g 
r 
e 
y 
v 
a 
| 
u 
e 
occlusion 
b= ft 
ars of interior 
ints. 
2K CCD frame 
are digitized 
order to avoid 
e CCD sensor 
Nhich are later 
era control and 
interface. The 
> which gives a 
rgets (Godding 
patterns 
96 
  
  
  
  
Figure 3: Images for testfield calibration 
The technical description of the acquired images is as 
follows: 
Camera Rollei 6008 / ChipPack 
Imaging device 2048 x 2048 pixels 
Sensor elements 15um x 15um 
Image format 30mm x 30mm 
Lens Distagon 40mm 
Size of testfield 80cm x 80cm 
Mean image scale 1:25 
Figure 3 displays the set of 8 testfield images. 
4 RESULTS 
41 Synthetic Point Patterns 
Theoretical investigations and practical experiments have 
shown that the position of a circular target can be 
determined with an maximum accuracy of 1/1000 pixel. 
Basically the resulting accuracy d depends on the size of 
the target s and the contrast k between target area and 
background. The precision has been determined 
emperically to 
d= nai (1) 
s-k 
This equation shows that accuracy improves with 
increasing size and/or contrast. The factor 4 results from 
differences between synthetic and real image data 
(Broers & Hemken 1996). Theoretical investigations by 
FORSTNER have shown similar results (Fórstner 1984). 
Figure 4 shows some of the results obtained by the test 
participants on the delivered imagery. In the case of 
IMAGE1 the best results have been achieved by method 
A, B and G (all ellipse operators). These methods show 
327 
mean differences to the nominal values between 0.003 
and 0.009 pixels. The results of center-of-gravity 
operators (F,l) and the line following approach (D) are 
between 0.015 and 0.03 pixels. Even better results (0.002 
pixels) have been produced with IMAGES (ideal patterns). 
0,16 
0,14 
0,12 
0,10 
0,08 
0,06 
0,04 
0,02 
0,00 
Image Image Image Image Image 
1 2 3 4 5 
Pixel 
  
  
—H—B 
—e—G m 
  
A 
F 
  
—— 
3 
  
Figure 4: Results on synthetic images 
IMAGE2 shows slightly worse results caused by noise 
added to the image greyvalues. Additive noise of 3 
greylevels leads to reduced accuracy if ellipse edge 
operators have been applied. 
IMAGE4 (additional occlusions) show equal results for the 
best operators in the range of 0.05 pixels. Although 
occlusions should be handled easier by structural 
approaches (i.e. ellipse methods) a higher number of 
points could not be measured. 
In IMAGES a variation of the background greylevel has 
been simulated. This effect leads to a decreased 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996