[gdal-dev] Bogus <gml:Face> interpretation

[Chaitanya kumar CH]

strk,

Thank you very much for pointing out the errors. I could blame it on the
lack of enough sample data but that's just laziness. I should have read the
specifications more throughly.

I see your first point but I need to dig deeper for the second.

Please file a ticket at http://trac.osgeo.org/gdal/newticket
I would be grateful if you can provide some sample data.

On Sun, Jan 30, 2011 at 4:39 PM, strk <strk at keybit.net> wrote:

> Hello,
> I'm writing GML output routines for Topologically-defined features
> in PostGIS and found what I think is a bug in how ogr interprets
> the <gml:Face> tag contents.
>
> Take this topology:
>
>  n1
>  +-----e1-->-----.
>  |               |
>  |      F1       |
>  |           n3  |
>  |   ,-e2->-+    |
>  |   |      |    |
>  |   |  F2  |    |
>  |   |      |    |
>  |   +-<-e7-'    |
>  | n4            |
>  |               |
>  `---<-e2--------+
>                   n2
>
> It occurs to me that face F1 above is bounded by all edges,
> not just the exterior ones, so I'd put _all_ edges inside
> one gml:Face tag:
>
>  <Face id="F1">
>  <directedEdge orientation="-" id="e1" />
>  <directedEdge orientation="-" id="e2" />
>  <directedEdge id="e3" />
>  <directedEdge id="e4" />
>  </Face>
>
> This is expressed clearly in the OGC 03-105r1 document (GML-3.1.1, 2004)
> and 07-036 (GML-3.2.1, 2007)
>
> <<
> The non-dangling edges in the boundary of a face comprise one or more
> topological rings. Each such ring consists of directedEdges connected
> in a cycle, and is oriented with the face on its left.
> >>
>
> Now, when encountering such a GML snippet, ogr2ogr (GML driver)
> insists in considering all edges as being part of the same ring
> thus producing an invalid polygon as a result.
>
> It basically interprets a <gml:Face> tag as if it was a ring, which
> I belive is wrong.
>
> Another example of such invalid intepretation follows:
>
>  +------+------+
>  | F1   |  F2  |
>  +------+------+
>
> A surface/polygon formed by the two faces above (F1,F2) should be
> represented as:
>
>  <TopoSurface id="P1">
>   <directedFace> <Face id="F1"/> </directedFace>
>   <directedFace> <Face id="F2"/> </directedFace>
>  </TopoSurface>
>
> Whereas the resulting feature geometry should be a single-ring polygon:
> the topological _union_ of the two faces (I haven't tested this but
> I belive GDAL would get this wrong as well).
>
> On the PostGIS side, I've so far implemented the "invalid" representation
> for the sake of interoperability with GDAL, but being a new implementation
> I'd rather get it right from the start...
>
> Note that using the "invalid" (but GDAL-compatible) representation also
> has the negative effect of making it _impossible_ to map PostGIS(ISO)
> topologies to GML in a lossless way (PostGIS topology faces would have
> no direct corrispondence to <gml:Face> tags).
>
> I hope, with this mail, to get some feedback from the authors of the
> GML reading capabilities of GDAL to plan actions towards interoperability
> of the two systems and adherence to the standard (if possible).
>
> --strk;
>
>  ()   Free GIS & Flash consultant/developer
>  /\   http://strk.keybit.net/services.html
> _______________________________________________
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>



-- 
Best regards,
Chaitanya kumar CH.
/tʃaɪθənjə/ /kʊmɑr/
+91-9494447584
17.2416N 80.1426E
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