The "Transformed Soil Adjusted Vegetation Index" (TSAVI), which is a transformation of the SAVI, was defined by Baret and collaborators (1989).
TSAVI is mostly used to describe both the change in the vegetative cover and the soil background and gives good result for low vegetation cover.
The first version of the TSAVI (Baret et al., 1989) was readjusted to minimize the effects of soil brightness by adding an "X" value equal to 0.08. This new improved version of the TSAVI is the following:
TSAVI = b(NIR-b.RED-a)/(RED+b(NIR-a)+X(1+b^2))
where a is the slope of bare soil line and b is the ordinate at the origin of the bare soil line.
This formula displays the dependency of TSAVI on : NIR (Near Infrared) and RED spectral ranges with constants a,b and X.
The semantic model gives explicit information to calculate a spectral index and TSAVI is one of the instances. The model decribes the dependencies of TSAVI which includes the best fit wavelength (RED : 0.6-0.7μm; NIR: 0.75-1.35 μm), the constants a and b which are based on the type sensor/platform used and the value of X which is always the same as given above. The model also lists some tasks related to TSAVI which are linked to a theme or subdomain as displayed here.