Both systems require high optical and mechanical
stability, which has to be tested and calibrated
periodically.
Using réseau technique numerical orientation of image
frames can be obtained with high precision and low
mechanical effort. The matrix sensors are positioned
in the manner that object information and at least one
grid of a réseau (glass plate containing raster-like
distributed targets) are projected.
To obtain the total image, a numerical perspective
retransformation of each image frame into the
corresponding réseau grid is performed. By that way
not only the spatial position of the sensor is
determined, but also the orientation of the local
frame coordinate system is refreshed at any time
the sensor was moved.
This technique can be used for sequential image
Scan as well as simultanious recording of a total
image realized in A/D - conversion of analogous
photographs and digital photogrammetric cameras
[WESTER-EBBINGHAUS 1984].
Based on this principle a réseau-scanner (Rolleimetric R51,
Fig. 1) and a digital semi-metric camera (Rolleimetric RSC,
Fig. 2) are developed by Rollei Fototechnic, Braunschweig,
in cooperation with the IPT.
After a survey of system concepts based on réseau-scanning
techniques, hard- and software components of the instruments
are discussed. First results of photogrammetric evaluations
are presented.
2. System concepts of réseau-scanning instruments
Fig. 3 shows the concept of an A/D-conversion system. CCD-
matrix sensor and optical system are moving parallel to
a réseau plate (scan-réseau), which is in contact with the
photograph. The sensor is orientated in the way that at
least one réseau grid is totally projected. Réseau targets
are located automatically and fitted to their nominal position
in the réseau coordinate system. The resulting trans-
formation parameter set is stored with the image
frame or used for rectification and composing of a total
digital image mosaic.
Variation of projection scale leads to different pixel
sizes in the image plane without change of signal-to-noise
ratio.
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