# Improving Spatial-Temporal Data Fusion by Choosing Optimal Input Image Pairs
## 4 Results and Analysis  
### Consistency between MODIS and Landsat  
The consistency between fine and coarse data sources can be a major factor impacting data fusion accuracy [9; 14; 15; 17]. The averaged RMSE and R from different dates based on all pair images for six MODIS and Landsat combinations (in Section 3.4) were calculated and are illustrated in Figure 4 for site 2.  
Results show that the RMSE between Landsat and MODIS reflectances is smallest for the red band, followed by NIR and then NDVI. The RMSE for NDVI may not be directly comparable to reflectance in the red and NIR bands since they have different data ranges. However, NDVI statistics can be cross-compared over different MODIS data sources (triangle symbols). On the other hand, R values for NDVI are higher than those for reflectance in red and NIR bands. One possible reason is that the study area is predominantly occupied by vegetation, which shows stronger vegetation signals in the VI image than in the reflectance bands, and thus higher correlation values.  
It can also be seen that RMSE and R from the combinations of different NBAR products with L7 (hollow signals) or L8 (solid signals) are different. Specifically, the combination of Terra NBAR and L7 produced the smallest RMSE and the highest R among all combinations, which implies better consistency between Terra NBAR and L7. This can be explained partly by sensor overpass time and the observation methods. Although Terra, Aqua, and Landsat are all sun-synchronous polar-orbiting satellites, their overpass times and swath paths are different. In these combinations, Landsat 7 is on the same orbit as Terra (with a \(\sim\)30-minute difference), therefore, Landsat 7 images always overlap with the central portion of the MODIS-Terra Swath with close overpass times on the same day. Therefore, Terra MODIS and Landsat 7 ETM+ have the similar solar-viewing geometries and atmospheric conditions. It is reasonable to deduce that the combination of Terra MODIS NBAR and Landsat 7 ETM+ has the best consistency among all of these combinations.  
Figure 4: Temporally-averaged RMSE (<RMSE>) (**a**) and R (<R>) (**b**) for different combinations of MODIS and Landsat pair images for site 2. NBAR, Terra, and Aqua in the x axis represent the combinations of the Terra and Aqua combined NBAR, Terra NBAR, and Aqua NBAR with Landsat, respectively. Labels in the plot show the combinations of MODIS (NBAR, Terra, Aqua) and Landsat (L7 and L8) image pairs.  
Additionally, RMSE and R between MODIS NBAR and Landsat are often in the middle of the other two combinations (Terra NBAR and Landsat and Aqua NBAR and Landsat) in red, NIR wavebands and NDVI. One possible reason is that the spatial resolution of the Collection 6 daily combined NBAR product is coarser (500 m) than our in-house Terra and Aqua daily NBAR (250 m) products used in this study. Another possible reason is that the daily MODIS NBAR product is computed using all available Terra and Aqua directional reflectance data from each 16-day period, which can improve the poor quality data on the retrieval date, but not necessarily surpass the good quality data collected on that date.  
In Figure 4b, the correlation coefficients (R) from the combined NBAR are similar for both Landsat 7 and 8. However, R values are very different from Terra or Aqua NBAR. This implies that the combined NBAR can produce more consistent results with either Landsat 7 or 8. The Collection 6 MODIS daily NBAR product is more stable and smoother, both temporally and spatially, than Terra and Aqua NBAR.