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

[Van Bakergem, William]

Stephen,

We may be talking apples and oranges.  Our project treats each city block as a separate object floating in a sea of public right of ways.  We generate the streets with a default width (18.3 meters) and then revise for streets with other widths.  The tool is designed for historic districts of about 500 city blocks.  So, we do not have any topological issues with left/right.  Everything is counterclockwise beginning in the most south west corner.  There are no branches, only single isolated polygons.  We always know what's inside and what outside.

Streets are generated on the fly when we need them - just the space between the blocks with sidewalk and curb typologies. Street attributes are managed via a street segments table whose geometry is the center line.  Is really simple, but works for a virtual 3d model generator.  

All of this is managed in a spatial temporal manager that keeps track of when objects appear and disappear in the urban landscape over the 200 year time interval.   

A timeline slide bar in the Xj3D viewer allows the user to see changes in real time.  Think H.G. Wells Time Machine.  

It's all open source.

Dave


-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net on behalf of Stephen Woodbridge
Sent: Thu 4/23/2009 3:35 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] How to create block polygons from street network?
 
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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