Why do I still get 0 values in ENVI's VI calculator when biophysical cross checking is turned off?
When using the ENVI Vegetation Index calculator, you have the option to use biophysical cross checking. Biophysical cross-checking compares indices against each other to validate their results. If conflicting values exist between indices, those data values are ignored. When biophysical cross checking is turned off, masking is not applied and ENVI stores all output data in the result. However, the results from the VI calculator might still contain '0' values. Why do 0 values still occur?
In addition to the biophysical cross check, ENVI's VI calculator has what is called the "range mask". The range masking, which cannot be disabled, automatically eliminates values that are not meaningful for specific indices. For example, if an NDVI value were to come out higher than 1 or less than -1, it would be masked because that is an invalid value for the NDVI index.
When values are masked due to this range mask, it could mean:
- poor atmospheric correction
- the use of radiance values as input
- an incorrect setting of the reflectance scale factor
It would be best to check the data for these items and correct accordingly.
Review on 12/31/2013 MM