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

[Roger Bivand]

On Mon, 13 Oct 2008, Dylan Beaudette wrote:

> On Thursday 09 October 2008, Roger Bivand wrote:
>> 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
>
> Hi Roger:
>
> Have the changes in the rgdal package made it into the mainstream code? I have
> submitted tickets to the GRASS trac site for the v.out.ogr patch. The changes
> you suggested appear to work well.

No sign of any change in the 6.3 or 6.4 trunks, unfortunately. 7.0 has the 
same bug. No response to the posting or your tickets. The only relevant 
thread was Markus' suggesting a name change in 7.0. Hamish: could you 
please prod the person(s) needed to move on ticket 333, and get them to do 
the same on 6.4 and 7.0 (the patch is for 6.3)?

Best wishes,

Roger

>
> Cheers,
>
> Dylan
>
>
>>>> 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