[netCDF #AEB-315486]: nf90_float too accurate...

Hi George,

> Hi there,
> 
> I was wondering if you could help me.  I have searched the mailing lists
> but can't find anything suitable.
> 
> I am using netcdf as the format for output from an atmospheric model.
> The data in the netcdf is used solely for color contour plotting using
> IDL. The problem I have is that the nf90_float data is much larger than
> it needs to be.  For instance a typical number written out from my idl
> code is:
> 
> 2.43866705894470210000e+000
> 
> There is way more accuracy in this number than needed for color
> applications - a simple:
> 
> 2.439e+00 would be perfectly adequate.
> 
> Why this is a problem is that the datafiles are very large - because
> they are large 2D arrays so the nf90_float accuracy is a problem.
> 
> What I would ideally like is something like an NF90_smallfloat or
> something.  Or is there another way of doing this ?
> 
> I am using netcdf 3.5 right now btw.

The nf90_float type is only a 32-bit float, with about 5 significant
digits of precision, as documented here:

  
http://www.unidata.ucar.edu/netcdf/old_docs/really_old/f90-html-docs/guide7.html

If you're seeing more precision than that, it may be the result of
printing the values as double-precision.  

Arrays of nf90_float values require 4 bytes of storage for each value
in a binary netCDF file.  If you want to use less storage than that,
you could pack the values into 2-byte nf90_short values, as described
here:

  
http://www.unidata.ucar.edu/netcdf/docs/BestPractices.html#Packed%20Data%20Values

using the add_offset and scale_factor attributes for the packed variable.

If you want to see the values with less precision when you print them out
with the ncdump utility, you can use the "-p float_digits[,double_digits]"
option to ncdump, as documented here:

  http://www.unidata.ucar.edu/netcdf/docs/ncdump-man-1.html

If we've misunderstood your question, please let us know.

--Russ

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



Ticket Details
===================
Ticket ID: AEB-315486
Department: Support netCDF
Priority: Normal
Status: Closed