What is NDVI? Satellite crop monitoring with Sentinel-2, explained
What NDVI measures, how often Sentinel-2 sees your field, what 10 m resolution means, its limits, and how AgriSync uses it for wheat, barley and OSR.
Last checked against gov.uk
- Written by
- The AgriSync team
- Published
- Reading time
- 4 minutes
Works for fields anywhere in the UK. Satellite pictures depend on clear skies.
Short answer
NDVI (Normalized Difference Vegetation Index) is a number that shows how green and dense a crop looks from space. It is worked out from how much red and near-infrared light the crop reflects. Thick, healthy green crops score higher. Satellites such as Sentinel-2 measure it field by field. So you can spot weak patches and walk those first.
What is NDVI?
NDVI compares two kinds of light that bounce back from the ground. As the U.S. Geological Survey (opens in a new tab) puts it, "certain pigments in plant leaves strongly absorb wavelengths of visible (red) light", while "the leaves themselves strongly reflect wavelengths of near-infrared light". The USGS (opens in a new tab) gives the formula:
“NDVI = (NIR − Red) / (NIR + Red)”
NDVI goes from −1 to +1. The USGS says bare rock, sand or snow usually score 0.1 or less. Thick plant cover scores roughly 0.6 to 0.9 (USGS (opens in a new tab)). On a farm, what matters is how a field compares with itself over time, and with similar fields.
How often does Sentinel-2 see my field?
Sentinel-2 is a pair of European satellites in the same orbit. The Copernicus mission page (opens in a new tab) says the pair gives "a high revisit frequency of 5 days at the Equator". Each satellite on its own comes back every 10 days.
Sentinel-2 works like a camera. It sees sunlight bouncing off the ground, so it can't see through cloud. In a cloudy spell you may go weeks without a clear picture of a field. AgriSync skips cloudy pictures. It shows the date of the last clear one.
What resolution is Sentinel-2?
It measures 13 bands of light: four at 10 m, six at 20 m and three at 60 m (Copernicus (opens in a new tab)). NDVI uses the red (B4) and near-infrared (B8) bands. Both are 10 m bands. A 10 m pixel (one dot in the picture) covers 100 square metres. So a hectare is 100 pixels.
| Sentinel-2 | |
|---|---|
| Satellites | Two, in the same orbit, 180° apart |
| Revisit | 5 days at the Equator with both satellites; 10 days for one |
| Bands | 13: four at 10 m, six at 20 m, three at 60 m |
| Bands used for NDVI | B4 (red) and B8 (near-infrared), both 10 m |
What can NDVI tell you, and what can't it?
| It can help you | It can't |
|---|---|
| See which fields, or parts of fields, look less green or less dense than usual | Tell you why a crop looks different: the cause needs checking in the field |
| Follow a crop through the season, pass by pass | Replace a soil test or tissue test |
| Decide which fields to walk first | Give a reading through cloud |
| Compare a field with itself last week | Tell you how much fertiliser to apply |
Treat a low reading as a reason to look. Then check the crop in the field, or with your agronomist.
What are NDRE, NDMI and GNDVI?
They are other readings worked out from different Sentinel-2 bands. AgriSync shows them as plain-English tabs on each field's chart:
| Index | Tab name in AgriSync |
|---|---|
| NDVI (Normalized Difference Vegetation Index) | Crop health |
| NDRE (Normalized Difference Red Edge) | Nitrogen risk |
| NDMI (Normalized Difference Moisture Index) | Moisture |
| GNDVI (Green NDVI) | Chlorophyll |
Which crops does AgriSync check by satellite?
Feed wheat, milling wheat, feed barley, malting barley and oilseed rape. Grassland and other crops show on the map. But they don't get a satellite status yet. Each checked field is marked On track, Watch or Needs attention, with the date of the last clear satellite picture.
Common questions
Can satellites see my crop through cloud?
Not Sentinel-2. It works like a camera, so cloudy pictures can't be used. In a cloudy spell there may be no new reading for a while.
Is a high NDVI always good?
Not on its own. NDVI shows how green and dense the crop looks from above. Compare a field with itself over time, and with similar fields. Check anything unusual on the ground.
Why does a field show no satellite status in AgriSync?
There are three likely reasons. Its crop isn't set. Or the crop isn't one AgriSync checks yet (wheat, barley and oilseed rape are). Or there hasn't been a clear picture lately.
How detailed is a Sentinel-2 picture?
The bands used for NDVI are 10 m. So each pixel covers 100 square metres, and a hectare is 100 pixels. That shows patterns across a field, not single plants.
Sources
Every source was checked on 25 October 2026. If something has changed, tell us at info@agrisync.co.uk.
- 1.NDVI, the Foundation for Remote Sensing Phenology (opens in a new tab), U.S. Geological Survey
- 2.Landsat Normalized Difference Vegetation Index (opens in a new tab), U.S. Geological Survey
- 3.Sentinel-2 mission (opens in a new tab), Copernicus SentiWiki (European Space Agency)
Get your SFI records in one place
The AgriSync phone app and web app keep your fields, dated field photos and an SFI to-do list together. Free during early access.