SAVI is the acronym for Soil Adjusted Vegetation Index that, as the name suggests, it is one of the many indices for the calculation of the vegetation index.
The general formula for the SAVI index is (indexdatabase.de):
800 nm - 670 nm
________________________ ( 1 + L)
800 nm + 670 nm + L
where L is between -0.9 and 1.6.
In the case of Sentinel 2 data, SAVI uses two bands, B8 and B4, that have the maximum available resolution for Sentinel 2 images, i.e., 10 meters, so the results have a quite high spatial resolution.
B8 is in the NIR (842 nm) while B4 registers the red-NIR transition (665 nm).
For Sentinel 2, the index is therefore calculated as:
B8 - B4
_______________ ( 1 + L)
B8 + B4 + L
The default value for L in indexdatabase is 0.5, and in fact testing for a few values (-0.9, -0.5, 0.0, 0.5, 1.0, 1.5) in the prescribed range, the most contrasted results are obtained for L = 0.5.
One information that is obtained from applying such an index to intensive farming landscapes is that it allows to delineate well the agricultural field limits, to compare the texture and the "spatial styles" of farming across different areas, for instances separated by rivers that may have acted as administrative or political borders.
In the following images, I present some examples of textures and styles of limits between fields for different areas in the Piedmont region, derived from a Sentinel 2B image, acquired on 2023/04/04 at 10:25. The SAVI index was calculated using the SNAP software by ESA.
The used L ("soil brightness correction factor") value is 0.5, chosen after some experimentation.
The figures below are generated from within SNAP, using a 8-4-3 composite for infrared view (upper window), while the SAVI index band is represented in the lower window. In the 843 composite, vegetation is red while rivers and roads are black.
