[R-sig-Geo] Setting FID values when saving GML files with writeOGR

[Roger Bivand]

On Fri, 26 Aug 2011, Roger Bivand wrote:

> On Thu, 25 Aug 2011, Karl Ove Hufthammer wrote:
>
>> Dear list members,
>> 
>> Is it possible to define which FID values should be used when saving a 
>> SpatialPointsDataFrame to a GML files using writeOGR (rgdal package)?
>> 
>> The FID values are stored as <ogr:layername fid="F0"> (and F1, F2, …). If I 
>> insert a new row in the SPDF, most features change their FID, which creates 
>> much unneccessary noise in our VCS,
>> so I would like to use an already existing ID as the FID.
>> 
>> I have tried change both the row.names and adding columns named
>> ‘fid’ and ‘gml_id’, the writeOGR still only uses F0, F1, … as the FIDs.
>
> No FID is set in writeOGR() in the underlying C++ function writeOGR(). One 
> could be set, but only as unique integers >= 0, not as the row.names() which 
> are strings. The result is that default FID are written. It would be possible 
> to modify the function to add integer FID, I think, but they would have to be 
> valid unique integers. I think that what gets written from there depends on 
> the driver internals, which the function does not reach. Would this help? 
> Patches welcome!

Indeed, the driver internals are a possible solution, see:

http://www.gdal.org/ogr/drv_gml.html

which suggests that some reading is possible in OGR >= 1.8.0, but that 
writing will first come with OGR >= 1.9.0, that is the development 
version, I believe. If this really matters for you, please look carefully 
at commits on the GDAL trunk in the GML driver. I think that there have 
been recent developments, with increased speed claimed. Consequently, 
there isn't much that can be done from the R side, you'll need an rgdal 
built against a future version of GDAL/OGR.

Roger

>
> Best wishes,
>
> Roger
>
>> 
>> -- 
>> Karl Ove Hufthammer
>> 
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>
>

-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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