[OSGeo-Discuss] multipatch shapefile FIRST_RING structure

[Eric Wolf]

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 <warmerdam at pobox.com>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,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
>
>
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