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digitize the whole photograph and since the blind, fully 
automated system is not easy to develop, we decided to 
use this RS1 to develop a kind of semi-automatic TP 
measuring system. We call it the semi-automatic system, 
because the area which should be digitized is determined 
by the operator not by the computer. That is, conjugate 
areas in the overlapping photographs are searched by the 
human operator. Each time only a small area is digitized by 
the CCD camera. This area will be called the tie area, 
because in it the aerial triangulation tie points should 
locate. Since the conjugate tie areas in different 
photographs are selected and digitized by human operator, 
the difficult blind searching of them in the whole block or in 
a whole photograph is avoided. 
After these areas have been digitized, TPs will be defined in 
them and transferred between them. The methods used in 
our system for defining and matching the TPs are mainly 
adopted from the Helava's proposal for the DCCS. The 
greatest change in our system is that instead of using one 
fixed window size, we use several different sizes for the 
Least Squares Matching window in order to increase the 
reliability and the accuracy of the matched results. Another 
change is that we match each time only one pair of tie 
areas. There will be no multiple image matching like in the 
DCCS. 
With this semi-automatic point measuring system, we 
combine the point transfer and the point measurement in 
one procedure. Both the speed and the accuracy of the 
aerial triangulation point measurement are increased. 
Since only small tie areas are digitized, the data amount is 
very small. There is no problem concerning the storing and 
the managing of the image patches. An ordinary PC486 is 
used for this system. 
After the tie area is digitized, image pyramids will be built 
up. From all the conjugate tie areas, one will be chosen as 
the master tie area and the rests as slave tie areas. In the 
highest level (with the lowest resolution) of the pyramid of 
the master tie area image, the Foerstner interest operator is 
used to find the places where the windows for defining the 
TPs should be. A number of such places should be chosen, 
in case that some would fail in the Subsequent matching. 
These places will be traced down through the pyramid to 
the lowest level (with the highest resolution). The actual 
TPs will finally be found there. 
While applying the Least Squares Matching technique, we 
have noted that the reliability is a problem. If the initial 
approximate position for starting the matching is not 
accurate enough, the LSM method could converge to a 
false place and there is no information from the matching 
itself to judge the reliability. Also the size of the matching 
window is a problem. There is no such thing as the best 
window size. Different window sizes converge to different 
positions, the difference can some times be larger than 0.2 
397 
pixel. Therefore we have developed a strategy to cope with 
these problems. Instead of one fixed window size, we use 
several different sizes for the LSM and by analyzing their 
normalized cross correlation coefficients (NCC) after the 
LSM matching, the best result is selected. When no reliable 
one could be found, that point will be abandoned. If there 
are not enough reliable points in a tie area, the system will 
switch automatically to the manual selection of the target 
and the search windows in the master and the slave tie 
areas respectively. 
2. System Configuration 
The whole system is built up on the basis of the Rollei RS1 
scanner running on a COMPAQUE PC. The RS1 is 
designed for digitizing the photographic film with very high 
geometric accuracy at low costs. Which is achieved by 
using a reseau plate (Luhmann and Wester-Ebbinhaus 
1986). The main parts ofthe RS1 area CCD camera anda 
glass picture carrier with 2mm spacing reseau crosses all 
over its entire surface. The CCD camera has an array of 
512x512 pixels and locates above the picture carrier. The 
pixel size in the film is approximately 8umx 6um. Therefore 
the total area of the film which can be digitized at one time 
is approximately only 4mm x 3mm. But the CCD camera 
can be moved to any place over the picture carrier by step 
motors, so that the whole picture can be digitized patch- 
wise. Since step motor is not very accurate, Luhmann and 
Wester-Ebbinhaus developed this cheap solution of using. 
reseau plate to obtain the high precision absolute reseau 
coordinates for each pixel. 
Two different light sources are built in the RS1 which can be 
Switched automatically in turn. When the bottom light is 
switched on, the light goes through the film into the CCD, 
and the film can be digitized. When the side light is 
switched on, only the reseau crosses are imaged into the 
CCD. In this case, the pixel coordinates of the reseau cross 
center can be obtained after an automatic matching of the 
cross image to the standard template. Then the 
transformation parameters from the pixel coordinates to 
the reseau coordinates can be calculated. Since the 
absolute coordinates of the reseau crosses are known, the 
absolute coordinates of each pixel can be calculated by the 
transformation parameters. 
The distance between the reseau crosses is 2mm and the 
CCD digitizes an area of 4mm x 3mm, so that it is 
guaranteed that there will always be at least 4 reseau 
crosses used for calculating the transformation 
parameters. The area encompassed by the 4 crosses has 
approximately 240x350 pixels. In order to reduce the error 
caused by extrapolation, only the pixels within the four 
crosses will be used for the TP measurement. 
The CCD camera can be driven by the reseau coordinates 
or by the photo coordinates. In our case we drive the CCD 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996