Landmask of ocean color product

[semaj545]

Last question!  Does anyone know what land mask data set is used to mask the SNPP VIIRS ocean color product and if this data is available? I'm particularly interested in the Canadian Arctic. I see in SeaDAS there are four coastline source data sets in the land mask dialogue box: a 50 and 150 m STRM_GC (which i believe only goes to 60N) and a 1 and 4 km GSHHS.  I'd like to know what kind of buffer exists around coastlines which will increase with decresing resolution of the land mask used in the processing.  thanks!

[OB.DAACx - SeanBailey]

The landmask is the same used for all missions we support, and it is currently based on the dataset provided with Generic Mapping Tools (GMT).
$ ncdump -h $OCDATAROOT/common/landmask_GMT15ARC.nc
netcdf landmask_GMT15ARC {
dimensions:
        lon = 86401 ;
        lat = 43201 ;
variables:
        double lon(lon) ;
                lon:long_name = "longitude" ;
                lon:units = "degrees_east" ;
                lon:actual_range = -180., 180. ;
        double lat(lat) ;
                lat:long_name = "latitude" ;
                lat:units = "degrees_north" ;
                lat:actual_range = -90., 90. ;
        byte watermask(lat, lon) ;
                watermask:long_name = "watermask" ;
                watermask:_FillValue = -128b ;
                watermask:actual_range = 0., 1. ;

// global attributes:
                :Conventions = "COARDS, CF-1.5" ;
                :title = "landmask_GMT15ARC.nc" ;
                :history = "grdlandmask -Glandmask_GMT15ARC.nc?watermask=nb -I15s -Rd -Df -N1/0 --IO_NC4_DEFLATION_LEVEL=5" ;
                :description = "Derived from the full resolution shorelinen" ;
                :GMT_version = "5.1.1 (r12968) [64-bit]" ;
}


...which is the GSHHS dataset. 

Regards,
Sean

[b.detracey]

What is the reason landmask_GMT15ARC.nc is used for land masking instead of watermask15ARC.nc(watermask.dat)? When does OCCSW use the watermask?

[gfireman]

The inputs to watermask15ARC.nc were developed by members of the MODIS Land Science Team, with the intent of keeping any water-containing pixels from being processed with land algorithms.  As a result, every river, small lake, and coastline was flagged as water - land-containing pixels which are non-optimal inputs for our ocean algorithms.

The GMT grdlandmask parameters used to create landmask_GMT15ARC.nc were chosen fairly conservatively, in order to exclude those marginal cases.

OCSSW does not use watermask15ARC.nc anywhere by default.  BTW, watermask.dat is an older-format file used only for shallow-water masking.

Compare landmasks for Chesapeake Bay:
From left to right: watermask15ARC.nc, landmask_GMT15ARC.nc, difference.

[b.detracey]

Thanks.