MOD13Q1 v006

• The 16-day composite VI is generated using the two 8-day composite surface reflectance granules (MOD09A1) in the 16-day period.
• This surface reflectance input is based on the minimum blue compositing approach used to generate the 8-day surface reflectance product.
• The product format is consistent with the Version 5 product generated using the Level 2 gridded daily surface reflectance product.
• A frequently updated long-term global Climate Modeling Grid (CMG) Average Vegetation Index product database is used to fill the gaps in the CMG product suite.

Validation at stage 3 has been achieved for the MODIS Vegetation Index product suite. Further details regarding MODIS land product validation for the MOD13 data products are available from the MODIS Land Team Validation site.

The QA bit flags for the "250m 16 days pixel reliability" layer are provided in Table 4 on page 15 and the "250m 16 days VI Quality" in Table 5 on page 16 of the User Guide.

In areas where no valid VI is retrieved, VI Quality bits 11-13 are correctly assigned to indicate the land/water type. The other bits are set to 1 which can create a false notion of the parameter description; however, since no valid VI is retrieved these values can be ignored.

Quality assurance information should be considered when determining the usability of data for a particular science application. The ArcGIS MODIS-VIIRS Python Toolbox contains tools capable of decoding quality data layers while producing thematic quality raster files for each quality attribute.

In addition to data access and transformation processes, AppEEARS also has the capability to unpack and interpret the quality layers.

The "250m 16 days VI Quality" layer is stored in an efficient bit-encoded manner. The unpack_sds_bits executable from the LDOPE Tools is available to the user community to help parse and interpret the QC layers.

The following issues have been detected:

• Unexpected missing data in the last cycles of each year.
• Incorrect instances of "NoData" and spikes in NDVI values.
• VI Usefulness Bits are not correctly assigned.

For instances where the VI Quality (bits 0-1) is flagged as good and the VI Usefulness (bits 2-5) indicates the same pixels have the lowest usefulness score, users are advised to disregard the usefulness score.

The incorrect representation of the aerosol quantities (low, average, high) in the Collection 6 MOD09 surface reflectance products may have impacted MOD13 Vegetation Index data products particularly over arid bright surfaces.

Corrections were implemented in Collection 6.1 reprocessing.

For complete information about the MOD13Q1 known issues please refer to the MODIS Land Quality Assessment website.