1. 
direction. The general accuracy of this model is about 0.8 meter 
in plain direction with slightly more than 0.7 meter for the best, 
0.9 to 1.6 meters in height direction with 0.9 meters for the best. 
• The second order affine model is the most complex 
model with 6 GCPs for the solution at least. The solution with 
only 6 GCPs available are instable, affected much by the distri 
bution of the GCP, thus not suitable for the accuracy improve 
ment. When 8 GCPs are used, the solution is much better with 
about 2 meters in plane direction and 4 meters in height. With 
more than 4 redundant GCPs, this model provides better accu 
racy than the models above, but at the same time this model is 
also less stable than others. With 16 GCPs distributed evenly in 
the test region, the highest accuracy can be obtained for this 
model with 0.56 meter in plain direction and 0.75 meter in 
height direction. 
4. CONCLUSIONS 
This paper presents experimental results of a study on accuracy 
assessment based on QuickBird across-track stereo imagery 
using GCPs and different transformation models applied on 
RFM in both object space and image space. Two QuickBird 
images acquired in the Shanghai area at different time and 
highly accurate GPS survey points as GCPs are used in the 
experiment. Different methods and GCPs distribution patterns 
are tested. From the analysis above we can conclude that 
although the QuickBird across-track stereo imagery was 
collected at different time, they can still meet the DigitalGlobe’s 
23CE90 standard. The addition of GCP distributed on both 
ground and the top of buildings greatly improved the 
positioning accuracy. Analysis of the results obtained by both 
object and image space models using different numbers and 
distributions of GCP shows that, with the same redundancy of 
conditions, the second-order models achieved better accuracy. 
If some modifications are conducted to the second-order models 
by removing some quadratic terms from plane and height for 
reducing their correlation, the number of GCP required by the 
geometric model could be less while maintaining comparable 
accuracy. The positioning accuracy has been achieved with the 
plane direction 0.7 meter and height direction 0.6 meter in the 
sample spots of the whole test region. 
ACKNOWLEDGEMENT 
The paper is substantially supported by National 863 High-tech 
Project “Complicated Features’ Extraction and Analysis from 
Central Districts in Metropolis”, ChangJiang Scholars Program, 
Ministry of Education, PRC and Tongji University “985” 
Project Sub-system “Integrated Monitoring for City Spatial 
Information”. The author would like to thank ChangJiang 
Scholars Dr. Ron., Li and Dr. Hongxing Sun for their 
invaluable help. 
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