[netCDF #TWD-935825]: Format of history attribute

Hi Phil,

> Thanks for replying to my recent comment against CF ticket 76 regarding
> encoding of multiple string values within the global history attribute.
> I'd forgotten that netcdf-3 only supports char vectors, and not string
> vectors.
> 
> Part of the reason for me overlooking that fact was the following
> partial ncdump header listing of a classic netcdf file which has been
> updated multiple times by the ncatted tool.  On the face of it, this
> appears to show a history attribute consisting of multiple strings (6 to
> be exact).  I just happened to use this file as a reference point when I
> was formulating my CF comment!
> 
> $ ncdump -h tasmean-1960-90-ll.nc
> ...
> // global attributes:
> :history = "Fri Aug  1 13:41:42 2008: ncatted -a
> long_name,air_temperature1,m,c,Temperature at 1.5 m
> tasmean-1960-90-ll.nc\n",
> "Fri Aug  1 13:39:52 2008: ncatted -a
> long_name,air_temperature1,c,c,Temperature at 1.5 m
> tasmean-1960-90-ll.nc\n",
> "Tue Apr  1 10:56:27 2008: ncatted -a axis,air_temperature1,d,c,
> boundsupdate.nc\n",
> "Tue Apr  1 10:56:12 2008: ncatted -a
> standard_name,air_temperature1,m,c,air_temperature boundsupdate.nc\n",
> "Tue Apr  1 10:54:26 2008: ncatted -a
> standard_name,air_temperature1,m,c,temperature boundsupdate.nc\n",
> "Tue Apr  1 10:54:00 2008: ncatted -a axis,air_temperature1,m,c,TZYX
> boundsupdate.nc" ;
> 
> $ ncdump -k tasmean-1960-90-ll.nc
> classic
> 
> And if I display the global attributes using ncks I get the following:
> 
> $ ncks -M tasmean-1960-90-ll.nc
> Opened file tasmean-1960-90-ll.nc: dimensions = 5, variables = 9, global
> atts. = 2, ID = 65536, type = NC_FORMAT_CLASSIC
> Record dimension: name = time, size = 1
> Global attribute 0: Conventions, size = 7 NC_CHAR, value = CF-1.0
> Global attribute 1: history, size = 590 NC_CHAR, value = Fri Aug  1
> 13:41:42 2008: ncatted -a long_name,air_temperature1,m,c,Temperature at
> 1.5 m tasmean-1960-90-ll.nc
> Fri Aug  1 13:39:52 2008: ncatted -a
> long_name,air_temperature1,c,c,Temperature at 1.5 m
> tasmean-1960-90-ll.nc
> Tue Apr  1 10:56:27 2008: ncatted -a axis,air_temperature1,d,c,
> boundsupdate.nc
> Tue Apr  1 10:56:12 2008: ncatted -a
> standard_name,air_temperature1,m,c,air_temperature boundsupdate.nc
> Tue Apr  1 10:54:26 2008: ncatted -a
> standard_name,air_temperature1,m,c,temperature boundsupdate.nc
> Tue Apr  1 10:54:00 2008: ncatted -a axis,air_temperature1,m,c,TZYX
> boundsupdate.nc
> 
> which looks as expected, i.e. a text string with newline chars honoured.
> 
> So is the first example simply a manifestation of the way in which
> ncdump (we're using v4.1.2) is programmed to display char attributes
> containing embedded \n characters, perhaps?

Yes, exactly!  I see the ncdump man-page documentation doesn't mention the
handling of "\n" characters when displaying character-variable values, so
there was no way you could have known this without reading the ncdump
source code, and this behavior goes away for netCDF-4 files that have real 
strings:

        case '\n':              
            /* Only generate linebreaks after embedded newlines for
             * classic, 64-bit offset, or classic model files.  For
             * netCDF-4 files, don't generate linebreaks, because that
             * would create an extra string in a list of strings.  */

I'll put adding information about this little idiosyncracy in ncdump
documentation on my list of things to do.  Thanks for pointing out the
problem!

--Russ

> PS:  Hope the netcdf workshop at ESRI this week is useful. I shall look
> forward to reading about any decisions/outcomes for enhanced ArcGIS 10.x
> capabilities.

It was a very interesting meeting for me, and I hope for ESRI, too.  If I
ever get around to using the Unidata Developers Blog to write some summary
notes, I'll let you know ...

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



Ticket Details
===================
Ticket ID: TWD-935825
Department: Support netCDF
Priority: Normal
Status: Closed