[Shapelib] Handling polygon rings in shapefiles
brianp at occinc.com
Mon Oct 10 16:40:13 EDT 2005
I was hoping someone could give me some insight into how to handle
polygon rings in shapefiles.....
I have looked at the ERSI spec. It says a polygon feature (made up of 1
or more parts) can have inner and outer rings. And the outer ring is
clockwise while the inner ring is counter clockwise. It also says the
order of the parts irrelevant. I guess then I would have to look at the
read shape to see if it is clockwise or counter clockwise to see if it
is an inner or an outer. A pain, but it works.
But then I also see that a polygon feature (made up of 1 or more parts)
can have multiple outer rings. Again, I can look at the
clockwise/counter-clockwise to figure out if is an inner or outer ring.
But if I have multiple outer rings and multiple inner rings and the
order doesn't matter, how do I figure out which inner ring goes with
which outer ring? Do I need to determine containment?
What if I have an outer ring containing and outer ring containing an
inner ring? How do I know which outer ring the inner ring is for? The
spec tells me a ring can not intersect, but it doesn't say anything that
an outer ring can not contain an outer ring.
Any insight into this would be extremely helpful. Any pointers on using
the shapelib library to help with this will also be helpful. Thanks in
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