Michaël Michaud wrote: > > ... about the second point, as you know SFS a no one, you may have an > answer to the following question : > why polylines may be made of lines sharing their boundaries > and multipolygons cannot be made of polygons sharing their edges (if > that's what "shells can touch at any finite number of points" means) > That *is* what I meant by "shells can touch at any finite number of points".
I can't see into the minds of the standards authors, but speaking as a developer who has implemented the standard, the restriction on how polygon shells touch makes for nicer topological assumptions, and hence simpler code. With this rule you can assume that all edges in polygon geometries are truly on the topological boundary - i.e that they divide the polygon interior from the exterior. Without this assumption, you have to basically do a "dissolve" to remove "interior" edges before doing any further processing. Polylines don't offer quite such a strong topological distinction (they don't divide the world into interior and exterior), so there aren't any unpleasant implications to just letting their linework "go anywhere it wants to". There are other assumptions which SFS could have made to make processing simpler - such as enforcing an orientation for polygon rings. But I guess they wanted to open the model up as much as reasonable, so as not to make a whole bunch of geometry invalid. Although then they could have allowed "inverted" polygons (self-touching at a point - ArcSDE style) as well. M -- Martin Davis Senior Technical Architect Refractions Research, Inc. (250) 383-3022 ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace _______________________________________________ Jump-pilot-devel mailing list Jump-pilot-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel
