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.
Peter,
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...
Indeed.
Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, warmer...@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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