NDVI (Normalized Difference Vegetation Index)

A simple math combination of red and near-infrared bands that measures how green/healthy vegetation is. Ranges roughly −1 (water/bare) to +1 (dense canopy).

Why it matters

NDVI is the most-used vegetation index in Earth science — drives drought, crop, deforestation, and phenology studies. A 25-year NDVI record exists from MODIS.

Where you’ll meet it

• MODIS MOD13Q1 is the canonical product — 250 m, 16-day composite NDVI — and is the workhorse behind the deforestation question in this atlas.
• It comes packaged as HDF granules you fetch with earthaccess, then open in xarray to compute trends over time.
• Landsat and the newer VIIRS sensors carry the NDVI record forward; Sentinel-2 (ESA) is the common higher-resolution complement when you need finer detail than MODIS gives.

In plain terms

A plant “health thermometer” from space — high NDVI = lush, low NDVI = stressed or bare.