## 3 Results  
### NDVI Intercomparison  
For the regression using all time periods over the 9 locations between 2001 and 2008, the correlations are higher than 0.9 between all products; however variations in correlations do exist (Figure 1). As expected the SPOT NDVI is correlated highest with Terra (0.97) as the sensors behind these two datasets have a higher spectral resolution and the NIR and red bands were designed for more appropriate vegetation monitoring than the AVHRR sensor. The NDVI datasets based on these sensors have also been shown to be more highly correlated to high-resolution ground-based [39] and Landsat NDVI [36] than the AVHRR-based NDVIg. There are also variations in the regression slopes between the products. Most comparisons between datasets have slopes near 1. Terra, VIP, and NDVI3g tend to have higher values compared to SPOT, NDVIg, and STAR. Even though the slope between Terra and NDVI3g is greater than one, the y-intercept indicates that NDVI3g has on average slightly larger NDVI values, which is consistent with results from [PERSON] and [PERSON] [23]. STAR NDVI values are substantially lower than all other products, because STAR is the only AVHRR-based dataset that has not been adjusted to match the range of MODIS as has been done with NDVIg, NDVI3g and VIP. Past research has shown that while AVHRR [58] and SPOT [38] ground-based NDVI sensors correlate very highly with MODIS sensors, the satellite-based correlations are lower and regression slopes exhibit large departures from one, suggesting that atmospheric correction is not as robust for the AVHRR and SPOT satellite-based NDVI than MODIS satellite-based NDVI. The lower correlation of STAR with Terra than those of NDVIig and NDVI3g with Terra could be due to a lack of atmospheric corrections used in STAR. The general relationships shown in Figure 1 vary significantly in space (Figure 2). In general, most individual points have a correlation lower than what was found in the 9-point correlation in Figure 1. While there are large areas with high correlations between products above 0.90, there are also locations where correlations are much lower. All products tend to have lower correlations when compared to Terra from the Rocky Mountains westward in the United States and in the evergreen broadleaf forests of southeastern Mexico. The three products based purely on AVHRR NDVI (NDVI3g, NDVIg, and STAR) have lower correlations with Terra over the Gulf Coast of the United States and northeastern Mexico. Based on the combination of AVHRR and MODIS data, VIP shows higher correlations in these areas than the other three AVHRR-based products, particularly over northeastern Mexico and the Southwest US SPOT shows the highest correlations of all products east of the Rocky Mountains, in the Southwest US and in the northern two-thirds of Mexico. Previous research suggests similar spatial variations in correlation and regression slopes between NDVIg and MODIS [37] and between NDVIg and MODIS [23]. However the analysis of [PERSON] and [PERSON] [23] showed slightly lower correlations over the northern Rocky Mountains of the USA. While the correlations for NDVI between each product and Terra were relatively similar, the regression slopes are quite different in their spatial patterns (Figure 2) consistent with the differences seen in Figure 1. NDVIg has a greater area of slopes less than one than area with slopes greater than one, but there are very high slopes greater than 2 over some areas (e.g., mountainous areas along the west coast of the US, the Mogollon Rim in northern Arizona, and parts of southeastern Mexico) which is consistent with recent research [23].