[postgis-users] Crossing roads

[Andrew Ross]

Regina,
Thanks for taking the time to look at my question, the answer is yes to both of your assumpions:
1. Segments are cut at junctions.
2. I am trying to find contiguous segments from the intersection, but not for every intersection in the dataset, just for the intersection resulting from the streets that I specify, I'll use python to capture a pair of street names and generate the query dynamically.

Thanks again,

Andrew




________________________________
From: "Obe, Regina" <robe.dnd at cityofboston.gov>
To: PostGIS Users Discussion <postgis-users at postgis.refractions.net>
Sent: Monday, December 15, 2008 8:58:30 AM
Subject: RE: [postgis-users] Crossing roads


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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