# Automatic Matching of Multi-Source Satellite Images: A Case Study on ZY-1-02C and ETM+
## 3 Experiments and Results
### Applicability Analysis of Methodology  
According to the proposed strategy, image resampling was a key step in the AMSIM case study. A match between the original and sub-sampled images was crucial for evaluating the accuracy and reliability of the method. Dataset 1 was used for this work. Considering the resolution of the ETM+ images, the ZY-1-02C image was down-sampled to resolutions of 15 and 30 m. For image matching, the down-sampled images were resampled to the same ground sampling distance (GSD) of the ZY-1-02C original image. Figure 5 shows the matching results of these two pairs of images. Table 2 summarizes the matching parameters of the experiment. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Dataset** & **Latitude/** & **Train** & **Sensor** & **Image Size** & **Resolution** & **Image** & **Acquired** \\
**ID** & **Longitude** & **Terrain** & **Sensor** & **(pixel)** & **(m)** & **Level** & **Time** \\ \hline
1 & 120.2/30.2 & City & \
\begin{tabular}{c} ZY-1-02C PAN \\ (0.51–0.85 μm) \\ \end{tabular} & 4200 \(\times\) 2100 & 5.0 & Level 1 &
\begin{tabular}{c} 19 February \\ 2012 \\ \end{tabular} \\ \hline
2 & 117.0/40.8 & Mountains & \
\begin{tabular}{c} ZY-1-02C PAN \\ (0.51–0.85 μm) \\ \end{tabular} & 12,000 \(\times\) 12,000 & 5.0 & Level 1 &
\begin{tabular}{c} 19 February \\ 2012 \\ \end{tabular} \\ \hline
3 & 116.5/40.5 & Mountains & \
\begin{tabular}{c} ETM+ \\ (0.52–0.90 μm) \\ \end{tabular} & 15,321 \(\times\) 14,921 & 15.0 & DOM &
\begin{tabular}{c} 28 January \\ 2003 \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of test datasets. Figure 5a,b,d,f,h,j presents the resampled images of ZY-1-02C. The GSDs of Figure 5a,d,f are 15 m, the GSDs of Figure 5b,h,j are 30 m. Figure 5c,e,g,i shows the original images of ZY-1-02C, where the GSDs are 5 m. The textures of Figure 5a,b,d,f,h,j are much fuzzier than that of the original images. Thus, the feature points in Figure 5c,e,g,i, for the most part, do not match the corresponding points in Figure 5d,f,h,j. Given that the matching image was actually the same image, the image translation was set as 15 \(\times\) 15 pixels. The approximate parameters of image transformation were detected using the initial value prediction model, as shown in Table 2. The grid size used in our approach to extract the Harris feature points was 50 \(\times\) 50 pixels, the template window size was 13 \(\times\) 13 pixels, the search window size was 21 \(\times\) 21 pixels, and the threshold of the correlation coefficient was 0.9. The total matches were few when matching the original image with the 30 m down-sampled image. Hence, fuzzier original images were more difficult to match. A method was used for directly calculating the accuracy of the matching experiment. In the image coordinate space, \(\Delta x_{i}=\left|x_{org}-x_{down}\right|\) and \(\Delta y_{i}=\left|y_{org}-y_{down}\right|\) were defined to describe the matching and resampling error. The RMSE and mean values of \(\Delta\)x and \(\Delta\)y were used to evaluate the matching accuracy, as shown in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Group ID**} & **Parameters of Transformation** & **Number of Feature Points** & **Total Number of Matches** \\  & **between Images** & & \\ \hline \multirow{2}{*}{1} & \(A=0^{\circ}\), \(k=3\), & \multirow{2}{*}{3124} & \multirow{2}{*}{456} \\  & \(\chi_{0}=15\) pixels, \(\chi_{0}=15\) pixels. & & \\ \hline \multirow{2}{*}{2} & \(A=0^{\circ}\), \(k=6\), & \multirow{2}{*}{3124} & \multirow{2}{*}{36} \\  & \(\chi_{0}=15\) pixels, \(\chi_{0}=15\) pixels. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chart of matching parameters. Figure 5: Results of image matching between the down-sampled and original images of ZY-1-02C. As the down-sampled images originated from the original images, \(\Delta\)x and \(\Delta\)y were the real errors in the matching. Table 3 shows the matching errors of the two groups of matching results.