(carrying over to grass-dev)
On Tue, Oct 7, 2008 at 8:40 PM, Frank Warmerdam <warmerdam@pobox.com> 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/528Dylan,
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.
Sounds good 
Does anyone know how to fix
vector/v.out.ogr/main.c
to support NULLs? I see db_set_value_null() in
lib/db/dbmi_base/value.c
which might be relevant.
Markus