A spectral index compresses multiband imagery into one diagnostic number per pixel. NDVI — the Normalized Difference Vegetation Index, computed from red and near-infrared reflectance — is the most famous: healthy vegetation scores high, bare soil low, water negative. Dozens of siblings target water (NDWI), built-up areas (NDBI), burn severity (NBR) and soil-adjusted vegetation (SAVI, EVI). Indices are simple, fast, physically motivated — and easy to over-interpret.
Why NDVI works
Chlorophyll absorbs red light; healthy leaf structure scatters near-infrared strongly. NDVI = (NIR − Red)/(NIR + Red) captures that contrast on a −1 to +1 scale: dense vigorous canopy typically 0.6–0.9, sparse or stressed vegetation lower, soil near 0.1–0.2, water below zero. The normalised ratio suppresses illumination differences, making values comparable across scenes.
Its power multiplies in time: an NDVI curve traces a crop’s season — green-up, peak, senescence — enabling crop identification, sowing-date estimation and stress alerts from trajectory anomalies.
The wider index family
EVI and SAVI correct NDVI’s saturation over dense canopy and soil-background sensitivity in sparse cover. NDWI variants highlight open water or canopy moisture; NBR differencing before/after fire grades burn severity; NDBI leans toward built surfaces; red-edge indices from Sentinel-2 sharpen crop nitrogen and stress signals.
Choosing an index is choosing which physical contrast to exploit — and acknowledging what else moves it.
Using indices honestly
Indices computed on uncorrected imagery inherit atmosphere and calibration noise — surface reflectance first. Thresholds are not universal: “NDVI > 0.4 is vegetation” varies by biome, season and sensor; calibrate locally. Mixed pixels blur meaning at coarse resolution, and NDVI measures greenness, not species, yield or health directly — those are inferences requiring ground truth.
Treated as calibrated evidence within a designed analysis, indices are superb; treated as universal truth-meters, they mislead with great confidence.