idl-NetCDF.tar.gz_idl gui_idl read cdf file_netcdf.tar.gz

NAME: ncdf_quickwrite SYNOPSIS: This is a routine for writing variables from an IDL session to a NetCDF file, in just a very few commands. It is designed to be quick to *use*, once you are familiar with the syntax. It may possibly also be slightly quicker to *learn* than writing NetCDF files the standard way, but this is not particularly the objective. DESCRIPTION: 1) Ensure that the supplied routines are installed in your IDL_PATH. ncdf_quickwrite.pro ncdf_quickwrite_helper1.pro ncdf_quickwrite_helper2.pro ncdf_quickwrite_helper3.pro 2) Set the string variable NCFILE to the NetCDF filename you wish to write, For example: ncfile='myfile.nc' The file should not already exist. If you wish to permit overwriting, put "!" before the filename. For example: ncfile='!myfile.nc' 3) Set the string variable NCFIELDS as follows. NB there are a number of options but you do not need to use them all! Skip down to "USAGE EXAMPLE" below for a simple example. a) For each variable, construct the VARIABLE SPECIFICATION as follows: => For an array, use the name of array, with the names of the dimensions in square brackets ([]) and separated by commas. For example: pressure[longitude,latitude] => For a 1-dimensional array, whose dimension name is the same as the array name, you can omit the dimension name if you want. For example: longitude[] => For each scalar, do not use square brackets, e.g.: g => If any of variables you want to use in the NetCDF file is not held in a variable of the same name in your IDL session, then put follow the variable specification with "=expression". Examples: 1) if pressure is in IDL variable "p", you have: pressure[longitude,latitude]=p 2) if g is not stored in a variable but should have value 9.8, you have: g=9.8 => If any of the variables have attributes to be stored, follow the specification with a colon (:) followed by the name of a variable containing a structure of attributes. For example: pressure[longitude,latitude]=p:patts where "patts" is a structure containing: {units:'pascal',missing_value:1e10} b) Join up all the variables with semicolons (;) to make the NCFIELDS string. For example: NCFIELDS = $ 'pressure[longitude,latitude]=p:patts;longitude[];latitude[];g=9.8' c) If there is a dimension, which you want to be of UNLIMITED size, then precede its name with "*" somewhere where it is mentioned (or where you omit the dimension name because it matches the variable name, nonetheless still include the "*" in the square brackets). For example the "time" dimension in any of the following: NCFIELDS = $ 'pressure[longitude,latitude,*time];longitude[];latitude[];time[]' NCFIELDS = $ 'pressure[longitude,latitude,time];longitude[];latitude[];time[*time]' NCFIELDS = $ 'pressure[longitude,latitude,time];longitude[];latitude[];time[*]' (Note that if you put "*" before the same dimension name more than once, that is accepted. But if you put "*" before more than one dimension name, that is an error, because only one dimension can be UNLIMITED.) d) If there are global attributes, follow the specification with an at sign (@) followed by the name of a variable containing a structure of attributes. For example: NCFIELDS = $ 'pressure[longitude,latitude];longitude[];latitude[]@globatts' where "globatts" is a structure containing: {source:'my program',version:2} e) You may insert whitespace in NCFIELDS for readability. 4) Type: @ncdf_quickwrite USAGE EXAMPLES: 1) SIMPLE EXAMPLE. => You have the following variables: LATITUDE FLOAT = Array[73] LONGITUDE FLOAT = Array[96] PRESSURE DOUBLE = Array[96, 73] => You issue the following commands: ncfile='my.nc' ncfields='latitude[];longitude[];pressure[longitude,latitude]' @ncdf_quickwrite => This makes "my.nc". "ncdump -h my.nc" reports the following: netcdf my { dimensions: latitude = 73 ; longitude = 96 ; variables: float latitude(latitude) ; float longitude(longitude) ; double pressure(latitude, longitude) ; } 2) MORE COMPLEX EXAMPLE. => You have the following variables: LATS FLOAT = Array[73] LONS FLOAT = Array[96] P DOUBLE = Array[96, 73, 100] UBAR DOUBLE = Array[73, 100] YR LONG = Array[100] => You issue the following commands. (NB The NCFIELDS assignment can be typed as one long line, but is split below for readability.) ncfile='!my.nc' angle_attr={units:'degrees'} wind_attr={units:'m s-1'} press_attr={units:'pascals',missing_value:1e10} g_attr={units:'m s-2'} globattr={source:'My program',version:2} ncfields = 'pressure[longitude,latitude,time]=p:press_attr; ' $ + 'longitude[]=lons:angle_attr; ' $ + 'latitude[]=lats:angle_attr; ' $ + 'ubar[latitude,time]:wind_attr; ' $ + 'year[*time]=yr; ' $ + 'g=9.8:g_attr @ globattr' @ncdf_quickwrite => This makes "my.nc", overwriting any existing my.nc. "ncdump -h my.nc" reports the following: netcdf my { dimensions: longitude = 96 ; latitude = 73 ; time = UNLIMITED ; // (100 currently) variables: double pressure(time, latitude, longitude) ; pressure:units = "pascals" ; pressure:missing_value = 1.e+10f ; float longitude(longitude) ; longitude:units = "degrees" ; float latitude(latitude) ; latitude:units = "degrees" ; double ubar(time, latitude) ; ubar:units = "m s-1" ; int year(time) ; float g ; g:units = "m s-2" ; // global attributes: :source = "My program" ; :version = 2s ; } (Note that in this example you have created a NetCDF file with verbose variable names even though the IDL variables have terse names.) BUGS: => Writing string variables may fail. LIMITATIONS: => An expression following an "=" sign should not contain ":" or ";" signs because these will be treated as separators. => An expression following an "=" sign gets evaluated twice. If this is computationally expensive, store it in a variable and reference the variable instead. => Attribute names are case-insensitive because they are structure tags. In fact, they are stored as lower case. AUTHOR: Alan Iwi <A.M.Iwi@rl.ac.uk> COPYRIGHT: I am putting these routines in the public domain. But the IDL program itself is subject to copyright restrictions.