The Problem: You Cannot Be Everywhere
As a farmer, you manage dozens or hundreds of hectares. Crops do not grow uniformly. Some zones in a field develop faster, others lag behind. Stress from drought, nutrient deficiency, disease, or compaction often appears in patches that are invisible from the field edge. By the time you notice a problem during a routine visit, it may have been building for weeks.
Walking every hectare regularly enough to catch problems early is not realistic. You need a way to see where your fields are performing differently, so you can focus your time where it matters most.
Satellite Imagery as a Screening Tool
This is where satellite-based vegetation monitoring comes in. The European Space Agency's Sentinel-2 satellites capture images of every field in Europe every five days, at a resolution of 10 meters per pixel, completely free of charge. From this data, vegetation indices can be calculated that highlight differences in crop vigor across your fields.
The most widely used index is NDVI: the Normalized Difference Vegetation Index. It has been in use since 1973, and organizations like NASA, USGS, and the USDA rely on it to monitor crop conditions worldwide. NDVI works by comparing how much red light your crop absorbs (for photosynthesis) with how much near-infrared light it reflects. Healthy, actively growing vegetation scores high. Bare soil, stressed plants, or sparse cover scores low.
An NDVI map gives you a quick overview of variation within and between your fields. Instead of guessing where to scout, you can see which zones deserve attention. Over a season, comparing NDVI maps from different dates lets you track how your crop develops and spot areas that fall behind.
What NDVI Cannot Do
Here is where honesty matters. NDVI tells you where to look, but never what is wrong. Oklahoma State University, the institution that developed the GreenSeeker NDVI sensor, puts it plainly: NDVI measures plant biomass but does nothing to tell you why the biomass differs from one zone to the next. A low reading could be drought, nitrogen shortage, disease, insect damage, or soil compaction. Without walking the field, you are guessing.
NDVI also has a saturation problem. Once your crop reaches full canopy cover, typically mid-season, the index stops showing meaningful differences. A stressed crop and a thriving crop can produce nearly identical readings because the red light band is fully absorbed in both cases. This is confirmed by NASA and by peer-reviewed research. NDVI becomes least useful precisely when you need the most detailed information.
Early in the season, the opposite happens. When vegetation cover is sparse, soil color and moisture dominate the signal, and readings become unreliable. Penn State Extension adds that even flowers, grain heads, and crop residues interfere with NDVI measurements.
And of course, optical satellites cannot see through clouds. In a cloudy growing season, Sentinel-2 may deliver only a handful of usable images. One European study recorded just seven clear observations in an entire season.
The Practical Solution: Combine Indices with Field Scouting
The answer is not to abandon NDVI but to use it for what it does well and complement it where it falls short. Sentinel-2 carries three red-edge bands that most other satellites lack. These enable NDRE (Normalized Difference Red Edge), an index that does not saturate in dense canopy and can detect nitrogen and chlorophyll stress one to two weeks before NDVI shows anything. When your crop closes canopy, switch from NDVI to NDRE.
For early season on bare soil, the Soil-Adjusted Vegetation Index (SAVI) compensates for soil reflectance. For cloudy periods, Sentinel-1 radar imagery can fill the gaps since radar penetrates clouds.
But no index, however sophisticated, replaces walking the field. Satellite data tells you where to look. Field scouting tells you what is actually happening. Recording both satellite observations and scouting findings in a platform like FarmDataViewer connects these data sources so you can track patterns over time and make better decisions season after season.
Where This Fits in FarmDataViewer
FarmDataViewer is built around the idea that satellite data and field observations belong together. In the web application, you view your fields on interactive maps with satellite imagery as the background layer. Public data layers such as WMS, WMTS, and PMTiles can be overlaid to add context from external sources, including vegetation index layers where available.
When an NDVI or NDRE map highlights a zone that needs attention, you can record what you find during your field scouting visit directly in the mobile app. Observations are geo-located, linked to the field and crop, and can include photos. Over time, this builds a history that connects what the satellite showed with what was actually happening on the ground, helping you recognise patterns and respond faster in future seasons.
When Not to Rely on NDVI
| Situation | Why NDVI Fails | Better Alternative |
|---|---|---|
| Dense canopy (mid/late season) | Saturates — cannot distinguish healthy from very healthy | NDRE |
| Early growth on bare soil | Soil reflectance distorts readings | SAVI |
| Diagnosing a specific problem | Shows that something is wrong, not what | Field scouting |
| Cloudy weather | Optical sensors cannot see through clouds | Sentinel-1 radar |
| Nitrogen management | Cannot isolate N deficiency from other stress | NDRE + soil sampling |
| Weed-infested areas | Weeds are green too — score appears healthy | High-resolution imagery |
Common Questions
What is NDVI?
NDVI (Normalized Difference Vegetation Index) compares near-infrared and red light reflected by plants. Healthy vegetation absorbs red light and reflects near-infrared, producing a value between 0 and 1.
Can NDVI tell me what is wrong with my crop?
No. NDVI indicates where variation exists, but not the cause. Field scouting is needed to determine whether the issue is drought, disease, nutrients, pests, or soil-related.
What is the difference between NDVI and NDRE?
NDVI uses red light, which saturates when the crop canopy is dense. NDRE uses red-edge light that continues to show variation in mid-to-late season crops.
Is Sentinel-2 NDVI data free?
Yes. Sentinel-2 data is freely available under the EU Copernicus open data policy, with 10-meter resolution and a 5-day revisit time.
Conclusion
NDVI is a proven, free, and simple screening tool that helps you find where your fields need attention. It has earned its place in agriculture over fifty years of use. But it is not a diagnostic tool and it is not a silver bullet. The farmers who benefit most are the ones who use NDVI to decide where to scout, switch to NDRE when the canopy closes, and record everything in a system that builds knowledge over time.
Contact us to learn how FarmDataViewer helps you combine satellite monitoring with field observations in a single workflow.