[postgis-users] Crossing roads

[Milo van der Linden]

Hello Andrew,

Can you give some more detail on the why part of this question? Why 
would you want to find all the elements that connect to elements that 
connect to an intersection? This sounds like you are either:
- Trying to do routing and prepare the network for that (in that case; 
check to see if pgRouting is working for you)
- Looking for an automated mechanism to group segments together

Kind regards,

Milo van der Linden

Obe, Regina wrote:
> Andrew,
>  
> Not sure I understand your question.  Are you saying your street 
> segments are cut such that they only connect at junctions.  Kind of 
> like tiger tlids.
>  
>  
> And given each intersection, you are trying to find continguous 
> segments from the intersection?
>  
>  
> e.g.
>  
>  
> -----------        -------------  ---------------   Intersection 
> -------------------------- -------------------------------  
> -----------------
>      t3                              
> t2                   t1                                        
> s3                 s4                               sn
>  
>                                                                                 |a1
>                                                                                 |a2
>                                                                                 
> |an
> So you would keep on collecting segments as long as they are connected 
> to the previous ones?
>  
> Thanks,
> Regina
>
> ------------------------------------------------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] *On Behalf Of 
> *Andrew Ross
> *Sent:* Saturday, December 13, 2008 11:05 AM
> *To:* postgis-users at postgis.refractions.net
> *Subject:* [postgis-users] Crossing roads
>
>
> Hi Everybody,
>
> I hope I'm posting to the right location (I just subscribed to this 
> list).  I have sent this question before but have not seen it listed 
> anywhere yet.
>
>
> In any case, I have a road network in which all of the street segments 
> are snapped together.  I need to group all of the line segments of two 
> streets that cross each other (street 1 and street 2 as shown below) 
> into 4 groups of segments defined as all of those located on either 
> side of the intersection, (street segments A, street segments B, 
> street segments C and street segments D, as shown at the bottom). 
>
>  
>
> Can anybody give me a high level idea of what functions / methods can 
> be used to achieve this?  
>
>  
>
> I’ve gotten as far as identifying the 4 segments actually connected to 
> the intersection using the ST_Intersection function.  
>
>  
>
> How would I get all other segments connected to these 4?  
>
>  
>
> Do I have to write a recursive function using ST_Touches ?  I’d rather 
> not if there is a more elegant way of doing this.
>
>  
>
> Thanks in advance,
>
> Andrew
>
>  
>
>  
>
>  
>
>                                     1
>
>                                     1
>
>                                     1
>
>                                     1
>
>             222222222222 X 22222222222222
>
>                                     1
>
>                                     1
>
>                                     1
>
>                                     1
>
>  
>
>  
>
>  
>
>                                     A
>
>                                     A
>
>                                     A
>
>                                     A
>
>             BBBBBBBBBBXCCCCCCCCCCC
>
>                                     D
>
>                                     D
>
>                                     D
>
>                                     D
>
>  
>
>  
>
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