I think this must be a bug in QGIS. In JTS the validity test produces
the expected results:
The MultiPolygon with two polygons touching at a single point is valid
The MultiPolygon with a component which self-intersects is invalid
My guess is that QGIS is not using the full GEOS validity test, since
the error message output is more explicit than produced by GEOS. Could
it be that they reimplemented it in order to get more explicit messages,
but introduced a bug? (And yes, this does indicate that it would be
nice to have more explicit error messages in JTS/GEOS!)
Martin
On 1/6/2014 4:00 PM, Martin Tomko wrote:
Dear all, and Martin in particular, I think,
We are encountering a behaviour that we believe is not following the
JTS documentation. This was first identify in PostGis ( therefore
using the GEOS port of JTS), and replicated in QGIS with a smaller
example. It is a variation on the classical bow-tie problem.
We have a MUlTIPOLYGON geometry with two member geometries, and these
have touching external LinearRings (in one vertex).
Now, following the JTS javadoc, we believe this is ok and satisfies
the interpretation of :
http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/geom/MultiPolygon.html
“As per the OGC SFS specification, the Polygons in a MultiPolygon may
not overlap, and may only touch at single points. This allows the
topological point-set semantics to be well-defined.”
It is also a common suggestion to change bow-tie geometries into
multipolygons ( here is a GEOS piece of documentation, but this has
been discussed on this list as well:
http://trac.osgeo.org/geos/wiki/GeometryCleaning
"POLYGON rings must not self-touch
* The classic "bow-tie" polygon would have to be re-written as a
"polygon with hole that touches once"
* A figure-8 polygon would have to be re-written as a MULTIPOLYGON
* Sometimes the figure-8 has one really, really small side, and it's
best to just lose those
* Similarly sometimes one half of the bow-tie is really small and
should just be dropped…
… and,
POLYGON rings must be properly nested and only touch once “
I have tested this on two very simple WKT examples:
1. Mutlipolygon where two member geometries touch in a single vertex
(10 10) - this is our target case
MULTIPOLYGON(((0 0,10 0,10 10,0 10,0 0)),((10 10,20 10,20 20,10 20,10 10)))
Fails the validity test.
1,segment 0 of ring 1 of polygon 0 intersects segment 0 of ring 0 of
polygon 1 at 10, 10
1,segment 0 of ring 2 of polygon 0 intersects segment 0 of ring 3 of
polygon 1 at 10, 10
1,Geometry has 2 errors.
I note that these errors are form the QGIS topology validator, and I
also believe that these are not correct reports ( just checking the
numbering of rings – there are only 2!). Nowe, I believe that GEOS is
used to do this check, hence it should be identical to JTS (and as a
member here, I report it here – if a Java example code is needed, I
can provide it but I do not currently have a project handy).
2. A Multipolygon where one of the member geometries has a bow tie
(vertex occurring twice) [this is a known, invalid case, for completeness]
MULTIPOLYGON(((0 0,10 0,10 10,20 10,20 20,10 20,10 10,0 10,0 0)),((30 30,40 30,
40 40,30 40, 30 30)))
Fails with the same errors.
1,segments 1 and 6 of line 0 intersect at 10, 10
1,segments 2 and 5 of line 0 intersect at 10, 10
1,Geometry has 2 errors.
My question is: is the doc of the JTS/GEOS insufficiently explicit and
we canot have touching member geometries, or is this a bug?
Thanks,
Martin
------------------------------------------------------------------------------
Rapidly troubleshoot problems before they affect your business. Most IT
organizations don't have a clear picture of how application performance
affects their revenue. With AppDynamics, you get 100% visibility into your
Java,.NET, & PHP application. Start your 15-day FREE TRIAL of AppDynamics Pro!
http://pubads.g.doubleclick.net/gampad/clk?id=84349831&iu=/4140/ostg.clktrk
_______________________________________________
Jts-topo-suite-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/jts-topo-suite-user
No virus found in this message.
Checked by AVG - www.avg.com <http://www.avg.com>
Version: 2013.0.3462 / Virus Database: 3658/6980 - Release Date: 01/06/14
------------------------------------------------------------------------------
Rapidly troubleshoot problems before they affect your business. Most IT
organizations don't have a clear picture of how application performance
affects their revenue. With AppDynamics, you get 100% visibility into your
Java,.NET, & PHP application. Start your 15-day FREE TRIAL of AppDynamics Pro!
http://pubads.g.doubleclick.net/gampad/clk?id=84349831&iu=/4140/ostg.clktrk
_______________________________________________
Jts-topo-suite-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/jts-topo-suite-user
