Flo, wow, way cool, upside down world!
re: contributing to the wiki - I would love help. I figured adding/refactoring this page would be the place to start: http://trac.mapnik.org/wiki/Troubleshooting I'm thinking this page because people need a place to land when data looks odd and there can be a number of reasons (though all tied up with proj4). Ideally we'd also have a place for people to read about projections to know what to do from the start, but... who does that? ;) Though we do have: http://trac.mapnik.org/wiki/IntroductionToGIS Ultimately I would like a write a python tool that parses your XML and identifies/makes recommendations. In this case this python tool could: 1) find all 'srs' values in the map xml and warn if any do not contain +over 2) calculate the full extent of all layers (using the robust 16 point transform) and then recommend using the cumulative value for the map maximum-extent 3) find any blank 'srs' parameters and inspect the data to auto-detect the best guess projection (using the ogr "osr" module) 4) lots more.... Dane On May 25, 2011, at 4:57 PM, Flo Ledermann wrote: > > > On 05/25/2011 03:16 PM, Dane Springmeyer wrote: >> On May 25, 2011, at 3:00 AM, Flo Ledermann wrote: > >>> I was experiencing some truncation of polygons towards the borders >>> in non-rectangular projections (like for example Robinson >>> projection) in Mapnik 0.7.0 >> >> By this do you mean the seemingly straight lines along the dateline >> on the right? I think reason that the left edge looks properly curved >> is that the shapefile has a high density of vertexes on the left side >> of the ocean polygons. The remaining edges (including the right side) >> lack vertexes, so when reprojected they continue to look straight. > > Excellent advice, that was indeed the case! > >>> Much to my surprise *it got much worse* in the trunk version, >>> truncating polygons heavily on both sides of the globe. >> >> Yes, this is because the move to clipping by the map extent means >> that you have one place (the map srs/bounds) to fix/handle out of >> bounds issues, but left unhandled very small bbox queries may be >> sent to the layer datasources which leads to very reduced features. I >> realize that you did not expect to have to "fix" them when upgrading >> and that this regression may be annoying, but I feel like it is an >> improvement both for reprojected and non-reprojected cases. > > Absolutely, no problem with that. If there's any wiki page or else where > I could contribute to documenting this, I'd be happy to write down what > I learned. > >> And there are three solutions to quickly fix: >> >> 1) As you noticed you can set the map.maximum_extent. But this extent >> must be in the bounds of the map srs. So, in your case you need to >> do: >> >> robinson_bounds = _get_projected_bounds(output_projection, >> bounds=bounds) mmap.maximum_extent = robinson_bounds >> mmap.zoom_to_box(robinson_bounds) > > OK, that worked great for me. > >> 2) Still, there is an even easier solution (which I need to document >> on the wiki better). It is that you can request that proj4 do sane >> things when out of bounds coordinates are passed and properly wrap >> them. Do do this add the: >> >> +over >> >> parameter to your proj4 string. This should fix things even without >> the use of maximum_extent. > > OK, good to know, thanks for the hint. > >> 3) The third option is to apply this patch: >> http://trac.mapnik.org/ticket/751. But, I do not wish to use this, at >> least for the mainline rendering in feature_style_processor.hpp, >> because it can slow down multithreaded servers slightly. Can you tell >> top performance is on my mind? > > I came across this but I don't see the need for it right now - solution > #1 worked great for me. > >> So, I would recommend #2, and then the mapnik2 image should render >> just like mapnik 0.7.x (but a bit faster if it were being rendered >> under high load). > > I'm going for the #1 option now, using the properly projected > maximum_extent. Together with a refined polygon edge it works like a > charm, thanks! > > New image attached, using the cool new +axis=esu parameter of Proj.4 > trunk on the way to put the world upside down ;) ! > > Best, > > Flo > > <world_robinson_.png> _______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
