# Fusion of Different Image Sources for Improved Monitoring of Agricultural Plots
## 3 Results
### Validation
#### 3.4.3 Similarity of the Sentinel 2 Images during the Overlapping Period  
Table 5 displays the similarity values of the RGI images obtained from Sentinel 2 during the overlapping period (September 2019). In this month, three images were used, acquired on the 3 rd, 18 th and 28 th. Data presented in this table utilize the image obtained on September 3 as a reference. _SSIM_ values are higher than in the previous cases, ranging from 0.964 to 0.983. Values of all parameters are higher than 0.900 in all bands. Analogous values were obtained when comparing September 18 and September 28 images, with _SSIM_ of 0.989, 0.986 and 0.995, respectively, for the RGI bands. 4.4 Similarity of NDVI, GNDVI and GCI Images Calculated from Sentinel 2 Images during the Overlapping Period  
NDVI, GNDVI and GCI images calculated from the Sentinel 2 series during the overlapping period also revealed a great level of similarity between them (data not shown), SSIM ranging from 0.986 to 0.998. 4.5 Similarity of NDVI, GNDVI and GCI Images Obtained from Merging the Original August and Original September Images during the Overlapping Period  
Table 5 displays crucial data for testing the validity of the whole procedure described in this paper. It compares two sets of resulting NDVI, GNDVI and GCI images: those obtained by merging the August image and those obtained by merging the September image during the three days of the overlapping period. Consequently, it provides information about how sensible the procedure is to changes imputable to differences between the original drone images. It can be observed that the resulting NDVI, GNDVI and GCI images obtained from the two original drone images had strong similarities, with SSIM ranging from 0.876 to 0.894. Major dissimilarities were caused by lower values of the correlation parameter (s). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**August Image**} & \multicolumn{3}{c}{**September Image**} \\ \hline
**Parameter/Band** & **R** & **G** & **I** & **R** & **G** & **I** \\ \hline \(c\) & 0.953 & 0.958 & 0.969 & 0.992 & 0.987 & 0.963 \\ \(s\) & 0.916 & 0.917 & 0.742 & 0.901 & 0.841 & 0.608 \\ \(l\) & 0.977 & 0.986 & 0.996 & 0.998 & 0.998 & 0.998 \\ \(SSIM\) & 0.852 & 0.866 & 0.717 & 0.892 & 0.829 & 0.584 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Similarity between homogenized drone images. Reference: July image. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Sentinel 2–September 18**} & \multicolumn{3}{c}{**Sentinel 2–September 28**} \\ \hline
**Parameter** & **R** & **G** & **I** & **R** & **G** & **I** \\ \hline \(c\) & 0.988 & 0.992 & 0.996 & 0.988 & 0.995 & 0.994 \\ \(s\) & 0.991 & 0.989 & 0.991 & 0.994 & 0.995 & 0.989 \\ \(l\) & 0.986 & 0.990 & 0.996 & 0.984 & 0.992 & 0.994 \\ \(SSIM\) & 0.964 & 0.972 & 0.983 & 0.967 & 0.982 & 0.977 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Similarity of the Sentinel 2 images during the overlapping period. Reference: September 3 image. These data suggest that this effect is linked to the dissimilarities of the original drone images with respect to this parameter before (Table 3) and after homogenization (Table 4). On the other hand, it must be highlighted that the similarity values of NDVI, GNDVI and GCI images obtained from the merged images with different sources (Table 6) are lower than those obtained from comparing analogous images from Sentinel 2 data on different days in the overlapping period (Section 3.4.4). This can be attributed to two factors: (a) Sentinel images have coarser resolution and (b) changes of these spectral indexes are relatively slow. 4.6 Similarity of the NDVI, GNDVI and GCI Images Obtained from Merging the Original August or the Original September Images Outside the Overlapping Period  
In this section, we provide data on the similarity of NDVI, GNDVI and GCI images calculated on two different days outside the overlapping period.