Re: reading raw (packed) data from NetCDF files and avoiding missing-value check

• To: netcdf-java@xxxxxxxxxxxxxxxx
• Subject: Re: reading raw (packed) data from NetCDF files and avoiding missing-value check
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Thu, 26 Oct 2006 11:17:48 -0600

Hi Jon:

One obvious thing would be to open it as a NetcdfFile, not a GridDataset. Is 
that a possibility?

Jon Blower wrote:

Hi,

I'm writing an application that reads data from NetCDF files and
produces images.  I've noticed (through profiling) that a slow point
in the data reading process is the unpacking of packed data (i.e.
applying scale and offset) and checking for missing values.  I would
like to minimize the use of these calls.

To cut a long post short, I would like to find a low-level function
that allows me to read the packed data, exactly as they appear in the
file.  I can then "manually" apply the unpacking and missing-value
checks only to those data points that I need to display.

I'm using nj22, version 2.2.16.  I've tried reading data from
GeoGrid.subset() but this (of course) performs the unpacking.  I then
tried getting the "unenhanced" variable object through
GeoGrid.getVariable().getOriginalVariable(), but (unexpectedly) this
also seems to perform unpacking and missing-value checks - I expected
it to give raw data.

Can anyone help me to find a function for reading raw (packed) data
without performing missing-value checks?

Thanks in advance,
Jon

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• References:
  • reading raw (packed) data from NetCDF files and avoiding missing-value check
    • From: Jon Blower