Earthdata Forum

Started to use SeaDAS 7.4 for MODIS Aqua and VIIRS NPP L2 data viewing (download from L1/L2 data browser).

Found both MODIS Aqua and VIIRS NPP L2 data have the exactly same L2_FLAGS attributes in meta data:

ATMFAIL LAND PRODWARN HIGLINT HILT HISATZEN COASTZ SPARE STRAYLIGHT CLDICE COCCOLITH
TURBIDW HISOLZEN SPARE LOWLW CHLFAIL NAVWARN ABSAER SPARE MAXAERITER MODGLINT
CHLWARN ATMWARN SPARE SEAICE NAVFAIL FILTER SPARE BOWTIEDEL HIPOL PRODFAIL SPARE

As SeaDAS 7.4 release document mentioned, 3 more flags added into group (Water, Quality_L2 and Quality_L3),
SeaDAS Mask Manager can find these 3 new flags but not in meta data attributes.  Also it seems SeaDAS
web help document hasn't included these 3 new flags yet.

Some Questions please help :

(1) Do those L2_flags attributes in meta data need update ? (not included 3 new flags in V2017246174733xx.L2_SNPP_OC.nc files)

(2) Why both MODIS Aqua and VIIRS NPP sensors have the same l2 flags, especially both have "Bowtiedel" ?

(3) Are SeaDAS 7.4 L2_flags bit description hard-coded for current interactive use only ? For all sensors ?

(4) How to read  & match l2 flag descriptions to every bit for all individual image pixels ? This is very helpful for programs to
search and read correct l2 flag info. Wish to have supportive info based on meta data attributes.

Thanks !

1 & 2) No.
Both MODIS and VIIRS (in fact all the L2 files we distribute) are generated using the same code (l2gen).  There is a common file format, and thus a common l2_flag array.
Obviously, for all but VIIRS the BOWTIEDEL flag is not set.
3)
The new "flags" in the SeaDAS GUI are generated by the GUI and are simply convenience functions that combine the L2 flags (well, "Water" is not a combination of the flags but the inverse of "LAND").  These will not be put in the L2 files.

4)
Reading the l2_flags and associating the bit values to given flag names is not hard to do.  How it's done will vary by language, though.  Attached is an example in Perl.

Regards,
Sean

CAVEAT:  the attached script hasn't been updated to the current flag set,  an exercise left to the reader :wink:
attachment 1

Thanks for the information !