It seems that your question in this new thread is related to this other thread you started in which @cjcoats provided feedback on using m3merge to create mask files for use by CMAQ ISAM.
The mask file(s) used by CMAQ ISAM are described in a section of Appendix B of the User’s guide. Specifically, they are I/O API-style netCDF files that “contain a separate variable for each spatial region of interest. Each variable is a gridded field of real numbers from 0.0 to 1.0, with 0.0 outside of the region of interest and 1.0 completely inside the region”.
Given that you may not be able to access the example GRIDMASK file with US states due to country-specific blocks on Google services, here is a ncdump of the relevant header portions of that example file:
dimensions:
TSTEP = 1 ;
DATE-TIME = 2 ;
LAY = 1 ;
VAR = 49 ;
ROW = 299 ;
COL = 459 ;
variables:
int TFLAG(TSTEP, VAR, DATE-TIME) ;
TFLAG:units = “<YYYYDDD,HHMMSS>” ;
TFLAG:long_name = "TFLAG " ;
TFLAG:var_desc = "Timestep-valid flags: (1) YYYYDDD or (2) HHMMSS " ;
float AL(TSTEP, LAY, ROW, COL) ;
AL:long_name = "AL " ;
AL:units = "fraction " ;
AL:var_desc = "AL fractional area per grid cell " ;
float AZ(TSTEP, LAY, ROW, COL) ;
AZ:long_name = "AZ " ;
AZ:units = "fraction " ;
AZ:var_desc = "AZ fractional area per grid cell " ;
float AR(TSTEP, LAY, ROW, COL) ;
AR:long_name = "AR " ;
AR:units = "fraction " ;
AR:var_desc = "AR fractional area per grid cell " ;
… " ;
float WV(TSTEP, LAY, ROW, COL) ;
WV:long_name = "WV " ;
WV:units = "fraction " ;
WV:var_desc = "WV fractional area per grid cell " ;
float WI(TSTEP, LAY, ROW, COL) ;
WI:long_name = "WI " ;
WI:units = "fraction " ;
WI:var_desc = "WI fractional area per grid cell " ;
float WY(TSTEP, LAY, ROW, COL) ;
WY:long_name = "WY " ;
WY:units = "fraction " ;
WY:var_desc = "WY fractional area per grid cell " ;
// global attributes:
:IOAPI_VERSION = "ioapi-3.2: Id: init3.F90 1 2017-06-10 18:05:20Z coats " ;
:EXEC_ID = "??? " ;
:FTYPE = 1 ;
:CDATE = 2019303 ;
:CTIME = 150353 ;
:WDATE = 2019303 ;
:WTIME = 150353 ;
:SDATE = -635 ;
:STIME = 0 ;
:TSTEP = 0 ;
:NTHIK = 1 ;
:NCOLS = 459 ;
:NROWS = 299 ;
:NLAYS = 1 ;
:NVARS = 49 ;
:GDTYP = 2 ;
:P_ALP = 33. ;
:P_BET = 45. ;
:P_GAM = -97. ;
:XCENT = -97. ;
:YCENT = 40. ;
:XORIG = -2556000. ;
:YORIG = -1728000. ;
:XCELL = 12000. ;
:YCELL = 12000. ;
:VGTYP = -9999 ;
:VGTOP = -9.e+36f ;
:VGLVLS = 0.f, 0.f ;
:GDNAM = "US12_459X299 " ;
:UPNAM = "cdf2uamv " ;
:VAR-LIST = "AL AZ AR CA CO CT DE DC FL GA ID IL IN IA KS KY LA ME MD MA MI MN MS MO MT NE NV NH NJ NM NY NC ND OH OK OR PA RI SC SD TN TX UT VT VA WA WV WI WY " ;
:FILEDESC = “” ;
:HISTORY = “” ;
}