[postgis-users] How to calculate the symmetric difference oflinestrings and polygons (i.e. linestrings which do notintersect polygons)

[Randall, Eric]

I guess what I meant was:

select st_difference(streets,st_intersection(streets,forests)) as streets
from streets, forests
where st_intersects(streets,forests)
union
select streets 
from streets, forests
where not st_intersects(streets,forests)



-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
Randall, Eric
Sent: Monday, January 31, 2011 6:41 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] How to calculate the symmetric difference
oflinestrings and polygons (i.e. linestrings which do notintersect
polygons)


Seems like the difference of the streets with the intersection of the forests and the streets should do that.


st_difference(streets,st_intersection(streets,forests))




Eric 



-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
Stefan Keller
Sent: Saturday, January 29, 2011 9:04 PM
To: PostGIS Users Discussion
Subject: [postgis-users] How to calculate the symmetric difference of
linestrings and polygons (i.e. linestrings which do not intersect
polygons)


Hi,

Given streets linestrings and forest polygons, think of all streets
segments which are *not* inside forest areas.
How can I calculate all segments from linestrings which do not
intersect polygons?

Any ideas?

- S.

Attempt 1: ST_SymDifference sounds good! But even this equivalent
produces nonsense:
# SELECT ST_AsText(
ST_CollectionExtract(ST_Difference(ST_Union(ls.way,po.way),
ST_Intersection(ls.way,po.way)),2) )
FROM linestring ls, polygon po

Attempt 2:
# SELECT ST_AsText(ST_Intersection(ls.way, po.way)) FROM linestring
ls, polygon po
gives me all streets *inside* forests. But I'd like get the
"remainder" in between...

Attempt 3:
# SELECT ST_AsText(ST_Difference(ls.way, po.way)) FROM linestring ls, polygon po
gives me two displaced lines for each street (because the resultse
contains the difference between one line and one polygon), Thesse two
could be intersected - but ST_Intersection of two overlapping lines
(with same nodes except end nodes) does'nt seem to work.

Attempt 4:
In a very desperate idea, I calculcated the extent of both tables in
order generate a huge boundary polygon with has as many wholes as
there are polygons. Then I could do an ST_Intersect of this big swiss
cheese polygon with the lines - but no luck.

Attempt 5:
I finally tried to dump all points from Attempt 2 (intersection) to
get hold of the boarder points where lines and polygons cross.
But when tried to split a line at a point and to sort out all lines
outside polygons things got again complicated..
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