Aqua MODIS NDVI data from the MYD13A1 product over part of Brazil, August 5 - 19, 2018.View full-size image
The MYD13A1 Version 6 product provides Vegetation Index (VI) values at a per pixel basis at 500 meter (m) spatial resolution. There are two primary vegetation layers. The first is the Normalized Difference Vegetation Index (NDVI), which is referred to as the continuity index to the existing National Oceanic and Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR) derived NDVI. The second vegetation layer is the Enhanced Vegetation Index (EVI), which has improved sensitivity over high biomass regions. The algorithm for this product chooses the best available pixel value from all the acquisitions from the 16 day period. The criteria used is low clouds, low view angle, and the highest NDVI/EVI value.
Provided along with the vegetation layers and two quality assurance (QA) layers are reflectance bands 1 (red), 2 (near-infrared), 3 (blue), and 7 (mid-infrared), as well as four observation layers.
|File Size||~28.49 MB|
|Temporal Extent||2002-07-04 to Present|
|Geographic Dimensions||1200 km x 1200 km|
|Number of Science Dataset (SDS) Layers||12|
|Columns/Rows||2400 x 2400|
|Pixel Size||500 m|
|SDS Name||Description||Units||Data Type||Fill Value||No Data Value||Valid Range||Scale Factor|
|500m 16 days NDVI||500m 16 days NDVI||NDVI||16-bit signed integer||-3000||N/A||-2000 to 10000||0.0001|
|500m 16 days EVI||500m 16 days EVI||EVI||16-bit signed integer||-3000||N/A||-2000 to 10000||0.0001|
|500m 16 days VI Quality||VI quality indicators||Bit Field||16-bit unsigned integer||65535||N/A||0 to 65534||N/A|
|500m 16 days red reflectance||Surface Reflectance Band 1||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|500m 16 days NIR reflectance||Surface Reflectance Band 2||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|500m 16 days blue reflectance||Surface Reflectance Band 3||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|500m 16 days MIR reflectance||Surface Reflectance Band 7||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|500m 16 days view zenith angle||View zenith angle of VI Pixel||Degree||16-bit signed integer||-10000||N/A||0 to 18000||0.01|
|500m 16 days sun zenith angle||Sun zenith angle of VI pixel||Degree||16-bit signed integer||-10000||N/A||0 to 18000||0.01|
|500m 16 days relative azimuth angle||Relative azimuth angle of VI pixel||Degree||16-bit signed integer||-4000||N/A||-18000 to 18000||0.01|
|500m 16 days composite day of the year||Day of year VI pixel||Julian day||16-bit signed integer||-1||N/A||1 to 366||N/A|
|500m 16 days pixel reliability||Quality reliability of VI pixel||Rank||8-bit signed integer||-1||N/A||0 to 3||N/A|
The QA bit flags for the "500m 16 days pixel reliability" layer are provided in Table 4 on page 15 and the "500m 16 days VI Quality" layer 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 "500m 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 quality layer.
The following issues have been detected:
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 MYD09 surface reflectance products may have impacted MYD13 Vegetation Index data products particularly over arid bright surfaces.
Corrections will be implemented in Collection 6.1 reprocessing in 2020.
For complete information about the MYD13A1 known issues please refer to the MODIS Land Quality Assessment website.