NDVI explained
NDVI stands for Normalized Difference Vegetation Index. It measures how much red and near-infrared light plants reflect. Healthy leaves absorb red light and reflect near-infrared light strongly, so a vigorous crop gives a high NDVI.
What the values usually mean
| NDVI | Typical meaning |
|---|---|
| Below 0.2 | Bare soil, water, or very poor cover |
| 0.2 to 0.4 | Young crop or sparse, stressed cover |
| 0.4 to 0.6 | Moderate growth |
| Above 0.6 | Dense, healthy canopy |
These ranges are a guide only. A healthy crop's NDVI changes with crop type and growth stage, so compare a field with itself over time and with the neighbouring plots.
Reading the map
Green and dark areas are more vigorous, and yellow or red patches are weaker. Look for patterns. A patch at the field edge may mean poor drainage, a straight stripe may point to uneven fertilizer or seeding, and a sudden drop across the field may mean pests or lack of water.
What to do with a low value
Walk to the weak patch and check the soil moisture, the roots and the leaves. Compare NDVI with NDMI for water stress, NDRE for nitrogen, and the weather forecast before you irrigate or fertilize.
Limits to keep in mind
Sentinel-2 sees the ground at 10 metres per pixel, so very small farms have few pixels. Clouds can delay a fresh image, and NDVI can flatten out in very dense crops, where NDRE is more reliable.