| on reading NCDF files [message #37673] |
Tue, 20 January 2004 00:01  |
leesw
Messages: 3
Registered: August 2003
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Junior Member |
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Hi everyone!
I'm gonna read a NCDF file. It may contain several variables within
itself. When I extract a variable named "image", the procedure is as
follows:
cid=ncdf_open('test.nc')
vid=ncdf_varid(cid,'image')
ncdf_varget,cid,vid,image
But suppose I don't know the name of each variable unfortunately. Is
there any way to figure out the details of the included
variables(name, dimension, etc.)? The second command fails when I put
a wrong variable. And is there any way to read the included variables
all at once? (something like /all keyword)
I wonder if there's any way to figure out all details of the included
variables from an NCDF file directly.
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| Re: on reading NCDF files [message #37756 is a reply to message #37673] |
Tue, 20 January 2004 15:50  |
Don Woodraska
Messages: 5
Registered: March 2001
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Junior Member |
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On 1/20/04 1:01 AM, in article
3ee6ff80.0401192300.534dc5b1@posting.google.com, "Sangwoo"
<leesw@astro.snu.ac.kr> wrote:
> Hi everyone!
>
> I'm gonna read a NCDF file. It may contain several variables within
> itself. When I extract a variable named "image", the procedure is as
> follows:
>
> cid=ncdf_open('test.nc')
> vid=ncdf_varid(cid,'image')
> ncdf_varget,cid,vid,image
>
> But suppose I don't know the name of each variable unfortunately. Is
> there any way to figure out the details of the included
> variables(name, dimension, etc.)? The second command fails when I put
> a wrong variable. And is there any way to read the included variables
> all at once? (something like /all keyword)
>
> I wonder if there's any way to figure out all details of the included
> variables from an NCDF file directly.
I'm the science data processing manager for a NASA instrument called the
Solar EUV Experiment. For what it's worth, we use a (semi-generic) routine
that my supervisor wrote called read_netcdf.pro and a companion routine
write_netcdf.pro for all levels of our daily routine science data
processing. We routinely read and write netcdf files containing structures
and/or arrays of structures up to a nested depth of 4.
These procedures are available at
http://lasp.colorado.edu/see/see_software.html
There are examples in the ftp directory where the code resides, too. It's
worked on every netcdf file I've come across, even those created by the
other 3 instruments on our spacecraft. Read_netcdf.pro returns an anonymous
structure (or array of structures) and an optional attributes string array.
These IDL routines insulate the end user from all the gory details of
reading and writing NetCDF files. They accept almost any IDL structure.
It's not perfect, and some of it may be darn ugly. The price for hiding the
user from the details is performance. It tends to be slow for files that are
larger than 30 MB or so. Also, you'll be much better off reading the data
file from a local fast drive, than a network drive. We routinely read and
write structures that are nested 4 structures deep, so that's one limit
(depth of 4). Also, objects and pointers are not allowed, but that's really
a file format restriction.
I hope these routines will save at least one person the trouble of writing
custom read and write routine.
Cheers,
Don Woodraska
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| Re: on reading NCDF files [message #37762 is a reply to message #37673] |
Tue, 20 January 2004 10:07 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Sangwoo wrote:
>
> Hi everyone!
>
> I'm gonna read a NCDF file. It may contain several variables within
> itself. When I extract a variable named "image", the procedure is as
> follows:
>
> cid=ncdf_open('test.nc')
> vid=ncdf_varid(cid,'image')
> ncdf_varget,cid,vid,image
>
> But suppose I don't know the name of each variable unfortunately. Is
> there any way to figure out the details of the included
> variables(name, dimension, etc.)? The second command fails when I put
> a wrong variable. And is there any way to read the included variables
> all at once? (something like /all keyword)
>
> I wonder if there's any way to figure out all details of the included
> variables from an NCDF file directly.
Yes, there is. For me at least, that's the point of netCDF.
I have a simple-minded read_ncdf() function that may do what you want. You can read all
the variable data by default:
IDL> error_status=read_ncdf('sndr_g12.EmisCoeff.nc',x,/quiet)
% Compiled module: READ_NCDF.
% Compiled module: IS_NCDF.
% Loaded DLM: NCDF.
IDL> help, x, /struct
** Structure <82417b4>, 10 tags, length=37856, data length=37856, refs=1:
RELEASE LONG 2
VERSION LONG 2
THETA_OFFSET DOUBLE 0.10000000
THETA_MAX DOUBLE 65.100000
NCEP_SENSOR_ID LONG Array[18]
WMO_SATELLITE_ID
LONG Array[18]
WMO_SENSOR_ID LONG Array[18]
SENSOR_CHANNEL LONG Array[18]
WIND_SPEED DOUBLE Array[13]
EMIS_COEFFICIENTS
DOUBLE Array[4, 5, 18, 13]
Or just the data you want:
IDL> error_status=read_ncdf('sndr_g12.EmisCoeff.nc',x,variable_li st=['Wind_Speed'],/quiet)
IDL> help, x, /struct
** Structure <82468d4>, 1 tags, length=104, data length=104, refs=1:
WIND_SPEED DOUBLE Array[13]
Or just the global attributes:
IDL> error_status=read_ncdf('sndr_g12.EmisCoeff.nc',x,/global_att ributes,/quiet)
IDL> help, x, /struct
** Structure <8240f34>, 7 tags, length=84, data length=84, refs=1:
WRITE_MODULE_HISTORY
STRING '$Id: write_emiscoeff_netcdf.pro,v 2.0 2003/06'...
CREATION_DATE_AND_TIME
STRING 'Tue Sep 16 16:53:38 2003'
TITLE STRING 'Emissivity fit coefficients for GOES-12 SOUND'...
HISTORY STRING '$Id: compute_emissivity_coefficients.pro,v 2.'...
SENSOR_NAME STRING 'SOUNDER'
PLATFORM_NAME STRING 'GOES-12'
COMMENT STRING 'Sensor emissivity created by Convolution:Spec'...
or all the variables and their attributes:
IDL> error_status=read_ncdf('sndr_g12.EmisCoeff.nc',x,/variable_a ttributes,/quiet)
IDL> help, x, /struct
** Structure <8241f2c>, 10 tags, length=38024, data length=38024, refs=1:
RELEASE STRUCT -> <Anonymous> Array[1]
VERSION STRUCT -> <Anonymous> Array[1]
THETA_OFFSET STRUCT -> <Anonymous> Array[1]
THETA_MAX STRUCT -> <Anonymous> Array[1]
NCEP_SENSOR_ID STRUCT -> <Anonymous> Array[1]
WMO_SATELLITE_ID
STRUCT -> <Anonymous> Array[1]
WMO_SENSOR_ID STRUCT -> <Anonymous> Array[1]
SENSOR_CHANNEL STRUCT -> <Anonymous> Array[1]
WIND_SPEED STRUCT -> <Anonymous> Array[1]
EMIS_COEFFICIENTS
STRUCT -> <Anonymous> Array[1]
IDL> help, x.emis_coefficients, /struct
** Structure <8240f3c>, 3 tags, length=37464, data length=37464, refs=2:
DATA DOUBLE Array[4, 5, 18, 13]
LONG_NAME STRING 'Emissivity model fit coefficients.'
UNITS STRING 'None.'
And you can use the tag_names routine to act on just the variables.
cheers,
paulv
--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC
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| Re: on reading NCDF files [message #37765 is a reply to message #37673] |
Tue, 20 January 2004 08:12 |
Liam Gumley
Messages: 473
Registered: November 1994
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Senior Member |
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"Sangwoo" <leesw@astro.snu.ac.kr> wrote in message
news:3ee6ff80.0401192300.534dc5b1@posting.google.com...
> Hi everyone!
>
> I'm gonna read a NCDF file. It may contain several variables within
> itself. When I extract a variable named "image", the procedure is as
> follows:
>
> cid=ncdf_open('test.nc')
> vid=ncdf_varid(cid,'image')
> ncdf_varget,cid,vid,image
>
> But suppose I don't know the name of each variable unfortunately. Is
> there any way to figure out the details of the included
> variables(name, dimension, etc.)? The second command fails when I put
> a wrong variable. And is there any way to read the included variables
> all at once? (something like /all keyword)
>
> I wonder if there's any way to figure out all details of the included
> variables from an NCDF file directly.
You may wish to try the following sample programs from my book:
NCDF_VARDIR: returns a list of variable names in a netCDF file
NCDF_ATTDIR: returns a list of attribute names associated with a given
variable
The programs are available at
http://www.gumley.com/PIP/About_Book.html
in the "Sample Programs" archive files. Reading and writing netCDF files in
IDL is explained in detail in Chapter 4.
Cheers,
Liam.
Practical IDL Programming
http://www.gumley.com/
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