trouble reading netcdf file with pack_nbits=12
James T. Potemra
jimp at HAWAII.EDU
Tue Mar 27 17:08:37 EDT 2007
Greetings all:
I have a netcdf file containing model output that I wish to read using
GrADS. The output from ncdump is included below. Since the (x,y)
coordinates are irregular I need to use a GrADS control file. With such
a control file, the file gets opened, but the displayed values are not
correct (salinity range should be about 30 to 38). I suspect the
problem is with the packed bits at 12. Can anyone confirm this, or
better yet, suggest a solution? Thanks in advance,
Jim
-----------------------------------------------------------------------------------------------------------------
ncdump:
netcdf ncom1 {
dimensions:
X_Index = 291 ;
Y_Index = 196 ;
level = 40 ;
variables:
float X_Index(X_Index) ;
X_Index:long_name = "X_Index" ;
X_Index:units = "none" ;
X_Index:FORTRAN_format = "i5" ;
float Y_Index(Y_Index) ;
Y_Index:long_name = "Y_Index" ;
Y_Index:units = "none" ;
Y_Index:FORTRAN_format = "i5" ;
float level(level) ;
level:long_name = "level" ;
level:units = "none" ;
level:FORTRAN_format = "i5" ;
float Salinity(level, Y_Index, X_Index) ;
Salinity:_FillValue = -1.e+34f ;
Salinity:missing_value = -1.e+34f ;
Salinity:valid_range = 33.01075f, 35.41651f ;
Salinity:long_name = "Salinity" ;
Salinity:units = "PSU" ;
Salinity:FORTRAN_format = "e13.6" ;
Salinity:coordinate_system =
"index_space_for_irregular_grid" ;
Salinity:dtgymd = 20070320 ;
Salinity:dtghms = 0 ;
Salinity:pack_nbits = 12 ;
Salinity:pack_valid_range = 4.960597e-43f, 41.5623f ;
Salinity:pack_min = 0.3415304f, 0.3402182f, 0.3386948f,
0.3370589f, 0.3353713f, 0.3336847f, 0.3320321f, 0.3304273f, 0.3288694f,
0.3273478f, 0.3258449f, 0.3243393f, 0.3228098f, 0.3212448f, 0.3196575f,
0.3181004f, 0.3166694f, 0.3154577f, 0.3146982f, 11.44224f, 12.32387f,
12.78005f, 13.60599f, 13.59866f, 21.76299f, 21.88346f, 21.98833f,
22.0575f, 22.13341f, 22.20853f, 22.28337f, 22.32251f, 22.32767f,
22.3273f, 22.32697f, 34.07007f, 34.07137f, 4.960597e-43f, 34.47794f,
4.960597e-43f ;
Salinity:pack_max = 41.5623f, 41.56229f, 41.56228f,
41.56227f, 41.56226f, 41.56224f, 41.56224f, 41.56224f, 41.56223f,
41.56222f, 41.56221f, 41.56221f, 41.56221f, 41.56221f, 41.56221f,
41.56221f, 41.56221f, 41.56221f, 41.56222f, 40.43718f, 40.50313f,
40.52974f, 40.55785f, 40.60667f, 40.62259f, 40.63385f, 40.66891f,
40.66811f, 40.74745f, 40.66326f, 40.62242f, 40.61849f, 40.61818f,
40.61217f, 40.58677f, 38.77753f, 38.75911f, 38.82295f, 38.82051f,
35.00489f ;
Salinity:pack_FillValue = -1.e+34f ;
Salinity:last_program_run = "regrid_gen from MODAS2" ;
Salinity:date_of_last_change = "Thu Mar 22 18:55:21 2007" ;
}
-----------------------------------------------------------------------------------------------------------------
sample control file:
ncom1.ctl
lii 33% cat ncom1.ctl
DSET ^ncom1.nc
DTYPE netcdf
UNDEF -1.0e+34 missing_value
*UNPACK scale_factor add_offset
UNPACK 12 pack_nbits
TITLE NCOM
XDEF 291 levels 169.614, ... (edited for this email)...., 220.59
YDEF 196 levels 9.796, ... (edited for this email)...., 40.335
ZDEF 40 levels -0.5, -1.594905, -2.897632, -4.44763, -6.291831,
-8.48608, -11.09682,
-14.2031, -17.89898, -22.29637, -27.52843, -33.75358, -41.16032,
-49.97294, -60.45827,
-72.93381, -87.77733, -105.4383, -126.4515, -151.4531, -181.2003,
-216.5939, -258.7054,
-308.8101, -368.4251, -439.3557, -523.7495, -624.162, -743.6338,
-885.7824, -1054.912,
-1256.144, -1495.572, -1780.446, -2119.391, -2522.671, -3002.498,
-3573.4, -4252.665, -5060.868
TDEF 1 linear 20mar2007 1dy
VARS 1
Salinity=>salt 40 t,z,y,x Salinity
ENDVARS
-----------------------------------------------------------------------------------------------------------------
my grads info:
Config: v1.9b4 32-bit little-endian readline sdf/xdf netcdf dods
dods-stn lats printim
Grid Analysis and Display System (GrADS) Version 1.9b4
Copyright (c) 1988-2005 by Brian Doty and IGES
Center for Ocean-Land-Atmosphere Studies (COLA)
Institute for Global Environment and Society (IGES)
This program is distributed WITHOUT ANY WARRANTY
See file COPYRIGHT for more information.
Built Mon May 23 13:17:11 EDT 2005 for i686-pc-linux-gnu
This version of GrADS has been configured with the following options:
o This is a 32-bit LITTLE ENDIAN machine version.
o Command line editing (readline) ENABLED.
o CIRES/CDC (http://www.cdc.noaa.gov) SDF/XDF interface ENABLED.
Use sdfopen/xdfopen to read NetCDF files.
o DTYPE netcdf is ENABLED; DTYPE hdfsds is DISABLED.
o OPeNDAP (a.k.a. DODS) gridded data interface ENABLED.
(http://opendap.org)
This version is configured to access gridded data on OPeNDAP servers
via the sdfopen or xdfopen commands, or using DTYPE netcdf.
o OPeNDAP (a.k.a. DODS) station data interface (gadods) ENABLED.
This version is configured to access station data on OPeNDAP servers
via the open command.
o PCMDI (http://www-pcmdi.llnl.gov) LATS interface ENABLED.
This version is configured to write GRIB and NetCDF files.
o DAO (http://dao.gsfc.nasa.gov) Athena Widget GUI DISABLED.
o NRL/DAO/PCMDI XA or ImageMagick Image Output DISABLED.
o printim command for direct png/gif output ENABLED.
(via the GD Library -- http://www.boutell.com/gd)
For additional information please consult http://grads.iges.org/grads/
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