Hi Peter, what you're describing sounds familiar. Ralf and I had been observing at least two characteristic types of failures: 1.) An airfield hole (or a road) cutting a landcover polygon into two parts of which the (much) smaller one was left without centroid after clipping. 2.) A centroid sitting in a very thin edge of a polygon which resulted from clipping at small angles, in a so-called "sliver" whereas the sliver was later removed (cleaned) from the polygon without relocating the centroid.
For the long-term I'm in favour of doing all this polygon-processing in GRASS because GRASS takes care of the topological consistency - at least as long as you don't force it not to do so. In preparation of a talk last month I did a few pictures from experiments (in PostGIS and GRASS) by creating a buffer around all the road lines - my experimental OSM line feature layer on the MapServer: http://mapserver.flightgear.org/map/?lon=6.5045&lat=51.23036&zoom=15&layers=00000BTFFFFTFTFFFFFF This turns all the various road lines into one single polygon layer, ready for clipping against the land cover: http://foxtrot.mgras.net/bitmap/FGFS/EDLN-OpenLayers-RoadCover.png .... which I'm then cutting out of the landcover. http://foxtrot.mgras.net/bitmap/FGFS/EDLN-OpenLayers-RoadCutout.png Images had been rendered via MapServer right from the PostGIS DB and delivered by OpenLayers. The underlying principle is very simple, indeed. Anyhow I'm still impressed that all this really works as advertized on real data ;-) Currently I'm (hopefully) running the last test-cycles for cleaning polygonal land cover at large scale in GRASS. Next step will be to verify wether "v.drape" yields a result which is suitable for our needs, thus by filling airport holes (via "poly2ogr" and some vertex elevation magic) and roads into the base land cover, just few steps remain until the stuff could be written into <your favourite scenery format>. Cheers, Martin. -- Unix _IS_ user friendly - it's just selective about who its friends are ! -------------------------------------------------------------------------- ------------------------------------------------------------------------------ Learn Windows Azure Live! Tuesday, Dec 13, 2011 Microsoft is holding a special Learn Windows Azure training event for developers. It will provide a great way to learn Windows Azure and what it provides. You can attend the event by watching it streamed LIVE online. Learn more at http://p.sf.net/sfu/ms-windowsazure _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
