# Improving Spatial-Temporal Data Fusion by Choosing Optimal Input Image Pairs
## 4 Results and Analysis
### Effect of Data Sources
Figure 6 shows composite images of red and NIR surface reflectance for Landsat and MODIS NBAR (Terra, Aqua and the combined) products over the two study areas for several dates. Visually, the images from Aqua or Terra NBAR with smaller VZA show greater spatial detail, more similar to Landsat. The MODIS images with large VZA are blurred due to the large pixel footprint size. The Collection 6 MODIS daily NBAR product improves image quality by integrating observations from both Terra and Aqua. Some cloud and cloud shadow pixels in Aqua and Terra NBAR images are corrected in the combined NBAR product. However, the combined NBAR images look much blurrier than either Terra or Aqua NBAR image alone due to the lower spatial resolution (500 m), especially within the growing season (such as NBAR in Figure 6B,D). Therefore, the MAD values based on the combination of combined NBAR + L8 in Figure 5a are even larger than those based on Terra + L8, which has a large VZA in the MODIS pair image. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Study Areas** & **Path/Row** & **Terra + L8** & **Terra + L7** & **Aqua + L8** & **Aqua + L7** \\ \hline
**Site 1** & 028031 & 57.77 & 7.25 & 27.62 & 44.64 \\
**Site 2** & 029031 & 60.05 & 4.06 & 38.30 & 40.11 \\
**Site 2** & 025036 & 63.66 & 0.66 & 12.39 & 59.15 \\ \hline \end{tabular}
\end{table} 
Table 3: Mean view zenith angle (VZA) from MODIS sensors in the study areas on Landsat 7 and 8 overpass dates. Figure 6: Comparison of the composite NBAR images from Terra (250 m), Aqua (250 m), and the combined products (500 m) with Landsat image for site 1 (**A**,**B**) and site 2 (**C**,**D**) from different Landsat dates. The images are composited using NIR, red and red bands as RGB (only red and NIR bands are available at 250 m resolution). The images for each date are linearly stretched using the same stretch range for better visualization. The view zenith angles (VZA) for Terra and Aqua images are the averaged VZAs from original directional reflectance. The figure shows that even after BRDF correction, the image quality and spatial detail in the MODIS data used as fusion inputs can vary significantly. Figure 7 demonstrates data fusion examples produced from different data source combinations. Predictions for the DOY 219 in 2015 were generated based with STARFM one input image pair on DOY 2015-027 generated from three different data source combinations (Terra + L8, Aqua + L8, and the combined NBAR + L8) (the original NBAR images are shown in Figure 6D). Comparing to the direct Landsat observation from the same date, we can see the best predicted image comes from the combination of Aqua + L8 (the highest R), while the poorest one is from the combination of Terra + L8 due to a larger discrepancy in viewing geometries between these two sensors. The larger VZA results in a lower effective spatial resolution for Terra NBAR image (such as Figure 6D), and generates a fused image with lower fidelity (Figure 7b). Although the spatial resolution of the combined NBAR image is only 500 m, the effective spatial resolution of the Terra-Aqua combined NBAR products can be better than that of the Terra NBAR products. The fusion result from the NBAR + L8 image pair also shows clear surface features, with R near 0.8 in comparison with the direct Landsat observation.