Re: [thredds] Incorrect values for elevation (WMS) [SEC=UNCLASSIFIED]

• To: Jon Blower <j.d.blower@xxxxxxxxxxxxx>
• Subject: Re: [thredds] Incorrect values for elevation (WMS) [SEC=UNCLASSIFIED]
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Tue, 03 Aug 2010 07:18:12 -0600

Jon Blower wrote:

The netCDF file stored the depths as single-precision floats, and the

NetCDF-Java lib reads these floats and converts to double-precision floats. If I would guess, that

appears to be the step that introduces the low bit errors in the values.

Hi Tim – yes, the conversion from 32- to 64- bit real numbers introduces spurious precision. I think this is a general feature of digitised floating-point numbers. In Java we don’t have to worry about zeroing out arrays and such-like – memory is “safe”. We could ensure that the numbers are only displayed to 32-bit precision even if they are 64-bit internally – I’ve filed a “todo” for this (http://www.resc.rdg.ac.uk/trac/ncWMS/ticket/184).

Cheers, Jon

 float f = (float) d;

im not sure what the JLS (java language spec) says about that - it may be JVM 
dependent.

• References:
  • Re: [thredds] Incorrect values for elevation (WMS) [SEC=UNCLASSIFIED]
    • From: Jon Blower
  • Re: [thredds] Incorrect values for elevation (WMS) [SEC=UNCLASSIFIED]
    • From: Tim Pugh
  • Re: [thredds] Incorrect values for elevation (WMS) [SEC=UNCLASSIFIED]
    • From: Jon Blower