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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
The first requirement can be met by releasing the shutters of the 
individual DMC camera heads with a precision of less than 0.01 
msec. This ensures that we can assume all eight images of one 
exposure instant to be taken synchronously and from the same 
location. Thus, we do not need to model the influences of 
different TDI shifts or to model the projection centre 
coordinates as a function of time. 
Due to the slightly different location of the projection centres of 
the individual camera heads, the generation of the central 
perspective image composites involves a systematic 
displacement of all pixels. In theory, this step requires the exact 
knowledge of the height of each point on the ground. Since 
such information is not generally available, the horizontal 
reference plane is used as an approximation instead. Thus, there 
is a residual relief displacement effect for all areas which do not 
exactly lie in this plane. The size of the relief displacement 
depends on the height variation in object space relative to the 
flying height above the ground. To this end, an investigation 
(Tang et al. 2000) was carried out which showed that the 
resulting error in the central perspective image composite could 
be neglected, even for very high accuracy requirements if the 
height variation is not extreme. The results of this investigation 
are shown in Figure 5. 
As a mechanical part inside an aircraft can never be constructed 
to be absolutely stable over short and long time periods, the 
camera mount for the DMC was designed to allow for angular 
deformations. This leads to a further assumption for our 
platform calibration model. Based on tie points determined 
automatically in the overlapping areas, we use a separate bundle 
adjustment for each imaging instant to compute the parameters 
of relative orientation between the individual camera heads. 
Since we only assume angular movements based on the 
mechanical design, we overcome the problem of high 
correlation between the unknowns to be estimated by solving 
only for the angular parameters. 
A typical accuracy for the tie point coordinates after the bundle 
adjustment is in the order of 1 or 2 pm, corresponding to 1/6 to 
1/12 of a pixel having a size of 12um. Our experience has 
shown that automatically determining tie points in this case 
does not pose a problem. Because of synchronous imaging, 
moving objects like cars and waves can also be used as tie 
points. As an example, the residuals of an arbitrary computation 
are shown in Figure 6. It should be noted that, in order to 
reliably compute the orientation parameters, a much coarser 
distribution of about 30 to 50 points is also enough. 
It is an interesting question how stable the camera configuration 
actually is, and thus an internal bundle adjustment often needs 
to be carried out to ensure an accurate generation of the image 
composites. We have investigated several DMC flights and 
have found only very small and random variations in the 
parameter values. Nevertheless, to be on the safe side, we 
currently recommend checking the stability of the camera head 
configuration at every exposure, since matching and parameter 
computation is very fast and thus negligible in terms of the 
overall computing time. 
  
0,5 
pixel 
DY|(Ah / hg =0.2) 
025 DY (^h / hg z0.1) 
DX (Ah 702) 
DX (^h/ hg 70.1) 
500 1000 2000 3000 4000m 
flying height above ground. ——» 
Figure 5. Influence of the projection centre offset on the image 
composite as a function of the height differences (Ah) in the 
imaged area to the flying height above ground (hg) 
  
  
— 2 0 microns 
  
  
  
Figure 6. Example of the residuals of the internal bundie 
orientation, oy = 0.82um based on 999 observations 
4. DMC IN PRACTCAL APPLICATIONS — RESULTS 
AND REACTIONS FROM USERS 
Because the DMC has been on the market for a number of 
years, there have been various reports about the accuracy of a 
DMC test flight and about the practical use of the DMC and its 
advantages over film images. As far as accuracy is concerned, 
we only give one example here and refer the interested reader to 
other publications for in-depth studies (Dórstel 2003). 
4.1 Short discussion of DMC accuracy potential 
The one example we mention here deals with imagery taken 
over Z/I Imaging's test field in Elchingen, Germany at a scale 
of approximately 1:13.000 and a flying height of 1500 m above 
ground. Thus, the pixel size of 12um corresponds to about 0.13 
m on the ground. Three overlapping strips were flown in an 
east-west direction, and another three in a north-south direction, 
providing a very stable block of about 20 images with 60% end 
and 60% side overlap. 
Tie-point coordinates were determined automatically using 
ISAT (Madani, et al, 2001) image coordinates of some GCPs, 
and a number of check points were measured manually. In the 
subsequent bundle adjustment, object coordinates for the check 
points were computed. The resulting standard deviation o, of 
the image coordinates amounted to 1.7 um or 0.14 pixels. A 
comparison with known values yielded an empirical standard 
deviation of 0.036 m in planimetry and 0.06 m in height. 
In comparison, film cameras regularly deliver a os of 
approximately 5 um. At the given scale of 1:13.000, this