# Automatic Matching of Multi-Source Satellite Images: A Case Study on ZY-1-02C and ETM+
## 3 Experiments and Results
### Accuracy Analysis of Matching between ZY-1-02C Images and ETM+ Images
#### 3.3.4 Accuracy Analysis of Image Matching  
The RPC of the ZY-1-02C images was modified using the matched points with the DOM of ETM+. To clarify the necessity for image matching, 20 checkpoints that were evenly distributed on the reference image were manually measured. The coordinate differences of the checkpoints between the input and modified images, which were calculated by RPCs, were analyzed. Figure 8 shows the checkpoints in the images.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Point ID** & **Triangle ID** & \(\mathbf{L}_{p}^{I}/\mathbf{L}_{p}^{r}\) & \(\mathbf{A}_{p}^{I}/\mathbf{A}_{p}^{r}\) & \(\boldsymbol{w}_{p}\) \\ \hline \multirow{4}{*}{\(1\)} & 1\_1 & 1.12 & 0.76 & 24 \\  & 1\_2 & 1.22 & 0.87 & 31 \\  & 1\_3 & 0.85 & 1.21 & 42 \\  & 1\_4 & 0.74 & 1.32 & 53 \\ \hline \multirow{4}{*}{\(2\)} & 2\_1 & 0.53 & 1.31 & 72 \\  & 2\_2 & 0.42 & 1.42 & 63 \\  & 2\_3 & 0.44 & 1.33 & 115 \\  & 2\_4 & 1.48 & 0.84 & 132 \\  & 2\_5 & 1.43 & 0.92 & 68 \\  & 2\_6 & 1.39 & 0.98 & 93 \\ \hline \multirow{4}{*}{\(3\)} & 3\_1 & 0.93 & 1.24 & 128 \\  & 3\_2 & 0.86 & 1.11 & 45 \\  & 3\_3 & 1.17 & 0.93 & 35 \\ \cline{1-1}  & 3\_4 & 1.25 & 0.97 & 113 \\ \cline{1-1}  & 3\_5 & 1.42 & 0.82 & 107 \\ \cline{1-1}  & 3\_6 & 0.79 & 1.03 & 41 \\ \cline{1-1}  & 3\_7 & 0.81 & 1.02 & 43 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Statistics of the error matches.  
Figure 7: Results of error matching. (**a**,**c**,**e**,**g**) show DOMs of ETM+, while (**b**,**d**,**f**) show ZY-1-02C images.  
The coordinates of both image points were then used to calculate the differences, i.e., \(\Delta x_{geo}\) and \(\Delta y_{geo}\), as follows:  
\[\left\{\begin{array}{l}\Delta x_{geo}=X_{ZY\_M}-X_{ZY}\\ \Delta x_{geo}=Y_{ZY\_M}-Y_{ZY}\end{array}\right. \tag{8}\]  
where \((X_{ZY\_M},Y_{ZY\_M})\) is the coordinate of the checkpoint in the modified image, and \((X_{ZY},Y_{ZY})\) is the coordinate of the checkpoint in the original image.  
Figure 9 shows the comparative results of the checkpoints. The revised image exceeds the original image in positioning precision by 100 m, in general, but differs from specific points. The refinement of point position after matching is significant compared to the original image.  
Figure 8: Checkpoint distribution on the DOM of ETM+.  
Figure 9: Comparative results of the checkpoints between the original and modified ZY-1-02C image.  
Directly clarifying the geo-referencing accuracy of ZY-1-02C Level 1 images and the DOM of ETM+ was difficult because of the topographic variations. Thus, after the processing, the RPCs of these ZY-1-02C images were modified by using matching models. To clarify the geo-referencing accuracy of the modified RPC, ZY-1-02C imagery must generate the DOMs by SRTM 90. Then, checkpoints were used to confirm the geo-referencing accuracy between ZY-1-02C and ETM+. Figure 10 shows the comparative results of the checkpoints on the DOMs of both ZY-1-02C and ETM+. The maximum value was less than three pixels, which indicates that the matching accuracy is typically eight meters in object space. Given the differences in terrain elevation, variances were bound to exist between different checkpoints.