I am trying to compare NDVI values computed from MODIS Surface Reflectance(MOD09) with the NDVI product (MOD13).
MODIS Surface reflectance Collection 5 products include a Data State QA field that I am using to estimate which pixels are best suited for computing NDVI.
A part of this field is labeled Aerosol quantity. The values in this field seem to significantly affect the result of NDVI computation.
The explanation of the possible values of this field in the MOD09 Users' Guide seems not entirely clear to me.
| 00 - climatology aerosol | 01 - low quantity | 10 - average | 11 - high
The last three values are clear, while the meaning of the climatology value is somewhat non-intuitive.
I have selected a crop field and calculated NDVI for all pixels clear of clouds or cloud shadows during one vegetation period and observed the following:
It seems that results with low aerosol quantity (green points) match well with the results obtained with the complex rating algorithm used in the standard NDVI MOD13Q1 product. While the data with climatology state (orange points) seems far off.
What does the climatology attribute mean and how to treat it?