[GRASS-dev] Issue with MOD11A2 QC bands

[Veronica Andreo]

Hi devs :)

The last couple of days Yann has helped me to understand a problem that
arose with QC bands in MOD11A2 product (maybe applies to other MOD11). I'm
just sharing what we learnt.

The problem arises because MOD11 products have 0 as no-data value (which is
reasonable given that temperature is in °K) and r.in.gdal automatically
reads that pre-defined no-data value and converts it into grass NULL [0].

The problem is that also QC bands appear with 0 as no-data (try gdalinfo
with the QC_band) - Maybe that's a problem of gdal, dunno... given that in
the Layers section of MOD11A2 [1], there's the following note:


> NOTE: there is no FillValue for the QC SDS. This SDS should be used
> together with the LST SDS. If LST has FillValue 0, the 0-1 bits in QC have
> values of 10 or 11, other bit fields with 0 are undefined.


Anyway, my workflow was as follows:

(pymodis to download, select bands, reproject and convert to tiff)
modis_convert.py  -s "( 1 1 0 0 0 0 0 0 0 0 0 0 )" -o
MOD11A2.A2015001_h12v11 -e 4326
MOD11A2.A2015001.h12v11.005.2015010065120.hdf

gdalinfo MOD11A2.A2015001_h12v11_QC_Day.tif
...
Lower Right ( -53.2090984, -30.0015506) ( 53d12'32.75"W, 30d 0' 5.58"S)
Center      ( -61.2455654, -25.0007753) ( 61d14'44.04"W, 25d 0' 2.79"S)
Band 1 Block=2327x3 Type=Byte, ColorInterp=Gray
*  NoData Value=0  <<<----- here!*

then,
r.in.gdal
i.modis.qc
... and so on...

So, when you intend to use i.modis.qc with mandatory_qa_11A2, for
example... you don't get zero in that QC band but the range goes from 1 to
3 (it should be 0-3).

The *solution* then would be, rather you keep LST pixels where
mandatory_qa_11A2 is null, or set null to 0 in QC band before using
i.modis.qc :)

Maybe using some tool other than GDAL to extract bands from MODIS HDF files
gives a different result (?).

Any other insights/suggestions are welcome :)

Cheers!
Vero

[0] https://grass.osgeo.org/grass73/manuals/r.in.gdal.html#NOTES
[1]
https://lpdaac.usgs.gov/dataset_discovery/modis/modis_products_table/mod11a2
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