The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part Bl. Beijing 2008 
effect of systematic errors. The distribution of GCPs in the test 
field is shown in figure 1. 
Figure 1. Distribution of GCPs in the test field 
3. DSM AND ORTHOIMAGE GENERATION 
A processing system named Pixel Factory, which is developed 
by Infoterra France, is used for DSMs and orthoimages 
generation. The Pixel Factory system is an integrated system for 
aerial and satellite imagery processing. It performs image 
matching for all pixels in the images, and can extract dense, 
precise and reliable DSMs, and generate orthoimages from 
single- or multi-sensor imagery. 
The Pixel Factory system has a fully integrated architecture that 
manages full processing workflow, resources and data. All the 
processing is performed in a fully automated way, including 
automated generation of thousands of tie points, automated tie 
point filtering to remove blunders, automated global multi 
sensor spatial adjustment to refine geometric models, automated 
dense DSM extraction, and automated orthoimage rectification 
using DSM information and taking into account occlusion. All 
image processing are parallelized in order to speed-up 
computation. Figure 2 shows the flow chart of Pixel Factory 
system for DSM and ortho-image generation. 
In this study, all the 15 stereo combinations of the IKONOS and 
QuickBird images are processed to generate DSMs and 
orthoimages automatically of the test field. A regular grid DSM 
with 4m spacing is generated from the raw measurement for 
each stereo pair. The computation of orthoimage is made by 
projecting the terrain geometry on the raw images using the 
DSM. The resolution of generated orthoimages is lm for in 
track and cross-track IKONOS stereo pairs, and 0.65m for 
cross-track QuickBird stereo and mixed satellite stereo pairs. 
Figure 2. Flow chart of DSM and ortho-image generation 
4. ACCURACY ASSESSMENT 
4.1 Geometric Stereo Model 
In conventional photogrammetry, the base-height ratio (B/H), 
which is defined as the ratio of the baseline of a stereo pair to 
the average flying height above ground level, is used to 
represent the stereo acquisition geometry and to predict the 
geometric accuracy. However, a major disadvantage is that this 
is valid only for stereo models which have no roll. Compared to 
standard in-track stereo imaging, high resolution satellite 
pointing camera systems provide a variety of stereo geometries. 
Thus, a geometric stereo model is necessary to analyze the 
relationship between the stereo acquisition geometry and the 
geometric accuracy in a variable stereo model geometric 
environment (Cain, 1989). 
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