If I have the product MOD13Q1.A2014001, does it represent the previous 16 or the 16 days after January 1st?
I.e., the product is made with 16 days, but how to determine if it was taken the previous or later 16 days?
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It represent from January 1st to January 16th. Here you got some information:
The day of year (DOY) for each MODIS image represents the first day of the period of the 8- or 16-day composite. The day during the composite period when the best observation is recorded is called the day of pixel composite.
If you want to know which DOY is composite day, check band number 11 (
250m 16 days composite day of the year).
138 | Day of year of data acquisition (In this case, day 138. The date in the granule is the first day of data in the eight-day file.)
It is the 16 days after January 1st (as mentioned by aldo_tapia).
MOD13Q1 products are collected with 16 day resolution from two spacecrafts: Terra and Aqua, which are phased produced to become a 8-day product (the goal is to improve the temporal change detection). From "MODIS Vegetation Index Product Series Collection 5 Change Summary" (pdf, (page 12):
Under phased production (implemented in collection 5.0 reprocessing) the algorithm will use the same compositing period, but starting at different days (one starts in the middle of the compositing period of the other).
The MOD13Q1 product description says (emphasis mine):
Phased production between Terra and Aqua products for improved temporal frequency (Terra 16-day period starting Day 001, Aqua 16-day period starting Day 009).
MODIS products are registered taking into account Julian days, within an yearly basis. The last three digits after the first dot on
MOD13Q1.A2014001 correspond to the acquisition (starting) date. For more info on this, see the MODIS Naming Conventions:
MODIS filenames ... follow a naming convention which gives useful information regarding the specific product. ... the filename MOD09A1.A2006001.h08v05.005.2006012234657.hdf indicates:
MOD09A1 - Product Short Name
.A2006001 - Julian Date of Acquisition (A-YYYYDDD)
.h08v05 - Tile Identifier (horizontalXXverticalYY)
.005 - Collection Version
.2006012234567 - Julian Date of Production (YYYYDDDHHMMSS)
.hdf - Data Format (HDF-EOS)