Extracting city blocks from OpenStreetMap data

One of the downsides of forming polygon from line segments in this way is that it implicitly assumes all lines have a vertex at the intersection of another line.

For example, something like this would fail ("o" represents a vertex):

    o      o  o
     \      \/
      \     /\
       \   /  \   o
        \ /    \ /
         /      /
        / \    / \
       o   \  /   \
            \/     \
            /\      o
           /  \
          o    \
                o

1) Intersect the lines before-hand, ensuring that you wind up with a vertex at every intersection. (In shapely's case, you'd call `shapely.ops.cascaded_union(lines)` before applying `polygonize`.)

2) Cut a bounding polygon with the lines, rather than forming polygons from individual line segments.

The second method has the advantage of ensuring that everything inside of the bounding polygon is split up into smaller, block-level polygons. Usually this is what you want, though for something like city blocks, you'd need to filter out non-urban areas (e.g. water).

I've always thought of city blocks as an anti-pattern in urban layout e.g. exploring New York on foot for the first time I tried 'using my nose' to find interesting places but had the problem that I got zero incremental information until the next intersection at which point the large block size meant it was too far to turn back.

I prefer to live in cities for which the concept of a 'block' has less relevance, and I'd caution against trying to 'blockify' cities worldwide.

[1] http://wiki.openstreetmap.org/wiki/Key:place

Proof might be an interactive map that shows the city block of a selected area in NYC.

Is there a visual proof in the walkthrough I'm not seeing?

thx