[Freegis-list] shapelib question

Timothy H. Keitt Timothy.Keitt at StonyBrook.Edu
Fri Apr 6 17:28:52 CEST 2001

Frank Warmerdam wrote:

> Multi-part polygons may be multiple islands (one feature, but disconnected
> outer rings), or one polygon with holes (or a mixture).  In theory you
> are supposed to be able to determine this from the winding direction of
> the rings (whether they are outer or inner edges) but in practice many 
> systems don't setup this distinction up properly (like most applications
> using Shapelib to write) so you pretty much have to determine yourself. 

(O.K. basic geometry here, but bear with me.)  If turning right is negative
and turning left is positive, then if the sum of turning angles is negative,
the winding direction is clockwise, no?  Is there a convention that inner
edges are digitized in a particular direction?

My dilema is whether to use an array of polygons in postgresql, or to use a
seperate table with multiple entries corresponding to a given "shape."  I don't
know if postgesql polygons can contain (or understand) holes or not.


Timothy H. Keitt
Department of Ecology and Evolution
State University of New York at Stony Brook
Phone: 631-632-1101, FAX: 631-632-7626

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