Reading NDVI without overreacting to one image

A satellite NDVI map of your paddock is one of the most useful tools agriculture has produced. It is also one of the easiest to misread. The trick is knowing exactly what it measures, and treating it as a question rather than an answer.

What NDVI is

NDVI, the Normalised Difference Vegetation Index, compares how much red light and near-infrared light a crop reflects. Healthy, actively growing plants absorb red light and reflect near-infrared strongly, so they score high. Bare soil, stressed plants and stubble score low.

The index runs from about minus one to one. In practice a vigorous broadacre crop tends to read somewhere around 0.6 to 0.9, while thin or struggling areas read much lower. Sentinel-2, the satellite behind most free imagery, revisits roughly every five days at 10-metre resolution.

What it is good at

NDVI shines as a prioritiser. It shows you the pattern of variation across a paddock and how that pattern changes over time. That makes it ideal for a few jobs.

• Deciding which paddocks and which zones to walk first.
• Spotting a problem developing before it is obvious from the ute.
• Comparing this season's vigour against the same week last year.

NDVI is a map of where to point your boots, not a diagnosis of what you'll find when you get there.

What it can't tell you

A low patch on an NDVI map is a symptom with many possible causes: a soil type change, waterlogging, nutrition, frost, disease or pest pressure. The index cannot tell them apart. It also can't see through cloud, can be thrown by the timing of a pass, and says nothing about grain quality or final yield.

Using it well

Use NDVI to decide where to look and what to watch over time, then ground-truth before you act. Treated as a scouting layer it sharpens every paddock walk. Treated as a verdict it will eventually send you spending money on the wrong problem.