| TSAVI Algorithm Specification | |
The Transformed Soil Adjusted Vegetation Index
algorithm was developed by Baret et al. (1989) and Baret and Guyot (1991)
This index assumes that the soil line has arbitrary slope and intercept, and it makes use of these values to adjust the vegetation index.
This would be a nice way of escaping the arbitrariness of the L in SAVI, if an additional adjustment parameter had not been included in the index.
The parameter X was adjusted so as to minimize the soil background effect. The value reported in the papers is 0.08.
The convergence point of the isovegetation lines lies between the origin and the usually-used SAVI convergence point (for L = 0.5).
The TSAVI results from the following equation:
TSAVI = s * (IR_factor * near_IR - s * red_factor * red - a) / (s * IR_factor * near_IR + red_factor * red - a * s + X * ( 1 + s * s ))
where:
- a is the soil line intercept
- s is the soil line slope
- X is the adjustment factor to minimize soil noise.
Not all soils are alike. Different soils have different reflectance spectra.
All of the vegetation indices assume that there is a soil line, where there is a single slope in RED-NIR space.
However, it is often the case that there are soils with different RED-NIR slopes in a single image.
Also, if the assumption about the isovegetation lines (parallel or intercepting at the origin) is not exactly right,
changes in soil moisture (which move along isovegetation lines) will give incorrect answers for the vegetation index.
The problem of soil noise is most acute when vegetation cover is low.
The following group of indices attempt to reduce soil noise by altering the behavior of the isovegetation lines.
All of them are ratio-based, and the way that they attempt to reduce soil noise is by shifting the place where the isovegetation lines meet.
WARNING: These indices reduce soil noise at the cost of decreasing the dynamic range of the index.
These indices are slightly less sensitive to changes in vegetation cover than NDVI (but more sensitive than PVI) at low levels of vegetation cover.
These indices are also more sensitive to atmospheric variations than NDVI (but less so than PVI). (See Qi et al. (1994) for comparisons.)
Also the processor computes an additional flags band called 'tsavi_flags' with the following bit coding:
| Bit Position | Description |
|---|---|
| Bit 0 | The computed value for TSAVI is NAN or is Infinite |
| Bit 1 | The computed value for TSAVI is less than -1 (minus one) |
| Bit 2 | The computed value for TSAVI is greater than 1 (one) |