Fiona and Shapely: spatially cleaning features

Inspired by comments I saw today on the internet, this is how you spatially clean features using Fiona and Shapely (for small values of dirty).

import logging
import sys

from shapely.geometry import mapping, shape

from fiona import collection

logging.basicConfig(stream=sys.stderr, level=logging.INFO)

with collection("docs/data/test_uk.shp", "r") as input:
    schema = input.schema.copy()
    with collection(
            "with-shapely.shp", "w", "ESRI Shapefile", schema
            ) as output:
        for f in input:

                # Make a shapely object from the dict.
                geom = shape(f['geometry'])
                if not geom.is_valid:

                    # Use the 0-buffer polygon cleaning trick
                    clean = geom.buffer(0.0)
                    assert clean.geom_type == 'Polygon'
                    assert clean.is_valid
                    geom = clean

                # Make a dict from the shapely object.
                f['geometry'] = mapping(geom)

            except Exception, e:
                # Writing uncleanable features to a different shapefile
                # is another option.
                logging.exception("Error cleaning feature %s:", f['id'])

Makes a neat example, I think. Shapely just happens to provide this handy zero buffer feature (via GEOS), it's not required at all. Fiona just reads and writes GeoJSON-like mappings (like Python's dict), it doesn't care what you do in between or what you do it with.

Update (2011-12-25): The error I noted in the comments is fixed above.


Re: Fiona and Shapely: spatially cleaning features

Author: Sean

Oops, there's an error above. I mean to assert that the cleaned geom is valid.