Setting FID values when saving GML files with writeOGR
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
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo
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