[gradsusr] Saving NetCDF lon parameter in geographic
Mark Hess
mhess at oceani.com
Fri Apr 13 19:36:05 EDT 2012
Thanks Chuck - Looks like that validates my suspicion that I cannot change
the lon coordinates from the 0-360 to world coordinates 180 to -180 when
extracting a subset of the original file to a smaller .nc file. I'll
consider different options. Thanks. Mark.
----- Original Message -----
From: "Charles Seman" <charles.seman at noaa.gov>
To: "GrADS Users Forum" <gradsusr at gradsusr.org>
Sent: Thursday, April 12, 2012 3:44 PM
Subject: Re: [gradsusr] Saving NetCDF lon parameter in geographic
> Mark,
>
> The documentation at
> http://grads.iges.org/grads/gadoc/gradcomdsdfwrite.html states:
> "By default, the output file will have a coordinate variable only for
> varying dimensions in the defined variable; non-varying dimensions will
> not appear as a coordinate variable with a size of 1. However, as of
> version 2.0.a5, options have been added to the set sdfwrite command to
> force the variable in the output file to have at least 4 or all 5
> dimensions. When either of these options to set sdfwrite are used, the
> output file will retain information about the values of the dimensions
> that were fixed when the variable was defined; non-varying dimensions
> will appear as a coordinate variable of size 1. For example, if your
> defined variable is 500mb height on14may2002 (a 2D variable that varies
> only in lon and lat), and you use the -4d option, the output file with
> show height as a 4D variable with a Z dimension of size 1 called 'lev'
> with a value "500 mb", and a T dimension of size 1 called 'time' with a
> value of "0 minutes since 2002-05-14 00:00".
>
> Seems the key phrase seems to be "the output file will retain
> information about the values of the dimensions that were fixed when the
> variable was defined"... I confirmed what I thought this meant on a file
> which has grid longitudes in the -180 to 180 range and found the
> following results:
> --------------------------------------------------------------------
> ...using "native" longitude range:
> ga-> sdfopen 3B42.1002.V6.nc
> Scanning self-describing file: 3B42.1002.V6.nc
> SDF file 3B42.1002.V6.nc is open as file 1
> LON set to 0 360
> LAT set to -49.875 49.875
> LEV set to 0 0
> Time values set: 2010:2:14:22 2010:2:14:22
> E set to 1 1
> ga-> q file
> File 1 : NASA TRMM 3B42 3-Hourly 0.25 degree TRMM and Other Satellites
> Precipitation Product (Version 6)
> Descriptor: 3B42.1002.V6.nc
> Binary: 3B42.1002.V6.nc
> Type = Gridded
> Xsize = 1440 Ysize = 400 Zsize = 1 Tsize = 1 Esize = 1
> Number of Variables = 2
> precipitation 0 t,y,x precipitation
> relativeerror 0 t,y,x relativeError
> ga-> set lon -180 180
> LON set to -180 180
> ga-> set lat -90 90
> LAT set to -90 90
> ga-> set sdfwrite precipitation.3B42.1002.V6.nc
> SDFWrite file name = precipitation.3B42.1002.V6.nc
> SDFWrite will replace an existing file
> ga-> sdfwrite precipitation
> SDFWRITE error: no defined variables
> ga-> define precipitation = precipitation
> Define memory allocation size = 8328992 bytes
> ga-> sdfwrite precipitation
> Wrote variable precipitation to precipitation.3B42.1002.V6.nc
> ...
> cjs: /tmp/data/precip/trmm/3B42.V6/pp/0.25x0.25/av/monthly/update/ -->
> \ncdump -c precipitation.3B42.1002.V6.nc
> netcdf precipitation.3B42.1002.V6 {
> dimensions:
> longitude = 1442 ;
> latitude = 722 ;
> ...
> longitude = -180.125,...,180.125
> latitude = -90.125,...,90.125
> ...
> ...so the output file runs from longitude = -180.125 to 180.125 and from
> latitude = -90.125 to 90.125 (a couple extra points added in each
> direction, so be careful)
> --------------------------------------------------------------------
>
> --------------------------------------------------------------------
> ...using a "global" longitude range, using x,y coordinates to define
> exact boundaries:
> ga-> sdfopen 3B42.1002.V6.nc
> Scanning self-describing file: 3B42.1002.V6.nc
> SDF file 3B42.1002.V6.nc is open as file 1
> LON set to 0 360
> LAT set to -49.875 49.875
> LEV set to 0 0
> Time values set: 2010:2:14:22 2010:2:14:22
> E set to 1 1
> ...
> ga-> set y 1 400
> LAT set to -49.875 49.875
> ga-> set x 721 2160
> LON set to 0.125 359.875
> ga-> set sdfwrite precipitation.3B42.1002.V6.nc
> SDFWrite file name = precipitation.3B42.1002.V6.nc
> SDFWrite will replace an existing file
> ga-> define precipitation = precipitation
> Define memory allocation size = 4608000 bytes
> ga-> sdfwrite precipitation
> Wrote variable precipitation to precipitation.3B42.1002.V6.nc
> cjs: /tmp/data/precip/trmm/3B42.V6/pp/0.25x0.25/av/monthly/update/ -->
> \ncdump -c precipitation.3B42.1002.V6.nc
> netcdf precipitation.3B42.1002.V6 {
> dimensions:
> longitude = 1440 ;
> latitude = 400 ;
> longitude = 0.125,...,359.875
> latitude = -49.875,...,49.875
> ...
> ...so the output file runs from longitude = 0.125,...,359.8755 and from
> latitude = -49.875,...,49.875
> --------------------------------------------------------------------
>
> Could you try defining the variable to sdfwrite using parameters
> appropriate to the netCDF file's original grid?
>
> Hope this helps,
> Chuck
>
> On 04/12/2012 03:57 PM, Mark Hess wrote:
>> I have a NetCDF file which I can ingest and display in GrADS with out a
>> problem. The LON parameter is set to a 0-360 grid as opposed to 0-180W &
>> 0-180E (or 0 to 180 for east and 0 to -180 for west). If I "set lon 239
>> 244" and then display my variable of interest (in this case U), the data
>> display properly showing the data between 116W-121W (116W equaling
>> -116=244-360 and 121W equaling -121=360-239). That all is fine until I
>> try to save my geographically subsetted area to a new .nc file using the
>> sdfwrite function. The file saves fine, but it saves the LON information
>> in the 0-360 format instead of the 0 to -180 format which I need for
>> another application. Is there a way to either 1) save the new, subsetted
>> .nc file with the lons converting to the in 0 to -180 format as part of
>> the process or 2) convert the lon format within GrADS first and then
>> save the new .nc file with the lon values in the format I want? Can I
>> subtract 360 from my "lon" variable to create a 'new' lon variable?
>> Thanks,
>> Mark
>>
>>
>> _______________________________________________
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>> gradsusr at gradsusr.org
>> http://gradsusr.org/mailman/listinfo/gradsusr
>
> --
>
> Please note that Charles.Seman at noaa.gov should be considered my NOAA
> email address, not cjs at gfdl.noaa.gov.
>
> ********************************************************************
> Charles Seman Charles.Seman at noaa.gov
> U.S. Department of Commerce / NOAA / OAR
> Geophysical Fluid Dynamics Laboratory voice: (609) 452-6547
> 201 Forrestal Road fax: (609) 987-5063
> Princeton, NJ 08540-6649 http://www.gfdl.noaa.gov/~cjs/
> ********************************************************************
>
> "The contents of this message are mine personally and do not reflect any
> official or unofficial position of the United States Federal Government,
> the United States Department of Commerce, or NOAA."
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