[postgis-users] Help with Query

[Clifford Snow]

I tried something similar, but the problem is streams are made of up
various length linestrings. Some have only two nodes but are part of what
makes up the entire stream. The ones I looking for are just short lengths
with only one end near the main stream. for example if the following is a
stream:

1             2                 3                    4                   5
          6
o---------o-------------o---------------o--------------o---------o
                                   |
                                   |
                                   o
                                   a

I'm looking to capture the extension "a" above. It's possible that I'll
also capture "6" but these short linestrings are always two nodes and are
less than 25' in length.  However, there are thousands of short two node
lines strings that are part of the stream.


On Fri, Jan 11, 2013 at 5:03 PM, Stephen Woodbridge <woodbri at swoodbridge.com
> wrote:

> On 1/11/2013 7:20 PM, Clifford Snow wrote:
>
>> First off, I'm a new at Postgis so please let me know if the right
>> response to my question is RTFM.
>>
>> I have the NHD streams for my state that I want to import into
>> OpenStreetMap. When I select just the streams that I'm interested in I
>> find that there are numerous short linestrings (2 nodes) that I'd like
>> to get rid of. These linestrings are only connected at one end.  It
>> seems like ST_Touches() would find them. I did a simple query of
>>
>> SELECT s1.gid, s2.gid from flowlines s1, flowlines s2 WHERE
>> ST_Touches(s1.geom,ST_PointN(**s2.geom,1) and s2.pointcount = 2;
>>
>> It returns 0 rows. Changing the point on s2.geom to 0 makes no difference.
>>
>> Anyone have a suggestion of how I find 2 node streams that connect to
>> another stream?
>>
>
> You might try creating a topology. I have not actually done this myself
> since I using pgRouting for a lot of stuff and it has its own topology
> tools for building graphs that I'm more familiar with.
>
> You also might try using st_dwithin( geom1, geom2, tolerance )
> where tolerance needs to be in the units of your data.
>
> select s1.gid, s2.gid
>
>   from flowlines s1, flowlines s2
>  where s2.pointcount = 2
>        and st_dwithin(s1.geom, s2.geom, 0.000001 )
>        and s1.gid != s2.gid;
>
> YMMV,
>   -Steve
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>



-- 
Clifford

OpenStreetMap: Maps with a human touch
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