Dear All,
i am having some issues with analysing MOD09 modis data in grass. I would like to mask out the cloud pixels using the below quality control band.
when i import this band the unsigned 16bit integer is converted in decimal and I am unsure how to use r.mapcalc to create a mask based on the different bits.
I am unsure how I should go about this (see *** section below). I would like to try different selections to see which would make the best mask but i am unsure how to do this against a binary field
eg. r.mapcalc MASK=“if(sur_refl_qc_250 == **** clear state **** , 1, null())”
how do I select out the different bits to create the mask?
At this stage i am thinking of trying to convert the decimal back to binary (not sure how to do this ) and then subselect columns (bits) eg 0-1 or 2-3 and the placing an if statement (eg. if column01 == “00” or if column23 = “00”).
Any guidance would be much appreciated.
kind regards
Andrew
sur_refl_qc_250m:
| Bit | Description |
|---|---|
| 0-1 | MODLAND QA bits; 00 – corrected product produced at ideal quality – all bands 01 – corrected product produced, less than ideal quality – some or all bands 10 – corrected product not produced due to cloud effects – all bands 11 – corrected product not produced for other reasons – some or all bands, may be fill value [Note that a value of (11) overrides a value of (01)] |
| 2-3 | cloud state; 00 – clear 01 – cloudy 10 – mixed 11 – not set, assumed clear |
| 4-7 | band 1 data quality, four bit range; 0000 – highest quality 1000 – dead detector; data has been copied from adjacent detector 1001 – solar zenith >= 86 degrees 1010 – solar zenith >= 85 and < 86 degrees 1011 – missing input 1100 – internal constant used in place of climatological data for at least one atmospheric constant 1101 – correction out of bounds, pixel constrained to extreme allowable value 1110 – L1B data faulty 1111 – not processed due to deep ocean or clouds |
| 8-11 | band 2 data quality, four bit range; (same values as above) |
| 12 | atmospheric correction performed; 0 – no 1 – yes |
| 13 | adjacency correction performed; 0 – no 1 – yes |
| 14 | different orbit from 500m; 0 – no 1 – yes |
| 15 | unused (spare) |
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