Welcome to the Earthdata Forum! Here, the scientific user community and subject matter experts from NASA Distributed Active Archive Centers (DAACs), and other contributors, discuss research needs, data, and data applications.
Hello - we had a user write in with a question that we think other users will be interested in the answer.
"Hello, I would like to ask you a question. I downloaded the data of MCD18A2 and extracted the three bands of PAR, but I found that most of the images were obviously missing, and the missing parts were not quite the same. What is the reason"
The MCD18A2 data product can contain multiple orbits, or individual overpasses, within the MODIS HDF file. The number of orbits can vary depending on where the MODIS tile of interest is at on the globe. Each day, data for each orbit is spatially and temporally clipped to MODIS tiles it intersects with, which can result in MODIS tiles that share an orbit and tiles that are only partially filled with data values for said orbit (i.e., orbit paths to not match one to one with MODIS tile boundaries). In the MODIS HDF file, the data for each orbit is stored in a third dimension named Orbit_Layers, along with the X and Y dimensions for the MODIS time. While the MODIS HDF file is referenced by date, the orbit data contained inside has a datetime association which can be found in the file metadata for the MODIS HDF file. Users can work with the data for individual orbits separately or combine orbits depending on the research or applications parameters.