X,Y,Z) as well
optional interior orientation parameters of the camera
by selfcalibration bundle adjustment. In the program,
the traditional model of bundle adjustment, ed.
photogrammetric observations and directly observed
coordinates (weighted control points) can be enhanced
with additional geodetic observations of the following
types : spatial distances, coordinate differences (height
differences), horizontal and vertical directions in object
coordinate system etc. For maximum precision of 3D
point positioning the additional parameters in the
functional model of the on the job calibration are
obligatory.
3.6 Utility functions
Generalizing, it can be stated that the elaborated
system fulfils the following basic user’s functions :
e modular structure of the system,
e programmable frame grabber,
e open architecture of the system resulting from its
work in the MS WINDOWS environment,
e generation of images using visual sensors - max. 4
CCD cameras,
e transmission of optical images and processing them
to obtain digital form in on-line mode,
e modules of digital processing of images,
interactive pointwise measure of pixel coordinates,
common determination of geometric and radiometric
errors in "on the job" mode,
3D positioning and reconstruction of non-topographic
objects in close range.
integration of the output data with standard formats
of the CAD/GIS systems environment.
4. TESTFIELD MEASUREMENT - FIRST EXPERIMENT
In the first experimental trials devoted to estimation of
accuracy of the system as well as its flexibility, a
simplified procedure of calibration and determination
points coordinates through triangulation was admitted.
The accuraccy potential of a proposed system for
photogrammetric point determination, using the
selfcalibration approach in conjunction with a small
close range testfield was studied. The testfield
consisted of a panel 0.6 x 0.8 m? in size, on which 35
control points were placed regular in a network of
squares and 4 more point were placed approximately
200 mm above the plane - the idea proposed by
Gustafson was applied (Gustafson, 1988; Gustafson,
1992). The testfield was placed horizontally. Accuracy
of points positioning was estimated to be 0.05 mm.
Size of the testfield was choosen to fulfil approximately
the field of sensor image. Ratio of the panel and image
scene amounted to about 1.4. The retro-reflective
targets are black circles with a diameter of 8 mm on
white background with a small white dot in the centre (
for centering and for theodolite measurements). The
testfield was imaged from eight different camera
positions ( Wester-Ebbinghaus, 1983). 4 convergent
and 4 normal case scenes taken with Panasonic CCD
camera cover the plate). The image scale was
approximately 1:200 for normal case and the average
object depth from the perspective amounted to 2.4 m.
Figure 3 shows the images configuration. The targets
are thus imaged onto 3 to 8 pixel with regard to
influence of the convergence angle.
Figure 3. Test object and CCD camera images
configuration.
In the first step the option of selfcalibration for single
camera was studied. Using a selfcalibrating bundle
adjustment , the parameters of interior and exterior
orientation were calculated. The additional parameters
for correction of the coordinates of the principal point,
camera constant of the sensor, first two parameters for
radial symmetric lens distortion and tangential
distortion (Heipke, 1992) were to be determined. Next,
multi images recording of test object from convergent
camera positions was performed. Determination of all
parameters, in this number X,Y,Z of targets coordinates
of the test object was carried out through common
bundle adjustment. Image coordinates of the signalized
points were measured manually, repeatedly, on the
screen of monitor. The measurement accuracy in image
516
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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