Frank and Peter, Maybe we we need an ogr_cleaner app that can read in poorly structured shapefiles and normalize them to the standard. Maybe even a general topology cleaner that's file-format agnostic. Sounds like a fun summer project for a CompSci student.
And are shapefiles really being used much in the 3D world? I figured they were simply the legacy of ArcView - which is why you never really encounter shapefiles with Z values or Multipatch. -Eric -=--=---=----=----=---=--=-=--=---=----=---=--=-=- Eric B. Wolf 720-209-6818 USGS Geographer Center of Excellence in GIScience PhD Student CU-Boulder - Geography On Mon, Jan 26, 2009 at 11:34 AM, Frank Warmerdam <[email protected]>wrote: > Rushforth, Peter wrote: > > The shapefile spec for multipatch shapefiles says this: > >> >> "When the individual types of rings in a collection of rings representing >> a >> polygonal patch with holes is unknown, the sequence must start with First >> Ring, followed by a number of Rings" (p 21). >> >> Figure 3 shows a "First Ring" as an inner ring with an exterior ring as a >> "Ring". >> >> Does this mean a receiving application must be prepared to figure out the >> (3 >> dimensional) topology of the rings it reads from a shapefile? Why? When >> would this ever be the case? >> > > Peter, > > 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. > > > I mean an application that doesn't know which > > is the exterior ring from the interior rings of an internal data > structure > > has no business writing them out and thus make them someone else's > problem, > > in my view. > > Everyone is entitled to an opinion... > > I would note that multi-part shapefiles are (in my experience) very rare. > > A more serious issue is that there are lots of applications that have > written out polygon shapefiles without following the specification properly > resulting in improper ring orientation and/or ordering meaning that an > application that wants to play it safe needs to do detailed geometric > analysis on polygon shapefiles too. > > In the past shapelib and OGR were quite careless about how they wrote > out shapefiles, and part of my penance is that they now have to do > geometric > analysis when reading which can be quite expensive. > > Best regards, > -- > > ---------------------------------------+-------------------------------------- > I set the clouds in motion - turn up | Frank Warmerdam, > [email protected] > light and sound - activate the windows | http://pobox.com/~warmerdam > and watch the world go round - Rush | Geospatial Programmer for Rent > > > _______________________________________________ > Discuss mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/discuss >
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