Shapefile Testing
This is a repo for testing how Mapnik will draw shapefiles that have (or have not) been clipped, segmentized, and/or re-projected.
The target projection is EPSG:3573, which causes issues with any source data that has data in the southern hemisphere.
Can I rely on Mapnik to transform a shapefile in an intermediary projection (like EPSG:4326)?
Source Data
"Simplified Land Polygons" can be downloaded from the following link:
http://data.openstreetmapdata.com/simplified-land-polygons-complete-3857.zip
Extract the zip archive into the "data" directory in this repo; you may have to create it. Once extracted, you can run the conversion script to use GDAL to create different versions of the shapefile.
GDAL Conversion
I use OGR2OGR from GDAL to convert the source shapefile to different projections:
- 3857 to 4326
- 3857 to 3573
- 4326 to 3573
- 4326 to 4326-clipped
- 4326-clipped to 3573-clipped
- 4326 to 4326 (fixed geometries) to 4326 (fixed and clipped)
I use 4326 for two reasons. The first is that I can specify the clipping in latitude/longitude; the second is that I want to use it as the intermediary projection and let Mapnik handle the final projection.
Note that GDAL requires libspatialite, and that libspatialite must be compiled with liblwgeom support. This provides ST_MakeValid() to GDAL, which is used to fix geometries.
Results
See RESULTS.markdown.
License
Preview images generated using data from OpenStreetMap contributors.
Author
James Badger (@openfirmware)