NDVI: Normalized Difference Vegetation Index-3rd generation: NASA/GFSC GIMMS

Normalized Difference Vegetation Index-3rd generation (NDVI) using the Global Inventory Monitoring and Modeling System (GIMMS):  Vegetation indices are radiometric measures of photosynthetically active radiation absorbed by chlorophyll in the green leaves of vegetation canopies and are therefore good surrogate measures of the physiologically functioning surface greenness level of a region. For 30 years, Compton J. Tucker  created the NDVI time series within the framework of the Global Inventory Monitoring and Modeling System (GIMMS) project. He carefully assemblied it from different AVHRR sensors and accounting for various deleterious effects, such as calibration loss, orbital drift, volcanic eruptions, etc.  The latest version of the GIMMS NDVI data set spans the period July 1981 to December 2015 and is termed NDVI3g (third generation GIMMS NDVI from AVHRR sensors). 

The USGS Remote sensing phenology states: "NDVI values range from +1.0 to -1.0. Areas of barren rock, sand, or snow usually show very low NDVI values (for example, 0.1 or less). Sparse vegetation such as shrubs and grasslands or senescing crops may result in moderate NDVI values (approximately 0.2 to 0.5). High NDVI values (approximately 0.6 to 0.9) correspond to dense vegetation such as that found in temperate and tropical forests or crops at their peak growth stage."