Re: [netcdfgroup] Precision an dSignificant digits in netcdf

• To: Eric Firing <efiring@xxxxxxxxxx>
• Subject: Re: [netcdfgroup] Precision an dSignificant digits in netcdf
• From: Ismail SEZEN <sezenismail@xxxxxxxxx>
• Date: Sat, 9 Jul 2016 00:11:38 +0300

Thank you Eric. This explanation was very good.

> On 08 Jul 2016, at 23:46, Eric Firing <efiring@xxxxxxxxxx> wrote:
> 
> On 2016/07/08 10:07 AM, Ismail SEZEN wrote:
>> If we put the data aside, there is a dilemma here between header outputs
>> of netcdf file by ncdump and R:ncdf4 package. In the ncdf4 oputput,
>> add_offset and scale_factor has extra extra digits after decimal point.
>> I don’t know how to interpret.
>> 
> 
> This is just the way floating point works.  The offset, for example, is 
> stored in the netcdf file as single-precision. R has no single-precision; 
> reals are double-precision.  When it converts the single-precision number to 
> double, it is no longer the closest representation of the original decimal 
> number.
> 
> Using python:
> 
> In [1]: import numpy as np
> In [2]: x1 = np.float32('187.65')
> In [3]: x2 = np.float64('187.65')
> In [4]: x3 = np.float64(x1)
> In [5]: print(x1, x2, x3)
> 187.65 187.65 187.649993896
> 
> In [6]: x1
> 187.64999
> 
> In [7]: x2
> 187.65000000000001
> 
> In [8]: x3
> 187.64999389648438
> 
> R is showing you x3; ncdump is showing you x1, recognizing that it is the 
> single-precision representation of 187.65, as in the print statement above.  
> As you can see, x3 is not the double-precision representation of 187.65; x2 
> is.
> 
> Eric
> 
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• References:
  • [netcdfgroup] Precision an dSignificant digits in netcdf
    • From: Roy Mendelssohn - NOAA Federal
  • Re: [netcdfgroup] Precision an dSignificant digits in netcdf
    • From: Chris Barker
  • Re: [netcdfgroup] Precision an dSignificant digits in netcdf
    • From: Ismail SEZEN
  • Re: [netcdfgroup] Precision an dSignificant digits in netcdf
    • From: Eric Firing