[gdal-dev] interpretation of nodata in OGR libraries

Dylan Beaudette dylan.beaudette at gmail.com
Tue Oct 7 17:00:48 EDT 2008


On Tuesday 07 October 2008, Frank Warmerdam wrote:
> Dylan Beaudette wrote:
> > Hi,
> >
> > I am not sure if this is a DBF issue, or something deeper in OGR, but
> > there does not appear to be a mechanism for preserving 'nodata' or NULL
> > values when exporting from GRASS to vector formats through v.out.ogr.
>
> ...
>
> > It seems that on export via OGR there is some kind of mangline of nodata.
> > This issue is a serious problem anytime you need to move vector data
> > between applications, through the use of an intermediate file (usually a
> > shapefile).
> >
> > Any ideas on what could be causing this problem? Fixing it would solve
> > several important issues. If need be I will file a new ticket on the
> > topic.
> >
> > Relevent tickets?
> > http://trac.osgeo.org/gdal/ticket/809
> > http://trac.osgeo.org/gdal/ticket/528
>
> Dylan,
>
> Generally speaking OGR understands about null attribute values, though
> there may be no way to represent them in some formats.  They should work
> fine in shapefile format though.
>
> Reviewing the mk_att() function in v.out.ogr's main.c I don't see
> any effort to distinguish between null and non-null attributes so
> I think the problem is just a limitation of v.out.ogr.  If possible,
> I'd suggest fixing it there.
>
> Best regards,

Thanks a lot Frank. You never fail to impress when it comes to finding bugs!
I will follow this up over on the GRASS-dev list. 

Cheers,
Dylan


-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341


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