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I am reducing the size of a world timezone shapefile,

http://efele.net/maps/tz/world/tz_world_mp.zip

But at a 0.01 tolerance, entire countries disappear

Anybody know a way round this?

QGIS shows the same error simplifying naturalearthdatamaps also; simplify with 0.01 tolerance and Italy disappears

underdark
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Paul Salber
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  • beta, but worth a try http://kelsocartography.com/downloads/gis/2012/natural_earth/ne_10m_time_zones_beta_3.zip – underdark Jun 16 '12 at 18:40
  • Shafefile is hard to reuse and is missing timezones, good start though – Paul Salber Jun 16 '12 at 19:14
  • Can anybody recommend alternate tool to simplify geometry and reduce size of shapefile? – Paul Salber Jun 17 '12 at 09:55
  • Check related questions such as http://gis.stackexchange.com/questions/178/simplifying-adjacent-polygons & http://gis.stackexchange.com/questions/3739/generalisation-strategies-for-building-outlines. If you cannot find an answer that works in your case, please open a new question. – underdark Jun 17 '12 at 10:35

1 Answers1

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Looks like a known problem http://hub.qgis.org/issues/4189

The reason for wrong behavior is a tiny ring. If it is removed, the polygon is simplified ok. The polygons with bigger rings simplify ok, but if any polygon has a ring that is smaller than tolerance, it breaks the whole polygon.

underdark
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  • Ah, thanks. Well found. Looks like I need to ogr2ogr until the fix the bug in QGIS – Paul Salber Jun 16 '12 at 21:55
  • I have tried GRASS and openjump on the timezone shapefile. The first simplifies but does not reduce filesize, the second crashes on an error. In order to progress, I am happy to contract help, anybody interested? – Paul Salber Jun 18 '12 at 09:36
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    mapshaper.com/test/MapShaper.swf simplifies shapefiles, and is robust to ocurrence of polygon rings smaller than the simplify tolerance. – Paul Salber Jul 01 '12 at 16:41