[Gdal-dev] UK OS Mastermap GML

[Martin Daly]

Roger, Frank,

> > The main problem with OS Mastermap data is that OGR's GML reader
looks 
> > specifically for featureMember elements to identify features.  When
I add
> > osgb:cartographicMember to the IsFeature() test, the Mastermap data
seems
> > to more or less work.

This approach will "more or less" work with one flavour of OS MasterMap,
Independent Polygons, where each feature defines its own geometry.
However, there is another flavour, Topological, where Polygons are
specified as in the following snippet:

<osgb:topographicMember>
<osgb:TopographicArea fid='osgb1000000756121907'>
<osgb:featureCode>10096</osgb:featureCode>
<osgb:version>1</osgb:version>
<osgb:versionDate>2001-11-07</osgb:versionDate>
<osgb:theme>Land</osgb:theme>
<osgb:calculatedAreaValue>50.956252</osgb:calculatedAreaValue>
<osgb:changeHistory>
<osgb:changeDate>1970-01-01</osgb:changeDate>
<osgb:reasonForChange>New</osgb:reasonForChange>
</osgb:changeHistory>
<osgb:descriptiveGroup>Landform</osgb:descriptiveGroup>
<osgb:descriptiveTerm>Slope</osgb:descriptiveTerm>
<osgb:make>Manmade</osgb:make>
<osgb:physicalLevel>50</osgb:physicalLevel>
<osgb:polygon>
<osgb:outerBoundaryIs>
<osgb:Ring orientation='anticlockwise'>
<osgb:ringMember orientation='-' xlink:href='#osgb1000000756121843'/>
<osgb:ringMember orientation='-' xlink:href='#osgb1000000756121831'/>
</osgb:Ring>
</osgb:outerBoundaryIs>
</osgb:polygon>
</osgb:TopographicArea>
</osgb:topographicMember>

Note also that the referenced osgb:ringMember-s may appear either
earlier or later in the stream, or, in the case of change-only updates,
might not appear at all.

Finally, there is a new "layer" of OS MasterMap Integrated Transport
Network (ITN) that makes even more use of topological structures, that
is unlikely to work well with Frank's short term fix.

Regards,
Martin