## 4 Evaluation of MODIS-Compatible VIIRS EVI  
The MODIS-compatible VIIRS EVI with the optimized coefficients (in Table 3) was evaluated using another dataset, _i.e._, MODIS and VIIRS global, daily CMG data of which acquisition dates did not overlap with those used in the calibration/optimization. The period of data for the evaluation spanned between 8/1/2013 and 11/30/2013 (four months). The total number of MODIS and VIIRS reflectance pairs, which fell in any of the angular bins described in Section 3.2, was more than 2.8 million from which an evaluation dataset was randomly extracted. The size of this subsample was 200,000. This evaluation dataset had different frequencies over angular and EVI bins from the one used in the previous section. A density plot of MODIS EVI minus VIIRS EVI (\(\delta_{1}\)) as a function of MODIS EVI is shown in Figure 5a. Trends of negative biases and high density in lower EVI values were similar to Figure 3a but the appearance of the plot (distribution of dots) was different because of different scenarios in extracting sample data. The mean of the error (\(-0.021\)) was similar to that obtained from the calibration dataset in Figure 3a (\(-0.019\)), whereas STD and RMSE (0.021 and 0.029) of this evaluation dataset were smaller than those in Figure 3a (0.028 and 0.034) as summarized in Table 4. Figure 5b shows a density plot of MODIS EVI minus MODIS-compatible EVI (\(\delta_{2}^{*}\)) as a function of MODIS EVI. The absolute average of \(\delta_{2}^{*}\) (0.003) was reduced from that of \(\delta_{1}\) (0.021). STD of \(\delta_{2}^{*}\) (0.020) was slightly lower than  
Figure 4: **(a) Spatial distribution of MODIS EVI minus VIIRS EVI (\(\delta_{1}\)) over geographic coordinate for the calibration; (b) Spatial distribution of MODIS EVI minus MODIS-compatible EVI (\(\delta_{2}^{*}\)). **  
that of \(\delta_{1}\) (0.021). RMSE of \(\delta_{2}^{*}\) (0.020) was reduced from that of \(\delta_{1}\) (0.029). These results indicate that the coefficients obtained in the previous section were reasonable. Figure 6a,b show the spatial distribution of MODIS EVI minus VIIRS EVI (\(\delta_{1}\)) and MODIS EVI minus MODIS-compatible EVI (\(\delta_{2}^{*}\)), respectively, for evaluation data. Spatial trends of errors in Figure 6 are very similar to those in Figure 4. The spatial coverage of Figure 6 is slightly wider than Figure 4 since the sample size of the evaluation dataset (200,000) was larger than that of the calibration/optimization dataset (137,278). The number of positive error occurrences in Figure 6b appears slightly smaller than that in Figure 4b because of the two different scenarios used in obtaining the evaluation and calibration datasets. For example, the southern part of the South American continent shows fewer dots of positive errors (red dots) compared to Figure 4b. The MODIS-compatible VIIRS EVI was further evaluated by examining its performance with respect to sun-target-sensor viewing angle, EVI dynamic range, and land cover type. Mean, STD, and RMSE of \(\delta_{2}^{*}\) and \(\delta_{1}\) were computed and those of \(\delta_{2}^{*}\) were divided by the respective counterparts of \(\delta_{1}\), referred to as the ratio of mean (RM), ratio of STD (RS), and ratio of RMSE (RR), for each angle bin, each EVI bin, and each land cover type (Figures 7-9 respectively). The MODIS Land Cover Type Yearly CMG (MCD12C1 [33], resolution of 0.05\({}^{\circ}\)-by-0.05\({}^{\circ}\)) for 2012 was used to identify the land cover type of each CMG pixel (the 17-class International Geosphere-Biosphere Programme (IGBP) classification) [34]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Mean** & **STD** & **RMSE** \\ \hline \(\delta_{1}\) & \(-0.021\) & 0.021 & 0.029 \\ \(\delta_{2}^{*}\) & \(-0.003\) & 0.020 & 0.020 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistics of MODIS EVI minus VIIRS EVI (\(\delta_{1}\)) and MODIS EVI minus MODIS-compatible VIIRS EVI (\(\delta_{2}^{*}\)) for the evaluation. Figure 5: Density plots of EVI differences using the data for the evaluation.