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[netCDF #VMR-409919]: HELP: netCDF data access and export in Python

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  • Subject: [netCDF #VMR-409919]: HELP: netCDF data access and export in Python
  • Date: Tue, 15 Mar 2011 15:44:56 -0600
Hi Manuel,

> I have to select and export oceanographic data from a netCDF file depending 
> on time slices.
> I am using the netCDF4 library together with numpy, scipy and gdal.
> So far I can already read the parts of the nc file (dimensions, variables, 
> attributes).
> 
> Now my questions:
> 
> 1) How can I access the "water surface elevation" variable values for a 
> certain time slice?
> I don't really understand the principle, because if I print the variable I 
> get all values, of course.
> So, may I have to join the time to the elevation values at first? I don't 
> know how the records are assigned to the time values.

Sorry, we can answer such a question for the C, Java, C++, or Fortran 
interfaces, but all the Python 
APIs for netCDF are developed and supported elsewhere, and they have different 
APIs.  For example,
if you're using Jeff Whitaker's netCDF-4 module for python: 
http://code.google.com/p/netcdf4-python/,
you should contact him for answers to questions about his API.

> 2) What kind of data type should I use? (multidimensional array, ... because 
> the file has >7GB)

I expect you'd want a 2D array for "water surface elevation" at a specified 
time.  It should
be possible to read in each time slice separately into such a 2D array.

> 3) I want to use GDAL for generating a Shapefile out of the selected time 
> slices. Is it better to cache the selected data (to ASCII file, ...) or doing 
> a direct shp creation? Considering the read/write performance.

Sorry, we didn't develop and don't support the GDAL library either, so I can't 
answer that
question.

--Russ

> Here's a part of the file structure:
> ----------------------------------------
> dimensions:
> time = unlimited (49 currently)
> zip2 = 1076093
> 
> variables:
> double time(time)
> time:long_name = "coordinate variables of time"
> time:units = "seconds since 2010-08-09"
> 
> float e (time,zip2)
> e:long_name = "water surface elevation from still water level"
> e:units = "meter"
> e:figure = "scalar"


Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: VMR-409919
Department: Support netCDF
Priority: Normal
Status: Closed