Agricultural EO Algorithms: A Guide

Back when satellite imagery only came in thirty metre square blocks and seven spectral bands, the Normalised Difference Vegetation Index – or ‘NDVI’ as every time-starved remote sensing student will tell you – was all you needed to monitor the health or otherwise of agricultural products.

Today, the number of agricultural algorithms that can be derived from the ever-expanding number of remote sensing sources, has grown as prolifically as the crops that they have been used to monitor.

To keep abreast of the earth observation developments in the Agriculture industry, we thought we would explore the names and uses of a few of the more common ones:

NDVI

Photosynthesis allows plants to convert the sun’s energy into chemical energy, and hence to grow.

So, plants absorb the wavelengths of sunlight that make this process happen (red light) and reflect those that don’t (near infra-red light).

The NDVI equals (infra-red light — red)/(infra-red light + red): a convoluted expression that provides a number between minus one and one. Zero means that there is no vegetation and one, a lush green field.

Slightly suprisingly, minus one tells us that the area is covered in water.

FCover

The Fraction of green vegetation Cover or FCover describes itself perfectly: i.e. how much of the area of interest is covered in green vegetatation.

Whilst its name is staggeringly prosaic, FCover percentages provide insight into the rate of crop development and how vigourously it is growing

The Leaf Area Index - LAI

The LAI is again a number without units, but in this case its value describes how much green leaf area there is any given square metre of a canopy of trees or crops.

In its simplest terms, the Leaf Area Index reflects the structure of the crop canopy which makes it an extremely useful parameter for predicting the growth and productivity of the crop.

Fraction of Absorbed Photosynthetically Active Radiation - FAPAR

As we learnt earlier, plants only use certain wavelengths of sunlight to perform photosynthesises – the FAPAR describes how much of this life-giving light the plant is using.

So, unlike the LAI which tells us how much foliage is present, the FAPAR tells us how much of the foliage is ‘working well’ and points at how to improve yields.

Many other indexes exist; Vegetation Condition, Vegetation Production, Soil Water etc. and believe us, etc. but hopefully this short blog post gives you a flavour of how imaging from above, be it from planes, satellites or drones, is helping farmers better understand the health of their crops.