| Re: Reading Multiple netCDF files at once [message #52250] |
Thu, 25 January 2007 12:42  |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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rita wrote:
> Hi all,
>
> I was wondering if anyone could enlighten me on the following:
>
> I have several netCDF files containing a standard set of data in which
> each file represents a time step.
> I want to perform a temporal mean, and hence, I need to be able to read
> all the files into memory and then perform operations on it.
>
> I know I could read each file in, export its contents to a save file ,
> repeat for each file, and then average; there must be a better way
> though!
Save files? Why would you do that? Why not accumulate a sum with each file read and then
average that with the number of files?
This is how I would do it with my IDL netCDF file reader:
files = file_search('*.nc')
nfiles = n_elements(files)
for i=0,nfiles-1 do begin
ierr=read_netcdf(files[i],mystruct) ; check ierr
if ( i eq 0 ) then $
avg = mystruct.varname $ ; or whatever the netCDF variable name is
else $
avg = avg + mystruct.varname
endfor
avg = avg / double(nfiles)
I can send you a copy of the read_netcdf stuff if you like. Below is the header docs.
cheers,
paulv
;+
;
; NAME:
; Read_netCDF
;
; PURPOSE:
; Function to read variable and attribute data from netCDF
; format files.
;
; CALLING SEQUENCE:
; result = Read_netCDF( ncFile, $ ; Input
; data, $ ; Output
; VARIABLE_LIST = variable_list, $ ; Input
; COUNT = count, $ ; Input
; OFFSET = offset, $ ; Input
; STRIDE = stride, $ ; Input
; VARIABLE_ATTRIBUTES = variable_attributes, $ ; Input
; GLOBAL_ATTRIBUTES = global_attributes, $ ; Input
; NO_VAR_BYTE_TO_STRING = no_var_byte_to_string, $ ; Input
; NO_ATT_BYTE_TO_STRING = no_att_byte_to_string, $ ; Input
; QUIET = quiet ) ; Input
;
; INPUTS:
; ncFile: The name of the NetCDF file to read
;
; INPUT KEYWORD PARAMETERS:
; variable_list: A string array of variable name to read from
; the NetCDF file. If not specified, ALL the
; variables are read.
; count: Set this keyword to a vector containing the
; number of points in each dimension that are
; required for a variable read. It is a 1-based
; vector and defaults to match the size of all
; dimensions so that all data is read.
; offset: Set this keyword to a vector containing the
; starting index position for each dimension of
; the variable required. It is a 0-based
; vector and defaults to zero for every dimension
; so that all data is read.
; stride: Set this keyword to a vector containing the
; strides, or sampling intervals, between accessed
; values of the required variable. It is a 1-based
; vector and defaults to one for every dimension
; so that all data is read.
; variable_attributes: Set this keyword to return variable
; attribute data. Using this keyword modified the
; the form of the output structure. See the
; OUTPUTS description below.
; global_attributes: Set this keyword to return global
; attribute data.
; no_var_byte_to_string: Set this keyword to prevent the
; conversion of BYTE variable data
; to STRING type. (IDL 5.2 and earlier only)
; no_att_byte_to_string: Set this keyword to prevent the
; conversion of BYTE attribute data
; to STRING type. (IDL 5.2 and earlier only)
; quiet: Set this keyword to suppress informational
; output.
;
; OUTPUTS:
; data: The data structure containing the file data
; requested.
;
; OUTPUT DATA STRUCTURE FORM
; --------------------------
; o The file dimensions are always returned,
;
; data.dim1
; .dim2
; .dim3
; .....
; .dimN
;
; o If variable data is read in, they are present in
; the output structure like so:
;
; data.var1
; .var2
; .var3
; .....
; .varN
;
; o If variable attributes are also requested, the variable
; portion of the output structure has the form:
;
; data.var1.DATA
; .att1
; .att2
; .....
; .attN
; .var2.DATA
; .att1
; .att2
; .....
; .attN
; .....
; .varN.DATA
; .att1
; .att2
; .....
; .attN
;
; where the capitalised tag DATA is the actual tag name
; used for the variable data.
;
; o If global attributes are requested, they are present
; in the output structure like so:
;
; data.gatt1
; .gatt2
; .gatt3
; .....
; .gattN
;
;
; FUNCTION RESULT:
; Error_Status: The return value is an integer defining the error status.
; The error codes are defined in the
; Error_Handling/error_codes.pro
; file.
; If == SUCCESS the netCDF data read was successful.
; == FAILURE an unrecoverable error occurred.
;
--
Paul van Delst Ride lots.
CIMSS @ NOAA/NCEP/EMC Eddy Merckx
Ph: (301)763-8000 x7748
Fax:(301)763-8545
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