[postgis-users] How to create block polygons from street network?

[Stephen Woodbridge]

Hi Dave,

I have a client that is interested in getting polygon blocks from a road 
network. I'm not sure what for but it seemed like something that should 
not be that hard to do given noded street data. I have used pgRouting so 
I know I can build a good adjacency graph of the data. The problem with 
an adjacency graph is that it does not tell you which node is right/left 
most so you have to do some analysis of the geometry unless there is 
some nifty graph analysis the finds all polygons in a graph.

How does your tool work?
Are you will to share it?
I was thinking that such tools might be a useful add-on to postgis.

-Steve

Van Bakergem, William wrote:
> Stephan,
> 
> I have a tool for generating city blocks from street center lines.  
> 1. Given centerlines, street widths and street names (from Google Maps), calculate intersections, blocks and block sides;
> 2. All geometries are inserted into PostGIS tables;
> 3. From these table I can generate a virtual city based on building typologies and development histories for any year over a 200 year time interval.  
> 4. Display in Xj3d or Google Earth (COLLADA).
> 
> I would like to hear more about your project.
> 
> Dave van Bakergem
> 
> 
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net on behalf of Stephen Woodbridge
> Sent: Thu 4/23/2009 2:29 PM
> To: PostGIS Users Discussion
> Subject: [postgis-users] How to create block polygons from street network?
>  
> Hi all,
> 
> Here is an interesting problem that I could use some suggestions and/or 
> sample code for. I have a street network where all the street segments 
> are noded at intersections. What I need to extract is all the blocks in 
> the coverage. A "block" is the smallest polygon bounded by segments that 
> does not have any segment crossing it. It could have a cul-de-sac 
> protruding into its interior as long as it does not divide the polygon 
> in two.
> 
> I could create a node adjacency list, but I'm not sure if that helps me.
> 
> I'm thinking that each edge needs to be tagged with a right and left 
> polygon id, then you can create polygons by grouping on the id.
> 
> while (current = get_edge_without_rl_pid) {
>    if current.right_side_edge has no pid {
>      get new_pid as this_pid
>      get other edges from end
>      find right most edge
>      if edge has id then {
>        update all new_pid to id
>        mark new_pid as unused
>        this_pid = id
>      }
>      else
>        mark both edges as this_pid
>    }
>    if current.left_side_edge has no pid {
>      get new_pid as this_pid
>      get other edges from end
>      find left most edge
>      if edge has id then {
>        update all new_pid to id
>        mark new_pid as unused
>        this_pid = id
>      }
>      else
>        mark both edges as this_pid
>    }
> }
> 
> Does this seem like it would work?
> Anyone interested in turning this into plpgsql?
> 
> It might be better to recursively pick an edge and recursively walk 
> connected right most edges back onto itself marking the edges as you go. 
> At some point you may still have edges not included in any of the above 
> that can only be marked by walking the left most edges.
> 
> Thoughts?
> 
> -Steve W
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