Hi strk,
sorry to disturb your discussion, but I am interested in this st_snap()
function you mention. I did not found it - is this something in PostGIS 2.0?
Regards,
Birgit.
On 16.02.2011 19:18, strk wrote:
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
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