Ticket #249 (assigned defect)

Opened 6 years ago

Last modified 1 year ago

[OGR-GML] GML driver doesn't recognize GML files with 0 features

Reported by: dmorissette Assigned to: warmerdam (accepted)
Priority: high Milestone:
Component: default Version: unspecified
Severity: normal Keywords: gml gfs
Cc: mloskot

Description (Last modified by mloskot)

The OGR/GML driver doesn't recognize files that contain 0 features. Everything goes well until GMLReader::PrescanForSchema?() gets called, it returns false since it doesn't find any feature class.

I guess this makes sense for datasets in general, but when we use the GML driver in the context of a WFS, receiving 0 features is a valid case.

It would be nice if OGR accepted to open GML file, and present it as a file with a single layer with 0 features in it.

What is an OGR driver expected to do if a file is empty? If nothing can be done at the OGR level for this then I guess I'll have to preparse the GML file before .

I try to open it with OGR, which is a bit of a waste of resources.

Any ideas, suggestions?

Change History

12/18/02 16:37:51 changed by warmerdam 

Daniel, 

I see your point.

I think it would be OK to treat a file as GML and return a datasource with
zero layers (since there would be no defined feature types) if the file is
clearly GML (it contains the opengis.net/gml and a featureCollection element).  

Do you really need the driver to return a dataset with one layer?  

Do you want to make these changes or do you want me to?

12/18/02 16:57:35 changed by dmorissette 

I just realized that I may have to open the XML anyway when the open() call 
fails in order to look for exceptions.  I'll get back to you on this one.

12/19/02 01:21:37 changed by dmorissette 

I modified MapServer to preparse the header of the WFS response and detect 
WFS exceptions, etc (a bit like a testopen) before calling OGR, so this is not 
an issue for me any more.

I'll let you decide if you want to mark this as invalid, or keep this as a 
potential enhancement for later.  I think it would be nice to return a 
datasource with zero layers as you suggested to indicate that the file format 
was recognized.  But this could just confuse applications too, I dunno?  I'll 
leave that up to you to decide.

Thanks for the quick reply earlier today BTW :)

02/19/04 16:28:43 changed by warmerdam 

Review for correction before GDAL 1.2.0 release.

02/27/04 14:10:40 changed by warmerdam 

I'm too nervous to fiddle with this aspect of the GML reader before the release.
Deferring.  This is more of an issue to be revisited on the next substantial 
effort to improve the GML reader.

04/09/07 13:11:39 changed by mloskot

  • description changed.

04/25/07 22:21:12 changed by mloskot

  • keywords set to gml gfs.
  • cc set to mloskot.