Re: [ferret_users] precision problem on a difference between a same variable read from 2 files with the same structure

["Ansley C. Manke" <ansley.b.manke@xxxxxxxx>]

Hi Patrick,

Yes, when the second file having the same name as an axis already in use by another file in the session, Ferret checks whether the axes are the same and if so, uses it in both grids.  I think you must be correct that this is the cause of the difference you're seeing.

I think you are correct that this also has to do with the reversing of the axis coordinates and that for some reason this is done differently in the two cases.

I'll look further into this.  Thank you for sending a detailed example.

Ansley

On 6/14/2019 12:31 PM, Patrick Brockmann wrote:

Hi ferreters,

I encountered a problem when I compare a variable

read from 2 different files with exactly the same structure (same axis in particular).

The difference calculated with ferret has the following range: 

   Minimum value: -2.1516798E-11

   Maximum value: 2.15167981E-11

but the difference calculated with ncdiff has  the following range: 

   Minimum value: -2.79521E-20

   Maximum value: 2.87991E-20

--> I was expecting the same order of magnitude with ferret than with ncdiff.

More strange, if I change the name of a coordinate axis of one of the input file

then the difference calculated with ferret and ncdiff have the same order

of magnitude.

--> I suspect that the variable read from the 2nd file uses a downward axis

set from the read of the 1st file.

Attached the minimized problem with 3 small nc files and 2 test scripts.

Please try:

$ ferret -script test1.jnl

$ ferret -script test2.jnl

Tested with ferret 7.44 and 7.50

Any help welcome.

Regards

Patrick

--
Data Analysis and Visualization Engineer / Project Manager
LSCE/IPSL, CEA-CNRS-UVSQ laboratory
LSCE - Climate and Environment Sciences Laboratory
IPSL - Institut Pierre Simon Laplace
--

--
Ansley Manke
NOAA/PMEL Science Data Integration Group
7600 Sand Point Way NE
206-526-6246