The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
1147 
Figure 3 shows the geometric stereo model which is used in this 
study. The stereo model can be expressed by four parameters, 
roll angle (RA), asymmetry angle (AA), convergence angle 
(CA) and bisector elevation angle (BIE). The roll angle is the 
angle between the epipolar plane and the local vertical, which 
represents the rotation of the epipolar plane about flight line 
between satellite cameras. The asymmetry angle is the angle 
between the bisector of the convergence angle and the 
projection of the local vertical onto the epipolar plane. The 
convergence angle is the angle in the epipolar plane between 
the two rays from the perspective centers of satellite cameras to 
the certain ground target point. And the bisector elevation angle 
is the angle between the horizontal plane at the target ground 
and the bisector ray in the epipolar. In this study, the four 
parameters are used together with the B/H ratio to analyze their 
relationship to the geometric accuracy of the DSMs and 
orthoimages. 
4.2 Results and Discussions 
22 GCPs measured by VRS-GPS are used as check points to 
evaluate the geometric accuracy of DSMs and orthoimages in 
vertical and horizontal directions. Table 2 shows the accuracy 
results of the DSMs and orthoimages generated from cross 
track stereo pairs, and the corresponding stereo acquisition 
geometry parameters. Table 3 shows the accuracy results of the 
DSMs and orthoimages generated from mixed satellite stereo 
pairs, and the corresponding stereo acquisition geometry 
parameters. From table 2 and table 3, it can be seen that high 
accuracy DSMs and orthoimages can be acquired by using the 
Pixel Factory system, and the effect of systematic error is 
removed by using 1 GCP. 
1112 
1113 
1114 
1214 
1314 
Q1Q2 
1213 
RA 
15.3 
9.7 
2.6 
1.5 
2.3 
9.0 
6.1 
AA 
18.8 
9.1 
12.6 
8.1 
8.2 
6.4 
3.8 
CA 
11.2 
35.8 
31.8 
24.1 
9.3 
27.7 
30.8 
BIE 
65.9 
76.7 
77.1 
81.8 
81.5 
78.9 
82.8 
B/H 
0.25 
0.67 
0.59 
0.43 
0.14 
0.51 
0.55 
STDV(Dxy) 
0.63 
0.86 
1.04 
0.69 
1.3 
0.59 
0.65 
RMS(Dxy) 
1.7 
1.42 
1.8 
1.46 
1.75 
1.4 
1.18 
STDV(Dh) 
3.04 
1.93 
2.34 
1.63 
8.76 
0.8 
1.18 
RMS(Dh) 
3.27 
1.96 
2.66 
2.74 
8.77 
1.45 
1.72 
Table 2. Accuracy results of generated DSMs and orthoimages 
from cross-track stereo pairs 
11Q1 
Il 02 
I2Q1 
I2Q2 
I3Q1 
1302 
I4Q1 
I4Q2 
RA 
11.0 
7.1 
12.1 
2.5 
8.8 
4.5 
4.1 
1.1 
AA 
3.6 
18.6 
1.2 
15.7 
5.3 
0.7 
10.4 
3.8 
CA 
45.8 
18.4 
34.8 
8.6 
30.0 
23.1 
22.7 
15.8 
BIE 
78.4 
70.2 
77.8 
74.1 
79.7 
85.4 
78.8 
86.1 
B/H 
0.88 
0.37 
0.67 
0.16 
0.53 
0.4 
0.44 
0.27 
STDV(Dxy) 
0.79 
0.81 
0.98 
3.46 
0.98 
0.6 
1.14 
0.75 
RMS(Dxy) 
1.43 
1.79 
1.57 
4.76 
2.04 
1.21 
2.05 
1.59 
RMS(Dh) 
2.52 
3.88 
3.09 
30.0 
4 
4.31 
2.86 
5.52 
6.16 
RMS(Dh) 
2.73 
4.06 
3.61 
29.7 
8 
4.88 
2.84 
5.4 
6.07 
Table 3. Accuracy results of generated DSMs and orthoimages 
from mixed satellite stereo pairs 
There are six cross-track stereo pairs in table 2 and 8 mixed 
satellite stereo pairs in table 3. In table 2, besides 5 IKONOS 
cross-track stereo pairs and 1 QuickBird cross-track stereo pair, 
1 in-track IKONOS stereo pair, i.e. 1213, is used to provide a 
reference for geometric accuracy comparison. 
The geometric accuracy is assessed from horizontal and vertical 
direction, respectively. Regarding the horizontal accuracy, it 
turns out that there is no significant difference among the in 
track IKONOS stereo pair, IKONOS and QuickBird cross-track 
stereo pairs, and mixed satellite stereo pairs. For cross-track 
stereo pairs, the 1114 shows relatively lower accuracy due to the 
difference of image quality caused by the large time interval of 
image acquisition. Also, it is confirmed that although the stereo 
acquisition geometry has the tendency to influence the accuracy 
to some degree, the accuracy does not degrade so much even if 
the B/H ratio is small. For mixed satellite stereo pairs, it is 
noticed that the accuracy changes considerably with the change 
of the stereo acquisition geometry. If the B/H is too small, the 
accuracy degrades drastically. Moreover, the influence of the 
bisector elevation angle can be confirmed in the case of that the 
B/H ratios of two stereo pairs are almost same. 
In comparison with the horizontal accuracy results, the stereo 
acquisition geometry exerts a remarkable influence on the 
vertical accuracy. In the case of cross-track stereo pairs, the 
accuracy degrades when the B/H ratio becomes small. In the 
case of mixed satellite stereo pairs, since the altitude of 
IKONOS and QuickBird satellite are much different, the 
geometrical relationship of a satellite stereo pair exerts a critical 
influence on the accuracy. Not only the B/H ratio is one of the 
main factors that affect the accuracy, the bisector elevation 
angle is also an important factor. Moreover, from the accuracy 
results, it is confirmed that if the stereo acquisition geometry is 
good, the same level accuracy compared with the in-track stereo 
pair can also be obtained by both the cross-track and mixed 
satellite stereo pairs. 
5. CONCLUSIONS 
In this study, the in-track, cross-track and mixed satellite stereo 
combinations of high resolution IKONOS and QuickBird 
satellite images are utilized to generate DSMs and orthoimages. 
The geometric accuracy for all combinations is assessed, and 
the relationship between the stereo acquisition geometry 
parameters and the geometric accuracy is analyzed. It is 
confirmed that if the stereo acquisition geometry is relatively 
good, the same level accuracy compared with an in-track stereo 
pair can be obtained by cross-track and mixed satellite stereo 
combinations. It is shown that although the factors such as large 
time interval of image acquisition, seasonal change of land 
cover, and the difference of image quality have the tendency to 
influence the accuracy to some degree, they are not the main 
factors. The stereo acquisition geometry of satellite stereo pairs 
exerts a critical influence on the geometric accuracy. Also, the 
effects on accuracy degradation exhibit different trends in 
cross-track stereo pairs and mixed satellite stereo pairs. 
Although further analysis is necessary by using more satellite 
images and test fields, the assessment results showed the great 
potential of using cross-track and mixed satellite stereo images 
for spatial information collection. 
References 
Cain, J., 1989. Stereomodel Acquisition Geometry. Ph.D. thesis, 
U.C. Berkeley.