How to Read an NDVI Map: Values, Colors & Scale
What an NDVI map shows you
An NDVI map is a satellite image of your field where every pixel has been converted from raw light into a single number — its Normalized Difference Vegetation Index — and then that number has been colored. The point of the color is simple: it lets your eye spot variation across the field in a glance. A uniform green field is healthy; patches of yellow or red inside a green field are where you need to walk.
The map does not tell you what is wrong (drought, disease, nutrient lack, pests all look similar). It tells you where and when — which is most of the value, because you cannot fix what you cannot locate.
The NDVI scale, explained
NDVI always falls between −1 and +1. Almost all agricultural land sits between 0 and 0.9. Here is what each band means:
| NDVI range | Color (typical ramp) | What it means | Typical for |
|---|---|---|---|
| 0.7 – 0.9 | Dark green | Very dense, vigorous vegetation | Peak-season crops, mature forest, closed canopy |
| 0.5 – 0.7 | Light green | Healthy, productive canopy | Well-developed crops, full-canopy vineyards |
| 0.3 – 0.5 | Yellow-green | Moderate / developing | Mid-growth, establishing crop, orchards |
| 0.1 – 0.3 | Orange / tan | Sparse or stressed | Early growth, drought stress, poor patches |
| −0.1 – 0.1 | Brown | Bare soil or dead vegetation | Plowed field, harvested, senesced |
| Below −0.1 | Blue / black | Water, snow, or cloud | Ponds, rivers, snow, dense cloud shadow |
Different tools use different color ramps (some go red-to-green, some brown-to-green, some use rainbow). The number is always the same; the color is just a legend applied on top. Always check the legend, because a “green” patch on a rainbow ramp is mid-range, not healthy.
How to actually read the map
Reading an NDVI map is about pattern and change, not absolute numbers:
- Look for variation, not averages. A field that is uniformly 0.4 might be perfectly fine. A field that averages 0.6 but has a 0.3 patch in the corner has a problem in that corner. Variation is the signal.
- Compare against time. One map tells you the current state. Two maps weeks apart tell you the trend. A patch dropping from 0.7 to 0.5 in two weeks is an alarm; a patch sitting at 0.5 for two months is probably just that soil type.
- Match the growth stage. A 0.3 reading is concerning in July but completely normal in April for a newly emerged crop. Always read the number against what is expected for the crop and date.
Spotting common problems
| What you see | Likely cause | Next step |
|---|---|---|
| Large green field with a yellow patch | Localised stress (compaction, drainage, pest, disease) | Walk the patch |
| Whole field dropping faster than neighbours | Drought, nutrient deficiency, systemic stress | Check NDMI + NDRE |
| Stripes or bands across the field | Application error, planter skip, irrigation failure | Check equipment logs |
| Persistent brown in a “green” season | Failed establishment, hail, herbicide damage | Ground-truth immediately |
| Blue/black patches | Waterlogging, ponding, or cloud contamination | Re-check next cloud-free image |
The limits of NDVI maps
NDVI saturates over dense canopy — above roughly 0.8 it stops telling you much, because all healthy dense vegetation looks the same. For mid-to-late season nitrogen questions, switch to an NDRE map, which keeps working after NDVI goes flat. For water questions, use NDMI. NDVI is the starting point; it is rarely the only map you need.
Clouds are the other limit. A sudden brown patch in a green field is often just cloud shadow, not dead crop. A good report filters cloud-contaminated pixels and tells you when the data is unreliable.
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Frequently asked questions
What do the colors on an NDVI map mean?
Colors are a legend applied to the NDVI number. In the most common ramp, dark green means dense healthy vegetation (high NDVI), yellow means moderate or stressed, and brown means bare soil or dead vegetation. Blue or black usually means water. Always check the legend for the specific tool you are using, because ramps differ.
What is a good NDVI value for crops?
For most annual crops at peak growth, 0.6 to 0.8 indicates a healthy dense canopy. Vineyards and orchards run lower (0.4–0.6 is normal) because rows and management keep the canopy open. More important than any single value is the trend and the spatial pattern — compare against your own field’s history.
Why is part of my field brown on the NDVI map?
Brown (low NDVI) usually means bare soil, dead or senescent vegetation, or a failed patch. But it can also mean cloud shadow or standing water. If the brown patch appears suddenly, re-check the next cloud-free image before assuming crop loss — cloud contamination is the most common false alarm.
How often does the NDVI map update?
Sentinel-2 satellites photograph every location on Earth every five days, weather permitting. Clouds can block the view for longer, which is why good reports use cloud-free composites and show trends over several images rather than trusting a single cloudy snapshot.