[postgis-users] linestring clipping problem
Brian Modra
brian at zwartberg.com
Wed Sep 30 12:37:34 PDT 2009
Create a new clip polygon which is the differenec between the blue one and
the outer rectangle. then aply that one as you did the blue one.
2009/9/30 James G Wilkinson <jgw at alpinegeophysics.com>
> Folks,
>
>
>
> I am having a problem determining the best way to clip out and disgrad a
> set of linestrings. Here is my setup:
>
>
>
> *****************
>
> *****************
>
> itn=# select postgis_full_version();
>
>
> postgis_full_version
>
>
> ------------------------------------------------------------------------------------------------------------------
>
> POSTGIS="1.3.2" GEOS="3.0.0-CAPI-1.4.1" PROJ="Rel. 4.4.9, 29 Oct 2004"
> USE_STATS (procs from 1.1.2 need upgrade)
>
> (1 row)
>
>
>
> itn=# select version();
>
>
> version
>
>
> ----------------------------------------------------------------------------------------------------------
>
> PostgreSQL 8.1.3 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC)
> 4.0.0 20050519 (Red Hat 4.0.0-8)
>
> (1 row)
>
>
>
> *****************
>
> *****************
>
>
>
> My linestring network has the following attributes (and a picture for
> visualization):
>
>
>
> *****************
>
> *****************
>
> itn=# \d itn.scaglinks
>
> Table "itn.links"
>
> Column | Type
> |
> Modifiers
>
>
> ------------------------+------------------+-------------------------------------------------------------------------
>
> record_id_alternate | integer | not null default
> nextval('itn.scaglinks_record_id_alternate_seq3'::regclass)
>
> a_node | integer |
>
> b_node | integer |
>
> dist | integer |
>
> taz_id | integer |
>
> .<clip>
>
> .
>
> .<clip>
>
> the_geom | geometry |
>
> Indexes:
>
> "idx_scaglinks_00" btree (a_node)
>
> "idx_scaglinks_01" btree (b_node)
>
> "idx_scaglinks_02" btree (a_node, b_node)
>
> "idx_scaglinks_03" btree (link_id)
>
> "pkgis_scaglinks" gist (the_geom)
>
> Check constraints:
>
> "enforce_dims_the_geom" CHECK (ndims(the_geom) = 2)
>
> "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) =
> 'MULTILINESTRING'::text OR the_geom IS NULL)
>
> "enforce_srid_the_geom" CHECK (srid(the_geom) = 26911)
>
>
>
> [image: scaglinks.GIF]
>
>
>
> *****************
>
> *****************
>
>
>
> My clipping polygon has the following attributes (and a picture for
> visualization):
>
>
>
> *****************
>
> *****************
>
>
>
> itn=# \d itn.itn_clip_polygon_1
>
> Table "itn.itn_clip_polygon_1"
>
> Column | Type | Modifiers
>
> ----------+----------+-----------
>
> cyid | integer |
>
> the_geom | geometry |
>
> Indexes:
>
> "pkgis_itn_clip_polygon_1" gist (the_geom)
>
>
>
> [image: scaglinks_clip_polygon.GIF]
>
>
>
>
>
> *****************
>
> *****************
>
>
>
> When I run the following command to clip the scaglinks linestrings, the
>
> results are what I would expect (again, a picture for visualization):
>
>
>
> *****************
>
> *****************
>
>
>
> itn=# create table clipped as select intersection(a.the_geom,b.the_geom) as
>
>
> new_geom, length(a.the_geom) as original_link_length, a.* from scaglinks a,
>
>
> itn_clip_polygon_1 b where intersects(a.the_geom,b.the_geom);
>
> SELECT
>
>
>
> itn=# create index pkgis_clipped on clipped using gist(new_geom);
>
> CREATE INDEX
>
>
>
> itn=# \d itn.clipped
>
> Table "itn.clipped"
>
> Column | Type | Modifiers
>
> ------------------------+------------------+-----------
>
> new_geom | geometry |
>
> original_link_length | double precision |
>
> record_id_alternate | integer |
>
> a_node | integer |
>
> b_node | integer |
>
> dist | integer |
>
> taz_id | integer |
>
> .<clip>
>
> .
>
> .<clip>
>
> the_geom | geometry |
>
> Indexes:
>
> "pkgis_clipped" gist (new_geom)
>
>
>
> [image: scaglinks_clipped.GIF]
>
>
>
> *****************
>
> *****************
>
>
>
> What I am really after are the links that fall outside the clipping
>
> polygon. So I attempted the following query:
>
>
>
> *****************
>
> *****************
>
> itn=# create table clipped2 as SELECT a.*,
> difference(a.the_geom,b.new_geom) as new_geom from scaglinks a, clipped b;
>
>
>
> ******** after 12 hours, the query throws the following error ********
>
>
>
> ERROR: could not extend relation 1663/164540/180960: No space left on
> device
>
> HINT: Check free disk space
>
>
>
> *****************
>
> *****************
>
>
>
> The scaglinks network consists of 108,000 links and the clipped network
>
> consists of 6,200 links. I have approximately 1.5 TB of free disk so I
>
> do not believe that disk space should be a problem.
>
>
>
> *****************
>
> *****************
>
>
>
> Again, what I had hoped is that the query to create clipped2 would return
>
> a table that results in the following graphic:
>
>
>
> [image: but_want_this.GIF]
>
>
>
> *****************
>
> *****************
>
>
>
> Can you offer any help in the query that will result in a table that
>
> looks something like the last graphic?
>
>
>
> Best Regards,
>
>
>
> Jim
>
> _______________________________________________
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>
>
--
Brian Modra Land line: +27 23 5411 462
Mobile: +27 79 69 77 082
5 Jan Louw Str, Prince Albert, 6930
Postal: P.O. Box 2, Prince Albert 6930
South Africa
http://www.zwartberg.com/
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