[GRASS-stats] Re: [R-sig-Geo] writing shapefiles / DBF files when input data contains NA

[Roger Bivand]

On Tue, 7 Oct 2008, Dylan Beaudette wrote:

> On Tue, Oct 7, 2008 at 6:26 PM, Hamish <hamish_b at yahoo.com> wrote:
>> Dylan:
>>> It looks like the limiting factor in this equation is the
>>> code used in v.out.ogr.

Following Dylan's posting on the GDAL list, and Frank's response, I 
suggest the following simple patch to vector/v.out.ogr/main.c (here 6.3):

$ diff -u main.c_old main.c
--- main.c_old  2008-10-09 10:54:19.000000000 +0200
+++ main.c      2008-10-09 11:30:34.000000000 +0200
@@ -625,7 +625,9 @@
                         colsqltype = db_get_column_sqltype(Column);
                         colctype = db_sqltype_to_Ctype ( colsqltype );
                         G_debug (2, "  colctype = %d", colctype );
-                       switch ( colctype ) {
+/* RSB 081009 emit unset fields */
+                        if (!db_test_value_isnull(Value)) {
+                         switch ( colctype ) {
                              case DB_C_TYPE_INT:
                                 OGR_F_SetFieldInteger( Ogr_feature, j, db_get_va
lue_int(Value) );
                                 break;
@@ -639,7 +641,8 @@
                                 db_convert_column_value_to_string (Column, 
&dbst
ring);
                                 OGR_F_SetFieldString( Ogr_feature, j, db_get_str
ing (&dbstring) );
                                 break;
-                       }
+                         }
+                        } /* RSB */
                     }
                 }
             }

In 6.4 this is after line 717. Essentially it just uses 
db_test_value_isnull() not to set values in OGR fields if the DB field 
value is NULL, and follows Frank's suggestion.

This matches code near line 939 in OGRGRASSLayer::SetAttributes()
gdal/ogr/ogr_frmts/grass/ogrgrasslayer.cpp in the vector plugin, which 
uses:

if ( !db_test_value_isnull(value) )

I'm sure the patch needs checking, but with changes in the R rgdal package 
to support vector null data correctly, it ought to improve the interface.

Best wishes,

Roger

>>
>> maybe a silly question, but is a 3rd party format even needed here?
>>
>> $ ogrinfo --formats | grep -i grass
>>  -> "GRASS" (readonly)
>>
>> at least in the one direction.
>>
>
> Excellent question. I had also wondered about this. It looks like
> there is a new argument in readVECT():
>
> # read in directly with GDAL/OGR -- no intermediate file:
> x <- readVECT6('xxx', plugin=TRUE)
>
> This is quite fast and depends on the GDAL-GRASS plugin... However,
> NULL data in a GRASS table is not imported correctly-- character
> fields are imported as '', and numeric fields as 0.
>
> So... Is the error in GDAL itself?
>
> I tried inspecting a vector from GRASS with NULL data in some of the
> columns from the table, using ogrinfo -al
> location/mapset/vector/xxx/head
>
> OGRFeature(1):23
>  cat (Integer) = 24
>  cat_ (Integer) = 24
>  str1 (String) = (null)
>  xyz (Real) = (null)
>  abc (Integer) = (null)
>  POINT (591583 4925280 0)
>
> ... which seems to correctly report the NULL values.
>
> This leads me to suspect that something in readOGR() and writeOGR are
> at fault in handling of NULL values.
>
> Unfortunately looking at the rgdal source code wasn't very productive
> (my fault).
>
> Cheers,
>
> Dylan
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no