Like many other studies the work presented here is based on the 8 km resolution
Normalized Difference Vegetation Index (NDVI) dataset
produced by the GIMMS (Global Inventory Monitoring and Modeling System)
group at NASA's Goddard Space Flight Center.
NDVI is calculated from reflectance in the near infrared (NIR)
and visible (VIS) regions:
NDVI = (NIR-VIS)/(NIR+VIS)
Healthy vegetation strongly reflects NIR radiation,
and absorbs VIS for driving photosynthesis.
