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

[strk]

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.

--strk;

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