Vegetation Indices : What, How and When?

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February 7, 2021

Many satellite insights beginners may have not heard of Vegetation Indices. What are they? How to best use them and when? We wrote this article below to clarify these questions!

What are Vegetation Indices?

Vegetation Indices (VIs) are combinations of two or more reflectance bands from satellite images.

Okay... but what are reflectance bands?

Well, first, you have to understand how the human eye perceives colors. Why oceans are blue, leafs are green... In fact, everything on Earth reflects back radiation (like light energy) at different wavelengths. Some of these wavelengths we can see with the human eye in the form of colors. Other wavelengths can be viewed by specific animals, cameras or special sensors. Satellites house these sensors and capture the specific wavelengths via reflectance bands.

Let’s take plant health insights for example. The graphic below illustrates which reflectance band on its own provides relevant insights to specific aspects of a plant depending on the wavelength.

Example of different reflectance bands available at varying wavelengths. Top row depicts the corresponding plant health insights.

Bands in the 1st zone are in the visible wavelength spectrum (RGB: red, green and blue) and provide information on leaf pigmentation, which is useful to determine the growing stage of the crop, and a basic indicator of crop health. During photosynthesis, plants rely on the green pigment chlorophyll to convert energy from the sun for fuel.

The 2nd zone near-infrared (NIR) wavelengths are a great way to detect healthy growing plants through chlorophyll detection, representing plant productivity more directly.

Bands in shortwave infrared 3rd zone provide information on the water content with the plants as well as biochemical components in leaves.

The Vegetation Indices superstar: NDVI

For over 30 years, the NDVI (Normalized Difference Vegetation Index) has been the reference index in agriculture. It combines visible and near infrared bands to provide information on canopy density and greenness.

NDVI is a great starting point to understand crop growth during a growing season, thus, the ‘go-to’ VI for many agricultural users. Older satellites have recorded NDVI at 30 meters resolution, and more recently at 10 meters. In recent years, we’ve witnessed a race to provide NDVI maps at the highest resolution possible, as if that was the ultimate solution. However, it should not be this way. Yes, higher resolution provides more precise insights. But instead of trying to see more by zooming in with the same index, why not change the measuring tool? Depending on your application, other indexes may be a better fit.

Let’s take a look at some different VIs.

A wide choice of Vegetation Indices

Below you can find a selection of VIs that we find quite relevant for today's agricuture. We grouped them into two categories: Crop Activity and Crop Productivity.

Category 1.Crop Activity Vegetation Indices

The following indices provide information on the current physical state of plants including coloration, density and water content.

  1. SAVI: Soil Adjusted Vegetation Index. Think of it as NDVI with a soil correction factor. Useful for crops with more visible soils (orchards, wineries) or for early growing stages.
  2. EVI : Enhanced Vegetation Index. For crops with high canopy density, NDVI tends to saturate. EVI is more robust to use for these types of crops or growing stages.
  3. LAI: Leaf Area Index. A measurement of the area of leaves per unit of land. Besides being able to obviously track the growth of a plant, LAI is also linked to plant productivity as leaves house the critical chlorophyll pigments for photosynthesis. LAI is commonly used for yield predictions.
  4. NDWI: Normalized Difference Water Index. Measures the relative water content in leaves. Provides a very clear indicator of the level of water stress in the crop

Category 2. Crop Productivity Vegetation Indices

While crop coloration and density is certainly linked to productivity, the following indices are better suited to estimate yields and provide insights on biochemical states of the plant.

  1. CHI: Chlorophyll Index. Measures the level of chlorophyll in the plant, indicative of nutrient requirements and yield.
  2. NDRE: Normalized Difference Red Edge. The best solution to monitor the nitrogen stress in the crop, and create prescription maps.
  3. NIRv : Near Infrared of Vegetation. Index which further isolates the vegetation signal from the satellite image and most related with plant productivity.

How & when to use Vegetation Indices?

From the list of nearly 250 available indices from Sentinel-2 alone (the most used earth observation satellite in Europe), over 130 can be useful for vegetation monitoring. So which index is the best for your client’s farm? Well, that depends on a number of questions including:

  1. What is the crop?
  2. What is the current growing stage?
  3. Is it even doable with indices?
  4. And most importantly, what specifically about the crop are you trying to understand?

Selecting the most appropriate solution for your application can easily become a daunting task.

At SpaceSense, we understand the intricacies of choosing the ‘right’ solution, therefore we made it our mission to guide you in unlocking the potential of satellite imagery, to enhance your solutions.

From very simple VIs to advanced AI models customised to your fields, we provide you with the best insights based on your needs and your satellite maturity.

Want to try it yourself? We have created an offer just for you!

With our Space Agriculture Starter Pack, you get access to the Vegetation Indices we just presented above, for free, up to 20 000 hectares! Reach out us to get started.

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