[postgis-users] Postgis overlapping two linestrings

[Ani Alamo]

Hi!

I need a specific function to get if two linestring overlaps. e.g. Linestring 1 is Point (0, 0) : Point (10, 0) Linestring 2 is Point (0, 0) : Point (3, 0)

In this case I need "true" result because LineString 1 overlaps in different points like 0,0 1,0 2,0 & 3,0. I dont need the common points, I only need if two linestring overlaps or not. I tried with this function in postgis...

overlaps(buffer(LINESTRING1, 0.001), buffer(LINESTRING2, 0.001))
I create a buffer on both linestrings... but muy problem is some times works better, some times not.

Can you help me please?

Thanks!

[James David Smith]

Hi Ani,

Have you looked into st_intersects() ? and st_intersection() ?

http://postgis.refractions.net/docs/ST_Intersects.html

James

> _______________________________________________
> postgis-users mailing list
> postgi...@lists.osgeo.org
> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
_______________________________________________
postgis-users mailing list
postgi...@lists.osgeo.org
http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users

[Ani Alamo]

Uhm... exactly I have this linestrings:

LINESTRING(2.118932325 41.398745991,2.168720219 41.385631938)

LINESTRING(2.142346041 41.398526436,2.111181541 41.409216161)

intersections gives me:

GeometryCollection: GEOMETRYCOLLECTION EMPTY

intersects gives me:

false

but when I see the line drawn on map I see both lines intersect in a little "section" (strectch). But really when I use intersects or intersection functions gives me "unavailable intersection"... and when I use OVERLAPS function 

overlaps(buffer(LINESTRING1, 0.001), buffer(LINESTRING2, 0.001)) also gives me "false".

Another ideas please??

2013/11/22 James David Smith <james.da...@gmail.com>

[Ani Alamo]

I tried this function:

intersects(buffer(LINESTRING1, 0.001), buffer(LINESTRING2, 0.001))

Always gives me "false" result.

I don't know what to do!

2013/11/22 Ani Alamo <alam...@gmail.com>

[Rémi Cura]

You may try
http://postgis.net/docs/ST_SharedPaths.html

Cheers,

Rémi-C

2013/11/22 Ani Alamo <alam...@gmail.com>

[Rémi Cura]

I hope you are not runnning into a very annoying precision issue.

If it were the case, you may want to try the patch I just submitted to GEOS, (https://github.com/Remi-C/libgeos).

and then you would check  like this :
decompose line A into points (ST_DumPPoints)
if any point is on line B : true

else
decompose line B into points

if any point of B in on line A
return true
else return false

(you chekc "point on line" with st_intersects)

Cheers,

Rémi-C

2013/11/22 Rémi Cura <remi...@gmail.com>

[Martin Feuchtwanger]

ST_Relate() is an interesting and versatile function: http://postgis.org/docs/ST_Relate.html
(also illustrated at http://workshops.boundlessgeo.com/postgis-intro/de9im.html )

But maybe your problem is one of tolerance/roundoff-error

_______________________________________________
postgis-users mailing list
postgi...@lists.osgeo.org
http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users

--

Martin Feuchtwanger   feu...@shaw.ca  
Vancouver, BC   V5N 1J6
http://members.shaw.ca/geomatics.developer

[Mike Toews]

On 23 November 2013 05:50, Ani Alamo <alam...@gmail.com> wrote:
> Uhm... exactly I have this linestrings:
> LINESTRING(2.118932325 41.398745991,2.168720219 41.385631938)
> LINESTRING(2.142346041 41.398526436,2.111181541 41.409216161)
>
> intersections gives me:
> GeometryCollection: GEOMETRYCOLLECTION EMPTY
>
> intersects gives me:
> false
>
> but when I see the line drawn on map I see both lines intersect in a little
> "section" (strectch).

No, these lines don't intersect, they are somewhat parallel to each
other, but not at all even close to touching. You might be confusing
another pair of lines.

> But really when I use intersects or intersection
> functions gives me "unavailable intersection"... and when I use OVERLAPS
> function
> overlaps(buffer(LINESTRING1, 0.001), buffer(LINESTRING2, 0.001)) also gives
> me "false".
>
> Another ideas please??

Avoid using buffer for proximity analysis. Try using ST_DWithin instead:

SELECT
ST_Distance(A::geography, B::geography) AS dist_m,
ST_Distance(A::geometry, B::geometry) AS dist,
ST_DWithin(A::geography, B::geography, 700) AS within_m,
ST_DWithin(A::geometry, B::geometry, 0.006) AS within
FROM
(SELECT
'LINESTRING (2.118932325 41.398745991, 2.168720219 41.385631938)'::text AS A,
'LINESTRING (2.142346041 41.398526436, 2.111181541 41.409216161)'::text as B
) as f;

-[ RECORD 1 ]-----------------
dist_m | 623.162866494591
dist | 0.00575141350513493
within_m | t
within | t


-Mike