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ropensci / MODIStsp

An "R" package for automatic download and preprocessing of MODIS Land Products Time Series
https://docs.ropensci.org/MODIStsp
GNU General Public License v3.0
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MODIS QA VI #184

Closed lerouxr-dev closed 4 years ago

lerouxr-dev commented 4 years ago

Hello, I downloaded the QA for vegetation index (VI) MOD13Q1_QA. I got 4 classes per tiles (0,1,2,3), but I can relate these value with MODIS documentation ? Any help to understand what each classe significate ? Regards, Renan

janeuhd commented 4 years ago

Take a look at the user guide of the data set (Section 5.5.2 QA Science data sets). In table 4 you will find the definition of the pixel reliability scores: https://lpdaac.usgs.gov/documents/103/MOD13_User_Guide_V6.pdf

lerouxr-dev commented 4 years ago

Thanks for your reply. I though it was this table. The problem I got is that most of my image is marked as covered by snow or ice, or I am working under tropical area. Does it means that ice can be Ice cloud ? Thanks

janeuhd commented 4 years ago

I cannot answer for your particular image, but that is surprising, I have not seen that issue in my (northern) part of the world. Usually 2 is a fairly good match to snow/ice and other extremely bright surfaces, and 3 to clouds, ocean etc. Have you checked the additional QA info (e.g. the QA_usef layer)? See table 5 in the user guide for how to interpret that layer.

lerouxr-dev commented 4 years ago

Here you can find wich layer I downloaded and en screen shot of the result over Brazil. Do I download the good QA ?

MODIStsp QA_Quality

lbusett commented 4 years ago

Hi @lerouxr-dev

Table 4 of the document linked by @janeuhd refers to the "Quality reliability" indicator, selectable on the left (see figure below).

image

In MODIStsp, the "Quality Indicators" in the middle are derived by interpreting the QA bits of the Quality Layers contained in the original MODIS hdf file. The table you should look at is therefore Table 5, and not table 4. For example, the table shows that the "VI Quality" indicator that you selected is derived from bit 0-1 of the "VI quality indicators" original layer. "VI usefulness" from bits 2-5, Aerosol quantity from bits 6-7, etcetera.

image

The interpretation is therefore in this case: 0 = Good quality, 1 = "intermediate" quality - Check other QA, 2 = Probably cloudy; 3 = NODATA for other reasons than clouds (i.e., in your case, Water pixels, apparently).

HTH,

Lorenzo

lbusett commented 4 years ago

Closing here. Feel free to reopen if needed