Re: [ferret_users] minor issue: "Ignoring erroneous point_spacing=even attribute."

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

    Hi Ryo,
    Yes, this is somewhat of a known problem; I've also occasionally
    seen things of this sort.  When a coordinates are read in on
    initializing a dataset, they're checked for regular spacing using
    the data type of the coordinates in the file, so a floating-point
    axis is checked using single-precision testing. And there's a sanity
    check on axes marked as regularly-spaced.  Apparently in the past
    there were egregious errors because a truly irregular axis was
    marked as regular. (Interestingly, point_spacing is not in the CF
    standard, but does seem to be widely used and goes back to early
    days of Ferret.) So datasets that are correct are treated correctly
    and any calculations using coordinates are fine when working with
    that file. But, on writing a netCDF dataset, Ferret/PyFerret always
    writes coordinates in double precision.  This is where we wind up
    getting a bit of inaccuracy.
    
    It looks as  though we changed the testing for regular-spacing
    somewhere at about version 7.4, and I'll have a look at that.  Also,
    we've been working along in general on changes so that any netCDF
    files that Ferret writes as subsets or otherwise based on netcdf
    files, match what is in the input files.  Things like upcasing
    variable names and ignoring attributes should at least be
    controllable. In recent versions we write data using the same data
    type as the input file, and it should be easy to do the same for
    axes, so for this example the y-axis would be written as single
    precision coordinate variable.  So we'll have a look at that too.
    
    Ansley

On 7/3/2020 6:51 AM, Ryo Furue wrote:

Ferret Team,

I searched our mailing archive but didn't find it there.

My original netCDF file has an x axis like

   float lat = 9.95, 10.05, 10.15, . . .

without the "point_spacing=even" attribute.

I open the file in PyFerret and save just one timestep (L) into a new file:

    save/file=tmp.nc/l=10 var

I then open this new file and PyFerret says,

    *** NOTE: Axis "LAT" is irregular. Ignoring erroneous point_spacing=even attribute.

Indeed the new file has gotten the "point_spacing=even" attribute.

I come across this problem from time to time.  (Today I reproduced this on PyFerret v7.5 on Linux.)

The axis is a single-precision real and the gridpoints may not be  precisely represented.  So, I'm not surprised that this axis isn't regarded as "even".  That is, PyFerret is doing the right thing when it reads the new netCDF file.  So, it's probably a mistake that it added the "even" attribute when it saved the data.

Strangely, the longitude axis -0.05, 0.05, 0.15, . . . , which originally didn't have a point_spacing attribute either, got a point_spacing = "uneven" attribute in the new file!

Cheers,

Ryo

-- 
Ansley Manke
Science Data Integration Group
NOAA Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle WA 98115

I am currently teleworking and am available Tue-Wed-Thu.