Frank and Peter,<div><br></div><div>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.</div>
<div><br></div><div>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.</div><div><br>
</div><div>-Eric</div><div><br></div><div>-=--=---=----=----=---=--=-=--=---=----=---=--=-=-<br>Eric B. Wolf 720-209-6818<br>USGS Geographer<br>Center of Excellence in GIScience<br>PhD Student <br>
CU-Boulder - Geography<br><br>
<br><br><div class="gmail_quote">On Mon, Jan 26, 2009 at 11:34 AM, Frank Warmerdam <span dir="ltr"><<a href="mailto:warmerdam@pobox.com">warmerdam@pobox.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div class="Ih2E3d">Rushforth, Peter wrote:<br>
> The shapefile spec for multipatch shapefiles says this:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
"When the individual types of rings in a collection of rings representing a<br>
polygonal patch with holes is unknown, the sequence must start with First<br>
Ring, followed by a number of Rings" (p 21).<br>
<br>
Figure 3 shows a "First Ring" as an inner ring with an exterior ring as a<br>
"Ring".<br>
<br>
Does this mean a receiving application must be prepared to figure out the (3<br>
dimensional) topology of the rings it reads from a shapefile? Why? When<br>
would this ever be the case?<br>
</blockquote>
<br></div>
Peter,<br>
<br>
An application that wants to process multi-patch shapefiles, including those<br>
using the FirstRing/Ring part types and that cares about nesting of rings<br>
must be prepared to do a geometric analysis to determine which are inner<br>
and outer.<div class="Ih2E3d"><br>
<br>
> I mean an application that doesn't know which<br>
> is the exterior ring from the interior rings of an internal data structure<br>
> has no business writing them out and thus make them someone else's problem,<br>
> in my view.<br>
<br></div>
Everyone is entitled to an opinion...<br>
<br>
I would note that multi-part shapefiles are (in my experience) very rare.<br>
<br>
A more serious issue is that there are lots of applications that have<br>
written out polygon shapefiles without following the specification properly<br>
resulting in improper ring orientation and/or ordering meaning that an<br>
application that wants to play it safe needs to do detailed geometric<br>
analysis on polygon shapefiles too.<br>
<br>
In the past shapelib and OGR were quite careless about how they wrote<br>
out shapefiles, and part of my penance is that they now have to do geometric<br>
analysis when reading which can be quite expensive.<br>
<br>
Best regards,<br><font color="#888888">
-- <br>
---------------------------------------+--------------------------------------<br>
I set the clouds in motion - turn up | Frank Warmerdam, <a href="mailto:warmerdam@pobox.com" target="_blank">warmerdam@pobox.com</a><br>
light and sound - activate the windows | <a href="http://pobox.com/~warmerdam" target="_blank">http://pobox.com/~warmerdam</a><br>
and watch the world go round - Rush | Geospatial Programmer for Rent</font><div><div></div><div class="Wj3C7c"><br>
<br>
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