## 4 Results and Discussion  
In Figure 4, \(NDVI\), \(EVI_{B}\), \(EVI_{P}\), and \(EVI_{C}\) derived from ASTER reflectances aggregated without MODIS PSF, and their differences (\(D_{NDVI}\) and \(D_{EVI}\)) and relative errors (\(RD_{NDVI}\) and \(RD_{EVI}\)) from the MODIS counterparts are plotted. The NDVI and all the three ASTER EVIs showed a good correlation with MODIS. R\({}^{2}\) values were very high (0.997) for all the three ASTER EVIs. An \(R^{2}\) value for ASTER NDVI was slightly lower (0.987) than those for ASTER EVIs. ASTER NDVI and EVIs were consistently lower than MODIS NDVI and EVI, with the slope estimates all being less than one. Considering that the aggregation method did not have any impacts on ASTER VI values as mentioned later, these results would be caused by the difference between ASTER and MODIS relative spectral responses. The intercept of \(EVI_{B}\) was close to zero. On the other hand, the intercepts of \(NDVI\), \(EVI_{P}\), and \(EVI_{C}\) were negative. Among them, the slope for the \(NDVI\) relationship (0.968) was the closest to one, whereas it had a larger intercept than the EVI relationships. The differences and relative errors of the NDVI and three EVIs were, on average, all less than zero (negatively large errors), and showed almost negative value, which means all ASTER NDVI and EVIs were less than MODIS NDVI and EVI (Figure 4). Both the differences and relative errors of \(EVI_{B}\) on evergreen needle-leaf forest, barren or sparsely vegetated, and urban and built-up land cover area, were very close to zero. On the other hand, the \(NDVI\) had larger difference and relative error for evergreen needle-leaf forest than for the other land cover types. The relative differences between ASTER and MODIS red reflectance ( (\(\rho_{ASTER,red}-\rho_{MODIS,red})/\rho_{MODIS,red}\) ) were much larger positive values than the others. This indicates that ASTER NDVI over evergreen needle-leaf forest was much lower than the ASTER-MODIS NDVI regression line. Figure 5 shows scatterplots of _NDVI_, _EVI\({}_{B}\)_, _EVI\({}_{P}\)_, and _EVI\({}_{C}\)_ derived from ASTER reflectances aggregated with MODIS PSF (Equation (10)), and their differences (_D\({}_{NDVI}\)_, _D\({}_{EVI}\)_) and relative errors (_RD\({}_{NDVI}\)_, _RD\({}_{EVI}\)_) from the MODIS counterparts. Trends depicted in these scatterplots agreed to those obtained with aggregated ASTER reflectances without MODIS PSF, except for _EVI\({}_{C}\)_. All _R\({}^{2}\)_ values were the same as the above results without MODIS PSF. This indicates that the differences of ASTER and MODIS EVI depend on the differences not of the aggregation methods, but of the spectral bandpasses in this study. However, the slope of _EVI\({}_{C}\)_ (0.913) with MODIS PSF was slightly closer to _EVI\({}_{B}\)_ than the slope of _EVI\({}_{C}\)_ without MODIS PSF (0.932). ASTER NDVI with MODIS PSF was mostly the same as that without MODIS PSF. The differences and relative errors for _EVI\({}_{B}\)_ were the closest to zero for the land cover types of urban and built-up, barren and sparsely vegetated, and evergreen needle-leaf forest, which were lower than MODIS EVI with or without MODIS PSF. Therefore, _EVI\({}_{B}\)_ had the best compatibility with MODIS EVI for all the land cover types examined in this study. Figure 4: Scatterplots of MODIS _vs._ ASTER VIs with ASTER reflectances aggregated without MODIS PSF. (**a**) ASTER Normalized Difference Vegetation Index (NDVI) _vs._ MODIS NDVI, (**b**) ASTER _EVI\({}_{B}\)_vs._ MODIS EVI, (**c**) ASTER _EVI\({}_{P}\)_vs._ MODIS EVI, (**d**) ASTER _EVI\({}_{C}\)_vs._ MODIS EVI, (**e**) _D\({}_{NDVI}\)_vs._ MODIS NDVI, (**f**) _D\({}_{EVI_{B}}\)_vs._ MODIS EVI, (**g**) _D\({}_{EVI_{P}}\)_vs._ MODIS EVI, (**h**) _D\({}_{EVI_{C}}\)_vs._ MODIS EVI, (**i**) _RD\({}_{NDVI}\)_vs._ MODIS NDVI, (**j**) _RD\({}_{EVI_{B}}\)_vs._ MODIS EVI, (**k**) _RD\({}_{EVI_{P}}\)_vs._ MODIS EVI, (**l**) _RD\({}_{EVI_{C}}\)_vs._ MODIS EVI.