[postgis-users] Retrieve the portion of line external to polygon

[Andrea Peri]

I found a solution using the ST_Split
this is a bit more long to write query, but work pretty well :)

select 1, ST_AsText(
    ST_Difference(
        ST_CollectionExtract(
            ST_Split(
                ST_GeomFromText('LINESTRING(10.9 2, 11 8)'),
                ST_ExteriorRing(ST_GeomFromText('POLYGON((12 2, 10 10, 20
21, 21 5, 12 2))'))
            ),
            2
        ),
        ST_Intersection(
            ST_CollectionExtract(
                ST_Split(
                    ST_GeomFromText('LINESTRING(10.9 2, 11 8)'),
                    ST_ExteriorRing(ST_GeomFromText('POLYGON((12 2, 10 10,
20 21, 21 5, 12 2))'))
                ),
                2
            ),
            ST_GeomFromText('POLYGON((12 2, 10 10, 20 21, 21 5, 12 2))')
        )
    )
)



2011/2/16 strk <strk at keybit.net>

> On Wed, Feb 16, 2011 at 07:08:03PM +0100, strk wrote:
> > On Wed, Feb 16, 2011 at 06:47:53PM +0100, Andrea Peri wrote:
> > > Look this simple example
> > >
> > > the difference between the line and the same line intersected with the
> > > polygon is equal to the line :)
> > >
> > > select 1, ST_Difference(ST_GeomFromText('LINESTRING(10.9 2, 11
> > > 8)'),ST_Intersection(ST_GeomFromText('LINESTRING(10.9 2, 11 8)'),
> > > ST_GeomFromText('POLYGON((12 2, 10 10, 20 21, 21 5, 12 2))')))
> > >
> > > the trick is that in the intersection point between polygon and line,
> the
> > > vertex added move the line so the difference fail to
> > > remove the internal portion.
> >
> > Ok, I think I know what's going on.
> > The original linestring isn't noded with the polygon boundary.
> > When computing the intersection, a node is added.
> > Such a node will NOT fall on the original line anymore, due
> > to precision constraints.
> >
> > See this:
> >  CREATE TABLE a AS SELECT
> >    'LINESTRING(10.9 2, 11 8)'::geometry as g;
> >  CREATE TABLE b AS SELECT
> >    'POLYGON((12 2, 10 10, 20 21, 21 5, 12 2))'::geometry as g;
> >  CREATE TABLE c AS SELECT
> >    ST_Intersection(a.g, b.g) FROM a, b;
> >  SELECT ST_Covers(a.g, ST_StartPoint(c.g)) FROM a,c; -- false
> >  SELECT ST_Covers(a.g,   ST_EndPoint(c.g)) FROM a,c; -- true
> >
> > What you could do is _snap_ the original line to the intersection,
> > or to node the input before proceeding.
>
> For a working example:
> select st_covers(a.g, st_startpoint(st_snap(a.g,c.g,1e-10))) from a,c;
>
> --strk;
>
>  ()   Free GIS & Flash consultant/developer
>  /\   http://strk.keybit.net/services.html
>



-- 
-----------------
Andrea Peri
. . . . . . . . .
qwerty àèìòù
-----------------
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