The MODIS/Terra Gap-Filled, Smoothed NDVI 8-Day L4 250m SIN Grid product, with short-name MOD09Q1G_NDVI is calculated from MODIS surface reflectance products (MOD09), at 250-m resolution. MODIS time series contains occasional lower quality data, gaps from persistent clouds, cloud contamination, and other gaps. Many modeling efforts, such as those used in NACP, that use MODIS data as input, require gap-free data. The procedure contains two algorithm stages, one for smoothing and one for gap filling, which attempt to maximize the use of high-quality data to replace missing or poor-quality observations.