Aqua MODIS NDVI data from the MYD13A3 product over part of Brazil, July 2018.View full-size image
The Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Indices (MYD13A3) Version 6 data are provided monthly at 1 kilometer (km) spatial resolution as a gridded Level 3 product in the sinusoidal projection. In generating this monthly product, the algorithm ingests all the MYD13A2 products that overlap the month and employs a weighted temporal average.
The MODIS Normalized Difference Vegetation Index (NDVI) complements NOAA's Advanced Very High Resolution Radiometer (AVHRR) NDVI products and provides continuity for time series historical applications. MODIS also includes an Enhanced Vegetation Index (EVI) that minimizes canopy background variations and maintains sensitivity over dense vegetation conditions. The EVI uses the blue band to remove residual atmosphere contamination caused by smoke and sub-pixel thin clouds. The MODIS NDVI and EVI products are computed from surface reflectances corrected for molecular scattering, ozone absorption, and aerosols.
Vegetation indices are used for global monitoring of vegetation conditions and are used in products displaying land cover and land cover changes. These data may be used as input for modeling global biogeochemical and hydrologic processes as well as global and regional climate. Additional applications include characterizing land surface biophysical properties and processes, such as primary production and land cover conversion.
Provided along with the vegetation layers and the two quality assurance (QA) layers are reflectance bands 1 (red), 2 (near-infrared), 3 (blue), and 7 (mid-infrared), as well as three observation layers.
|File Size||~8.29 MB|
|Temporal Extent||2002-07-01 to Present|
|Geographic Dimensions||1200 km x 1200 km|
|Number of Science Dataset (SDS) Layers||11|
|Columns/Rows||1200 x 1200|
|Pixel Size||1000 m|
|SDS Name||Description||Units||Data Type||Fill Value||No Data Value||Valid Range||Scale Factor|
|1 km monthly NDVI||1 km monthly NDVI||NDVI||16-bit signed integer||-3000||N/A||-2000 to 10000||0.0001|
|1 km monthly EVI||1 km monthly EVI||EVI||16-bit signed integer||-3000||N/A||-2000 to 10000||0.0001|
|1 km monthly VI Quality||VI quality indicators||Bit Field||16-bit unsigned integer||65535||N/A||0 to 65534||N/A|
|1 km monthly red reflectance||Surface Reflectance Band 1||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|1 km monthly NIR reflectance||Surface Reflectance Band 2||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|1 km monthly blue reflectance||Surface Reflectance Band 3||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|1 km monthly MIR reflectance||Surface Reflectance Band 7||N/A||16-bit signed integer||-1000||N/A||0 to 10000||0.0001|
|1 km monthly view zenith angle||View zenith angle of VI Pixel||Degree||16-bit signed integer||-10000||N/A||0 to 18000||0.01|
|1 km monthly sun zenith angle||Sun zenith angle of VI pixel||Degree||16-bit signed integer||-10000||N/A||0 to 18000||0.01|
|1 km monthly relative azimuth angle||Relative azimuth angle of VI pixel||Degree||16-bit signed integer||-4000||N/A||-18000 to 18000||0.01|
|1 km monthly 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 "1 km monthly pixel reliability" layer are provided in Table 4 on page 15 and the "1 km monthly 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 "1 km monthly 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 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 MYD13A3 known issues please refer to the MODIS Land Quality Assessment website.