NDVI: Broken down
What is NDVI?
NDVI stands for normalized difference vegetation index
To put it simply NDVI is a standard way of measuring how healthy vegetation or plant crop is. This is done by measuring the difference between near infrared and red light.
So how does NDVI actually work?
NDVI works because when sunlight reaches a plant, certain wave lengths are absorbed while the rest are reflected. Classically healthy vegetation (chlorophyll) reflects more NIR (near infrared), but it absorbs more red and blue light. This is why when a human eye looks at vegetation it appears green to them. When a plant becomes dehydrated or stressed the spongy layer of the plant collapses, and its leaves reflect less NIR, yet they still reflect the same amount of light in the visible range.
NDVI ranges from -1 to +1, so for example if you seem to have a negative value it’s more likely to be water, if your NDVI measures close to +1 this indicates dense green leaves. When it hits zero you are likely to be looking at an urbanised area. The following table below shows some examples of the values on the scale might represent:
-1 – <0
Negative values are mainly formed from inanimate dead material such as soil/roads.
Unhealthy plant material
Barren areas of rock
0.2 – 0.3
Shrub and grassland
0.4 – .5
0.6 – 0.8
0.9 – 1+
Crops at their peak growth stage
Satellites can measure red and infrared waves reflected by land surfaces, using algorithms scientists then use these figures to calculate vegetation indices. There is an equation which was developed to make use of the satellite imagery which can be seen below:
The algorithm allows you to define between two indistinguishable areas of vegetation which may have different NDVI values, due to one being in bright light and another in more cloudy conditions.
So to summarise when you have high NDVI values, you have healthier vegetation.
When you have low NDVI, you have less or no vegetation
Generally, if you want to see vegetation change over time, then you will have to perform atmospheric correction.
How do we use NDVI?
Okay, so now we know how NDVI works, but ‘what do I want to use NDVI’?
In agriculture, farmers use NDVI to measure biomass, whereas in forestry it can be used to quantify forest supply and leaf area index. Additionally NASA has found that NDVI is a strong indicator of drought – because when water limits growth it has a relatively low NDVI value.
Drone mapping makes it easier for farmer and agronomists by creating maps that transform the -1 to 1 scale into colours, which can be interpreted quicker and then evaluated more efficiently. What happens is that NDVI values are mapped to a set of colours – these can vary and there isn’t a set colour used.
In the last 40 years or so, advances in technology have allowed for the incorporation of high-quality NDVI images onto manned aircraft and this is where we step in…