See the effects of soil color...

NDVI images are plagued by issues with uneven soil color and tend to overstate vegetation density and vigor for areas with soils that are relatively darker. These dark areas are commonly caused by varying soil wetness, texture, or organic matter content.

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Notice the difference in soil color due to varying wetness between these two bare soil fields in the natural color image to the right. In both the NDVI and C3 Vegetation Index images below, shown in black & white, we would expect the indices to be lighter where there is more vegetation and darker where there is less vegetation.

 

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The median NDVI value for each block is shown along with the magnitudes of the NDVI value errors (expected NDVI value for bare soil is 115). The NDVI values for the block on the right indicates the presence of vegetation when there was no vegetation present. Note that variation in soil color causes errors in NDVI values even for cases where the amount of vegetation is moderate to high.

 

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With the calibrated C3 Vegetation Index we would expect fields with no vegetation to have a value of 100. Despite the uneven soil moisture and color, the C3 map accurately reveals the true condition of these blocks - they are all just bare soils.