What Is a Vegetation Index?
What is a vegetation index?
A vegetation index is a number, computed from satellite reflectance data, that summarises the state of vegetation on the ground. Instead of looking at raw light values (which are hard to interpret and vary with sun angle, haze, and sensor calibration), an index combines specific bands of light into a single figure that correlates directly with something a farmer cares about: how green, how dense, how wet, or how well-fed the canopy is.
The most famous is NDVI — but NDVI is one of many, each tuned to a different aspect of plant health and each with its own strengths and blind spots.
How satellites measure vegetation
Every vegetation index is a formula that takes two or more bands of satellite reflectance and produces a number, typically between −1 and 1. The bands are chosen for what they reveal:
- Red light — absorbed by chlorophyll. Healthy leaves absorb it; stressed leaves reflect more.
- Near-infrared (NIR) — reflected strongly by healthy leaf cell structure. Stressed or sparse canopies reflect less.
- Shortwave-infrared (SWIR) — absorbed by water in the leaf. Dry leaves reflect more.
- Red-edge — a narrow transition band sensitive to chlorophyll concentration; shifts with nitrogen status.
- Green — reflected by water surfaces; used in water-body detection.
The indices combine these bands as ratios (so the result is independent of scene brightness) or as more complex formulas with correction factors for soil, atmosphere, or canopy structure. The key insight: a ratio cancels out the raw brightness of the image, so the index means the same thing whether the scene was captured on a bright or hazy day.
Common indices at a glance
| Index | Measures | Best for | Read more |
|---|---|---|---|
| NDVI | Canopy greenness and density | General health monitoring, most crops | What is NDVI? → |
| EVI | Dense-canopy biomass | High-biomass crops, tropical forest, hazy regions | What is EVI? → |
| SAVI | Sparse-canopy vegetation (soil-corrected) | Young crops, arid land, early growth | What is SAVI? → |
| MSAVI2 | Sparse-canopy vegetation (self-adjusting) | Heterogeneous fields, very sparse cover | What is MSAVI2? → |
| NDMI | Leaf water content | Drought monitoring, irrigation, fire risk | What is NDMI? → |
| NDWI | Surface water bodies | Flood mapping, drainage, rice paddies | What is NDWI? → |
| NDRE | Chlorophyll / nitrogen status | Variable-rate nitrogen, mid-late season | What is NDRE? → |
| NBR | Fuel condition / burn severity | Fire risk and post-fire damage assessment | What is NBR? → |
Choosing the right index
Most monitoring starts with NDVI — it is the universal baseline. From there, add indices based on your question:
- “Is the canopy dense enough?” → NDVI, and switch to EVI if it saturates
- “Is the crop well-watered?” → NDMI (water stress appears here before NDVI)
- “Is the crop well-fed (nitrogen)?” → NDRE (chlorophyll, not just biomass)
- “Is the young crop establishing properly?” → SAVI or MSAVI2 (soil-corrected)
- “Is there standing water on my field?” → NDWI (surface water detection)
- “Is my land at fire risk?” → NBR and NDMI (fuel condition + moisture)
- “How badly did the fire burn?” → dNBR (differenced NBR, pre vs post)
No single index answers every question. The best reports combine several and interpret them together. And never trust a single satellite snapshot — cloud cover can make a healthy field look dying. A trustworthy system uses cloud-masked imagery, looks at trends over multiple dates, compares against seasonal history, and tells you when the data is unreliable.
Free report: Check your field’s vegetation health — free satellite analysis, no signup. Works on any land worldwide. Check my field’s vegetation indices →
Frequently asked questions
What is the most commonly used vegetation index?
NDVI (Normalized Difference Vegetation Index) is the most widely used. It measures canopy greenness and density from the ratio of near-infrared and red reflectance. It is simple, universal, and an excellent default — but it saturates over dense canopies and does not directly measure water or nitrogen. The other indices address these gaps.
Can vegetation indices detect crop disease?
Indirectly. Disease causes stress, which indices can detect as a drop in chlorophyll, moisture, or canopy density. NDVI, NDRE, and NDMI may all decline before visible symptoms appear. But no satellite index can identify the specific pathogen — it tells you where and when stress is occurring, not what caused it. Walking the field remains essential for diagnosis.
What satellite do vegetation indices come from?
For agricultural monitoring, the most common source is Sentinel-2 — two satellites (Sentinel-2A and 2B) operated by the European Space Agency that photograph every point on Earth every five days at 10–20 m resolution. The data is free and publicly available. Landsat (NASA) is also used, though at lower revisit frequency. AgroReports uses Sentinel-2 because of its 5-day revisit and 10 m resolution.