Bugs item #3607970, was opened at 2013-03-13 21:12 Message generated for change (Comment added) made by jratike80 You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=679906&aid=3607970&group_id=118054
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: General / Other Group: None Status: Open Resolution: None Priority: 5 Private: No Submitted By: Sean (bigrodent) Assigned to: Nobody/Anonymous (nobody) Summary: Generalization will remove full polygons for high tolerance Initial Comment: Take for instance a US Zip and set it to .01 or even as low as .0001 and you will end up with less polygons than the original file. You will actually remove full zip codes. As you turn the tolerance down they gradually appear back. Upon physical inspection of the shapefile it is clear that it is the smallest zips in the file that get stripped first. Other opensource tools have this issue as well. ---------------------------------------------------------------------- >Comment By: Jukka Rahkonen (jratike80) Date: 2013-03-14 05:46 Message: Both Spatialite functions created 84031 polygons. Of those 3663 were invalid when Simplify was used, and 24 when SimplifyPreserveTopology was used. However, for the latter also the original features had topology errors and SimplifyPreserveTopology seems to be reliable. ---------------------------------------------------------------------- Comment By: michael michaud (michaudm) Date: 2013-03-14 05:30 Message: Thanks for the tests Jukka, Simplify : did you check that you had 84031 polygons in the result. SimplifyPreserveTopology : I know there are cases where a small hole finishes out of the exterior ring after simplification. If you have stranger cases, it maybe worth pointing them to Martin. Simplyfy Polygon Coverage : the flaw I identified is about polygons which have only two edges in the graph, and the simplification of both edges give the same result. The polygon disappears. Agree to add a warning for simplify plugin. It i more difficult with Analysis>Geometry functions ---------------------------------------------------------------------- Comment By: Jukka Rahkonen (jratike80) Date: 2013-03-14 02:31 Message: I made some tests. I had originally 84031 polygons and 28 of them has self-intersections. Simplify Polygon Coverage gave only 82970 simplified polygons in 35 minutes. There were no topology errors in those. Spatialite (or geos and thus fundamentally JTS) found the same 28 invalid polygons from source data. Function Simplify converted all the 84031 polygons in 15 seconds and in the resultset there were 3663 invalid polygons. Function SimplifyPreserveTopology took 95 seconds and result contained 24 invalid polygons. People tend to believe that SimplifyPreserveTopology preserves layer topology. However, it preserves topology only feature by feature and the resulting layer may have gaps and overlaps. If OJ will use this JTS function some day I would suggest showing a comment about layer topology for the users. ---------------------------------------------------------------------- Comment By: michael michaud (michaudm) Date: 2013-03-14 00:51 Message: Jukka is right about the difference between DouglasPeuckerSimplifier and TopologyPreservingSimplifier. However, I don't remember I have seen geometries disappearing with DouglasPeuckerSimplifier. On the other hand, I just check our Simplify Polygon Coverage plugin and found a flaw which could explain that small polygons disappear (just added some comments in the code of SimplifyPolygonCoveragePlugIn). I've no patch yet and I'll try to find a solution with the help of Stefan. ---------------------------------------------------------------------- Comment By: Jukka Rahkonen (jratike80) Date: 2013-03-13 22:27 Message: I can confirm that this happens with Simplify Polygon Coverage and OJ snapshot r3322. Douglas Peucler simplification is not guaranteed to preserve the number of polygons http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/simplify/DouglasPeuckerSimplifier.html There are another algorithms which keep all the polygons like http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/simplify/TopologyPreservingSimplifier.html I am not sure if our Simplify Polygon Coverage is supposed to preserve the topology or not and if this is a bug or a missing feature. I used Spatialite and SimplifyPreserveTopology function for my data with good result. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=679906&aid=3607970&group_id=118054 ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_d2d_mar _______________________________________________ Jump-pilot-devel mailing list Jump-pilot-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel
