vith 
‘he 
;D- 
to 
  
303 
3.3. Data processing 
We only mention the more important tasks of the program in this respect. 
3.3.1. Edge detection: In the picture the edge of a scale mark is signified by a change in grey-value. But due to noise 
not every change in grey-value is an edge. The algorithm we use for edge detection is based on the Canny-operator 
(Canny, J., 1986). 
3.3.2. Radiometric camera calibration: A linear correction model is used to eliminate the influence of different light- 
sensitivity of different pixels. In the process of radiometric calibration (s. Fig. 3) the correction parameters are 
determined and stored to a calibration-file. 
3.3.3. Geometric camera calibration: We have considered it more convenient for a linear CCD-camera to use an 
integrated way to determine calibration-data and store them to a look-up-table, rather than to aim at physical calibration 
parameters. To collect the values for this table an edge of one scale mark is moved through the field of view of the 
CCD-camera in small steps. At each step a picture is taken and picture- and interferometer-reading are recorded to 
the table. Later on, in the course of calibration, an arbitrary point can be transformed from the picture- into the object- 
system by linear interpolation. 
3.3.4. Calculation of the scale mark position: As mentioned above the position of a scale mark is defined by its two 
edges. All positions refere to the first edge of the first scale mark. The position is determined by the distance 
information supplied by the laser-interferometer and the position supplied by the CCD-camera. 
3.3.5. Final data processing, parameter estimation: After the physical process of calibration, the measuring, has 
been completed the scale of the rod, the standard deviation of the scale marks and the precision of the measurement 
are calculated. 
4. PERFORMING A TYPICAL CALIBRATION-JOB 
  
alignment 
laser/reflector 
alignment 
rod or scalebar 
alignment 
camera 
radiometric 
camera 
calibration 
geometric 
camera 
calibration calibration-file 
  
  
  
  
  
  
  
file with theoretic 
scale mark positions 
  
  
  
  
  
rod & scalebar 
calibration 
create 
outputfiles 
  
  
  
    
  
- numeric output 
- graphic output 
    
  
  
  
  
     
Schmid/Meissl 2.95 
Fig. 3 Performance-scheme for a calibration-job 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995