Soil-Adjusted Vegetation Index

A transformation technique to minimize soil brightness from spectral vegetation indices involving red and near-infrared (NIR) wave lengths.

The transformation involves a shifting of the origin of reflectance spectra plotted in NIR-red wavelength space to account for first-order soil-vegetation interactions and differential red and NIR flux extinction through vegetated canopies.

Near InfraRed (NIR)Multispectral

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In general, most vegetation indices rely in the existence of a “soil line” in red and NIR wavelength space, i.e., there is a principal axis of soil spectral variation extending outward from the origin with increasing brightness.

Since most of the soil spectra fall on or close o the soil line, and since the intercept of such a line is close to the origin, RVI and NDVI values of bare soils (ratios) will be nearly identical for a wide range in soil conditions.

Source: Huete, A.R. 1988

Figure 2

Source: Huete, A.R. 1988

1

2

lred

lNIR

12

12

lredlNIR

lredlNIR

Since the soil line has slope close to 1, the adjustment factors, l1 and l2, would be nearly equivalent. Shifting the red and NIR data equally (l1=l2) and utilizing the NDVI format

LredNIR

redNIR

LLredNIR

redNIRSAVI

1

Where L=l1+l2=2l. Thus a soil adjustment index(SAVI) would only involve an addition of a constant, L, to the denominator of the NDVI equation.

However, in order to maintain the bounded cinditions ot the NDVI equation (NDVI can vary from –1 to +1), a multiplication factor (1+L) is needed in eq. 3

Figure 3

Source: Huete, A.R. 1988

Figure 4

Source: Huete, A.R. 1988

Figure 5

Source: Huete, A.R. 1988

2

81212 5.02 redNIRNIRNIRMSAVI

MODIFIED-SAVI

Reference

• Huete, A.R. 1988. “A soil-Adjusted Vegetation Index (SAVI)”, Remote Sensing of Environment, 25:295-309

•Huete, A.R., Lui, H.Q. 1994. “An Error and Sensitivity Analysis of the Atmospheric and Soil- Correctin Variants of the NDVI for he MODIS-EOS.” IEEE Transactions on Geoscience and Remote Sensing, 32(4), 897-905