[OSGeo-Discuss] multipatch shapefile FIRST_RING structure

Frank Warmerdam warmerdam at pobox.com
Mon Jan 26 11:10:45 PST 2009

Rushforth, Peter wrote:
> 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.


I would not expect to do the analysis in 3D as for my purposes I
am only interested in 2.5D geometry sets.  I believe multipatch
files are normally used to represent TINs which can be safely
analysed in 2D, and the Z values just carried along.

> 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
> near-spaghetti.

I agree.

>> I would note that multi-part shapefiles are (in my 
>> experience) very rare.
> I guess it depends on where you're swimming...


Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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