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The reference targets have been made by punching out discs of retro-reflective material from a sheet. The glass 
spheres at their edges are not masked and they do not exhibit the shifts described here. The position of the target 
under test can therefore be compared with the left and right punched target as the target carrier is tilted from 
- 40° to + 40°. 
The relation between object and image plane can be expressed by (see Figure 6) :: 
F *t1 * cos(«) 
Xe) D + t1 * sin(a) 
(3) 
The following assumptions have been made: 
« The glass spheres of all three targets lie in the same plane, otherwise additional shifts are introduced. 
e Aretro-target with a punched edge will not suffer a shift. 
e The centre of the tilted target lies on the bisector of the smallest angle subtended by the target contour. 
Two different experimental setups were evaluated: 
In set 1, the center target was a punched target. This comparison will show the zero effect. 
In set 2, the center target was replaced by a test target, i.e. one with a painted mask. 
3.2. Results 
The difference between the theoretical values x(x) and the actual measured values X(a) is displayed in Figure 8 (set 1) 
and Figure 9 (set 2) for circular targets @ 8mm, with a painted mask. 
These results are representative of all measurements. Different combinations of the following parameters were tested: 
Retro-Targets : @ 2mm, & 4mm of a target tape (thin paper mask), @ 8mm (painted mask). 
Punched targets: ^ Circular and line targets of different sizes punched out of above retro-targets. 
Distances: 1.69m, 2.26m, 3.28m. 
  
  
  
  
  
  
  
X(a.20?): 80 to 220 pixels on the CCD. 
Target spacing: t1 = 15mm. 
Pixel spacing: sum. 
SERENE ee mm right target 
X(a) - X(a) in pixels i i 
(a) - X(a) inp jet target with LSQ fit 
0.5 T 
0.4 
0.3 |- 
0.2 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
—40 —30 — 20 —10 0 10 20 30 40 
angle a [deg] 
Figure 8 Theoretical minus measured distance between the centre target and the left or right 
punched reference target. The centre target is a punched target. 
The target-center in the image is evaluated by intersecting two fitlines. One line is given by midpoints of horizontal 
chords, the other by midpoints of vertical chords through the target ellipse. Raising and falling edge of each chord is 
measured by subpixel edge detection with the moment preservation method proposed by Tabatabai and Mitchell 
(Tabatabai, 1984). 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences’, Zurich, March 22-24 1995