Trouble handling a NetCDF4 file

Question

I have trouble working with a NetCDF4 file. Can anyone please help me. Reading it in seems to work.

import netCDF4
f = netCDF4.Dataset('mydata.nc', 'r')

When I try to investigate what's inside the file (>>> print f), I get some information:

<type 'netCDF4.Dataset'>
root group (NETCDF4 file format):
dimensions: soundings, levels
variables: 
groups: Retrieval, Sounding

...

print f.dimensions 

gives me:

OrderedDict([(u'soundings', <netCDF4.Dimension object at 0x2bd24b0>), 
(u'levels', <netCDF4.Dimension object at 0x2bd2500>)])

From what I read in tutorials I should be able to check the length of the different dimensions by typing

print len(soundings) 

But I receive the error message "name 'soundings' is not defined". Does anybody know what I might do wrong? Thanks.

Answer 1

You're getting the error because you haven't defined a variable named soundings. If you define this variable by

soundings = f.dimensions[u'soundings']

then you should be able to find the length of soundings using print len(soundings).

Alternatively, you can access the length of the 'soundings' dimension directly by using

print len(f.dimensions[u'soundings'])

I have to admit, I've not used netCDF4, so I read the netCDF4 documentation briefly. In the section 'Dimensions in a netCDF file' it contains the following example of displaying the dimensions of a netCDF4 dataset:

>>> print rootgrp.dimensions
OrderedDict([('level', <netCDF4.Dimension object at 0x1b48030>),
             ('time', <netCDF4.Dimension object at 0x1b481c0>),
             ('lat', <netCDF4.Dimension object at 0x1b480f8>),
             ('lon', <netCDF4.Dimension object at 0x1b48a08>)])

(For brevity, I've omitted the details about where rootgrp comes from.) The next line of code in the following code fragment is this:

>>> print len(lon)

What you might have missed is that the variable lon was declared further up, as

>>> lon = rootgrp.createDimension('lon', 144)

The above section of tutorial deals with creating new dimensions in a netCDF file, whereas you are reading existing dimensions from a netCDF file. You must therefore fetch the dimensions out of the netCDF file.