writing shapefiles / DBF files when input data contains NA

On Tuesday 07 October 2008, Roger Bivand wrote:

On Mon, 6 Oct 2008, Dylan Beaudette wrote:

Hi,

I have noticed that saving data to files that include a DBF, result in
bogus data where there were NA. Using the write.dbf() function from
the foreign package seems to work a little better, but I still get odd
results in numeric columns. Writing to GRASS with the methods in the
spgrass6 package results in some thing that looks like this:

Dylan,

I'm afraid that there is no good solution for this at all. DBF does not
seem to have a clear and uniform NA treatment (or even !is.finite()
treatment). The only work-around is to preprocess the data.frame in the
output object to insert known NODATA values, and to replace those flags
manually on the GRASS side. This could possibly be written as a wrapper
around writeVECT6(). The help page does say:

     "Please note that the OGR drivers used may not handle missing data
      gracefully, and be prepared to have to correct for this manually.
      For example use of the 'readOGR' PostGIS driver directly may
      perform better than moving the data through the DBF driver used in
      this function - or a PostgreSQL driver used directly or through
      ODBC may be a solution. Do not rely on missing values of vector
      data moving smoothly across the interface."

I did try to look at the SQLite driver on the GRASS side, which might be
more robust, but did not see how to proceed.

One possibility is not to recode, but to build an NA mask on the R side,
and then loop over fields on the GRASS side for the chosen driver
inserting NAs in the correct rows (whatever the syntax for that might
be). Would this be db.execute with an insertion of SQL NULL?

Can we redirect this discussion to the statgrass list, because GRASS
developers follow that list?

Best wishes,

Roger

Sorry for the cross-posting. Wanted to clarify where this thread is
going/went.

Hi Roger--

It looks like the limiting factor in this equation is the code used in
v.out.ogr.

From the GRASS-dev + Frank W's help:

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,

Once you establish which GRASS attributes are NULL, you can ensure they
are pushed out to OGR as null by just skipping the step that sets them.
Perhaps that will help a bit.

So, once v.out.ogr is fixed, this should clear up several issues:

1. import of vector data into R via spgrass6 methods
2. better compatibility of vector data exported from GRASS

I still do not know why writeOGR() does not create correct DBF files... it
may be related to the code in v.out.ogr....

Cheers,

Dylan