The Land surface phenology is defined as the seasonal pattern of variation in vegetated land surfaces observed from remote sensing. Land surface phenology is distinct from the traditional definition of phenology. The traditional definition refers to specific life cycle events using in-situ observations of individual plants or species, while the land surface phenology observed from remote sensing is aggregated information at the spatial resolution of satellite sensors. Currently, satellite sensors with a frequent repeat cycle provides synoptic information to study land surface phenology across the landscape. Both the temporal frequency of the data and the moderate spatial resolution make this sensor well suited for studying continental/global phenology related to the timing of vegetation cycles at seasonal and interannual time scales.

The MODIS time series contain occasional lower quality data, gaps from persistent clouds, cloud contamination, and other gaps. Many modeling efforts, such as those used in the North American Carbon Program, that use MODIS data as inputs, 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.

The MOD09Q1P_EVI is a Gap-filled Smoothed NDVI created from the MOD09A1 8-day Surface Reflectance product. The spatial resolution is 500 m. MOD09A1P_NDVI is stored in Hierarchical Data Format (HDF) with sinusodial projection, same as other standard MODIS land products.

For details of temporal smoothing and spatial gap filling algorithms used, see the User Guide for MOD09PHN and MOD15PHN products.