I am trying to understand why certain vegetation indices do not fall into the -1, 1 theoretical range.
I generated Modified Soil Adjusted Vegetation Index II values in Google Earth Engine using surface reflectance images. My MSAVI II values are all below 1, but can drop down to -7 (-288 in one extreme case). When I calculate the zonal statistics for a given polygon in QGIS 2.18, the extreme negative values impact the mean.
Based on a response by Jeffery Evans (Soil Adjusted Vegetation Index Values (MSAVI2) are Not Within Expected Range), I understand that MSAVI II is one of the slope corrected indices that will have values greater than 1 or less than -1; however, I cannot find information with a full explanation even after reading the two cited articles.
Could anyone provide documentation that explains this phenomenon, or suggestions on how to correct this issue? If negative values are theoretically acceptable, do I just have to ignore questionable means and go with the median?