Getting Started with MODIS Version 6 Vegetation Indices Data Part 3
This video focuses on the National Aeronautics and Space Administration’s (NASA) Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Version 6 Vegetation Indices data distributed by NASA’s Land Processes Distributed Active Archive Center (LP DAAC). Information about MODIS Vegetation Indices quality information, including how to decode quality bits, tools for working with quality data, and where to find additional information, will be provided. To learn more about MODIS Version 6 Vegetation Indices data and other data products distributed by the LP DAAC please visit https://lpdaac.usgs.gov/.
The LP DAAC is one of twelve NASA Earth Observing System Data and Information System (EOSDIS) DAACs. It is located at the United States Geological Survey (USGS) Earth Resources Observation Systems (EROS) Center in Sioux Falls, South Dakota. The LP DAAC ingests, archives, processes and distributes NASA Earth science land processes data and information.
Location Taken: Sioux Falls, SD, US
Video Credits: Content created by Danielle Golon (Contractor to USGS EROS)
Getting Started with MODIS Version 6 Vegetation Indices Data Part 3: Interpreting Quality Information. Presented by the Land Processes Distributed Active Archive Center or LP DAAC. This is part 3 of the “Getting Started with MODIS Version 6 Vegetation Indices Data” video series. This video will cover the Quality information associated with Vegetation Indices products. We will discuss the information the quality layers contain, tools that can be used to expose the quality information for further use, and where to find out more information on the LP DAAC’s website. Each Vegetation Indices product is distributed as a Hierarchical Data Format or HDF file. Each MODIS HDF file contains associated Quality science dataset layers, which provide users with information regarding the usability and usefulness of the data products. Here is an example of the science dataset layers for the Terra MODIS Version 6 16-day Vegetation Indices 500 meter level 3 gridded product, MOD13A1.The information extracted from these quality science datasets can be used to filter data based on different levels of quality. There are two Quality Assurance or QA science dataset layers that can come with the MODIS Version 6 Vegetation Indices data products. The first is the Vegetation Indices or VI --Quality layer, which provides pixel-level quality assignment information. The other is the Pixel Reliability, which provides simple ranks that capture the overall pixel quality. The Pixel Reliability can be directly interpreted and requires no detailed post-processing and interpretation. More detailed information about specific QA layers for each product can be found in the product User Guide and Algorithm Theoretical Basis Document or ATBD. These documents are available in the “Documentation” section of each Digital Object Identifier or DOI Landing Page. The User Guide provides a more in-depth and up-to-date explanation of the product and the ATBD provides descriptions of the algorithms and calculations that are fundamental to the product. User Guides are updated for each new product version, while ATBD’s are not. The VI Quality science dataset layers are binary encoded and bit-packed. Meaning each pixel in the QA layer contains an integer value that must be converted to a bit binary value for interpretation. The bit-string interprets various parameters, as shown by this chart. Here is an example of how to break down a bit binary value using the Terra MODIS 16-Day Vegetation Indices 250 meter level 3 gridded MOD13Q1 data product. Here is the integer value of one pixel from the 250 meter 16-Day VI Quality layer, this number will convert to this 16-bit binary value. This can be done using a calculator or conversion tool. Once the binary value is found, the value can be interpreted using the chart. The bit-string is read from right to left. For example, the Bit-Word for Bit-No. 0-1, VI quality, is 00 which means the pixel produced with good quality and no issues. Bit-No. 2-5, the VI usefulness, is identified using the Bit-Word 0001, which means the pixel is of lower quality, based on the 16 level rank from 0000 to 1111. Follow the same pattern to interpret the additional bits. There are several tools that can be used to interpret quality information. One of those tools is the Land Data Operational Products Evaluation or LDOPE QA “unpack_sds_bits” tool, which can be executed from the command line or invoked via scripts. The unpack_sds_bits tool decodes requested bit fields and writes them to an output HDF file containing a new science dataset that can be viewed with software supporting the HDF or HDF-EOS data formats. More information about the LDOPE Tools can be found under the “Tools” section of the LP DAAC website. Another option for examining QA information is the Application for Extracting and Exploring Analysis Ready Samples or AppEEARS. This application can be used to visualize the QA information and allows users to filter data within AppEEARS by Modland QA bit. Furthermore, AppEEARS also provides users with fully decoded MODIS quality information in a .csv file. Finally, the MODIS Python Toolbox provides users with a simple way to decode and interact with quality layers in ArcGIS. One of the tools in the toolbox is the Decode Quality tool. This tool takes the Quality science dataset as an input and creates individual GeoTiff files corresponding to each bit word or quality category described in the Quality science dataset. The Pixel Reliability QA information does not require any post-processing unpacking of bits. Pixel Reliability is an 8-bit integer that provides simple ranks that capture the overall pixel quality. Ranks -1 to 3 apply to all of the Vegetation Indices products, rank 4 relates directly to the Vegetation Indices Climate Modeling Grid (CMG) products. Detailed materials about Vegetation Indices Quality Information are available on the E-Learning page on the LP DAAC website. Search for “Quality” in the table to learn more about the QA information and the AppEEARS and Python tools mentioned in this video. Thank you for watching our video. For more information on MODIS Version 6 Vegetation Indices data please visit the LP DAAC website at lpdaac.usgs.gov.