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Could anyone explain why the vegetation indices are not working in the desert region?

I used Landsat 8 image which has been already radiometric/atmospheric corrected.

I want to see the fractional vegetation cover in super arid environment in part of middle east country. Most of the area is covered by sand sheet, but there has a few trees and I wish I could extract them.

So far, NDVI, SAVI, and EVI seems not working well. NDVI and SAVI output are quite similar, and somehow both have high value in some place which do not have any vegetation at all (blue circles in the image below). In contrast, the area contain a few trees has lower value (red circle). Only the value seems matching with real land cover types is agricultural area which shows in red colour.

The SAVI image below is displayed as a rainbow scale, from low (-1): dark blue to high (+1): red.

True colour landsat image and SAVI output

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The sensitivity of the normalized difference vegetation index (NDVI) to the soil background and atmospheric effects has generated an increasing interest in the development of new indices. The NDVI index is saturated in high biomass and it is sensitive to a number of perturbing factors, such as atmospheric effects, cloud, soil effects, and anisotropic effects etc.

The modified soil adjusted vegetation index (MSAVI) and its later revision, MSAVI2, are soil adjusted vegetation indices that seek to address some of the limitations of NDVI when applied to areas with a high degree of exposed soil surface because of the reflectance of light in the red and near-infrared (NIR) spectra can influence vegetation index values.

The soil adjusted vegetation index (SAVI) was developed as a modification of the NDVI to correct for the influence of soil brightness when vegetative cover is low. The problem with the SAVI is that it required specifying the soil brightness correction factor L through trial and error based on the amount of vegetation.

The problem with the original soil adjusted vegetation index (SAVI) is that it required specifying the soil brightness correction factor L that ranges from 0, for very high vegetation cover, to 1 for very low vegetation cover. Most researchers use 0.5 for L, which is for intermediate vegetation cover.L to 0 makes SAVI equivalent to NDVI (Huete, 1988)

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    Hi, thank you so much for your helps. I tried MSAVI 2 today. The result is still same, higher value in no vegetation area and lower value in scattered vegetation area. Even agricultural area became lower value although it suppose to be higher veg density. I guess the soil (=sand) is too dry in this place, therefore the NIR reflectance became as high as OR higher than scattered vegetation? – Kyoko Sep 6 '18 at 5:48
  • In more humid regions, senescent vegetation tends to be brighter than live vegetation, and dry soils are brighter than wet soils maybe that's why.NIR reflectance seems to be a better answer in this aspect. I will look more into this or will try to find other indices for a better analysis. – Damini Jain Sep 6 '18 at 15:30
  • I really appreciate it, Damini. I am still looking for the solution for this but if I could find any, I will let you know. Thanks again. – Kyoko Sep 7 '18 at 8:20

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