[OSGeo-Discuss] multipatch shapefile FIRST_RING structure

Rushforth, Peter Peter.Rushforth at NRCan-RNCan.gc.ca
Mon Jan 26 10:50:54 PST 2009

Hi Frank,

Thanks for your response.

> An application that wants to process multi-patch shapefiles, 
> including those using the FirstRing/Ring part types and that 
> cares about nesting of rings must be prepared to do a 
> geometric analysis to determine which are inner and outer.

This is in 3d we're talking right, so 2d libraries won't work
unless the rings are transformed to the horizontal prior to analysis.

The odd thing is that there are also OUTER_RING and
INNER_RING structures.  FIRST_RING/RINGis used seemingly to signal
a potential polygon in which the rings must be analysed.  I say
potential, because there's no guarantee that the rings don't
overlap, for instance, so this structure must be used to signal

> I would note that multi-part shapefiles are (in my 
> experience) very rare.

I guess it depends on where you're swimming...

Thanks again,

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