Hi,
I have downloaded MOD09A1 V006 surface reflectance data. I have an issue when using the QA flags to filter those pixels with a low quality. It seems that there exist numerous lines with this flag: 1075838976.
I think that the binary number point out that there is no atmospheric correction. However, according to bit number 0-1 it is "00" "corrected product proudced at ideal quality --all bands". Additionally the word bit for band 6 is "0001", which is a binary code not considered in Table 10 of the User Guide.
Binagy flat is:
10 0000 0001 0000 0000 0000 0000 0000 00
I would be grateful if you could give me a clue to interpret this or to decide if I should filter out those pixels or not. I am mosaicking and computing NDVI values, so atmospheric correction is important.
Thanks
Victor
The digital flag for each band also has a descriptor, or two. Good quality data will say something like 'high quality'. Poor quality data will include the reasons it is poor. For example, problems with the detector, cloud cover, etc. The quality flag numers will have a descritor. From where did you download the data? Try the Appeears site.
Russ ell, thanks for your answer. I do understand the use of quality flags. My problem is that the code that I have obtained for many lines cannot be explained by the user guide as it presents a bit word for which there is no equivalence. It is strange because the first bit word (from right to left) says that the all the bands have the highest quality. However, the bit word for band 6 is a code for which there is no equivalence in the user manual (0001). Additionally, the second digit from left to right points out that the atmospheric correction was NOT performed.
I see that you are using V006, but I found the attached document describing bit errors in V005. Perhaps it is possible that these bit errors were carried over to V006. Is the affected data in 2000, 2001, and/or 2002?
Dear Katarzyna his is the same table as the one I checked in the user guide. There is no translation for bit code 0001 in band 7. :(
Katarzyna thanks for your answers. I think I am reading it correctly. According to this video tutorial: https://www.usgs.gov/media/videos/getting-started-modis-v6-surface-reflectance-data-part-3
The bit sentence must be read from right to left and the bit word from left to right. It is shown in the minute 3:31 of the video tutorial
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