Comparison and validation of GIMMS, SPOT-VGT and MODIS global NDVI products in the Loess Plateau of northern Shaanxi Province, northwestern China

To select an optimum time series in remote sensing data for monitoring vegetation changes, especially before and after implementation of the Sloping Land Conversion Program, we analyzed spatial distributions, seasonal and annual variations of vegetation in the Loess Plateau of Shaanxi Province, based on the normalized difference vegetation index (NDVI) derived from three kinds of remote sensing data, i.e. GIMMS, SPOT-Vegetation (SPOT-VGT) and MODIS. The NDVI data were compared using linear correlation and root mean-square error (RMSE) analyses. The results showed that: 1) there were no significant differences in large scale spatial distributions among the GIMMS/NDVI, the SPOT-VGT/NDVI and the MODIS/NDVI data sets. However, the MODIS/NDVI data was more efficient in monitoring spatial vegetation distributions, with high spatial resolution and a wide range in the NDVI; 2) In seasonal changes of vegetation, significant correlations were found among the three remote sensing data sets. RMSE between the MODIS/NDVI and GIMMS/NDVI was higher than that between the MODIS/NDVI and SPOT-VGT/NDVI. We also found that differences among the three remote sensing data sets varied between seasons, with the largest difference occurring during summers due to more rainy and cloudy days; 3) In annual changes of vegetation, significant increases in both MODIS/NDVI and SPOT-VGT/NDVI were shown from 1999-2007, but no significant variation in GIMMS/NDVI, suggesting that this data set has some limitations in monitoring the annual vegetation changes. The correlation coefficient of annual variation between the MODIS/NDVI and GIMMS/NDVI decreased with an increase in vegetation coverage, and became even negative for coniferous and broad-leaf mixed forests, indicating that the GIMMS sensor was less sensitive to the variation of vegetation with high coverage due to changes of atmospheric water vapor and clouds. 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