Folks,
I am having a problem determining the best way to clip out and disgrad
a set of linestrings. Here is my setup:
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*****************
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)
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My linestring network has the following attributes (and a picture for
visualization):
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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)
scaglinks.GIF
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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)
scaglinks_clip_polygon.GIF
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*****************
When I run the following command to clip the scaglinks linestrings, the
results are what I would expect (again, a picture for visualization):
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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)
scaglinks_clipped.GIF
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What I am really after are the links that fall outside the clipping
polygon. So I attempted the following query:
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*****************
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
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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.
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Again, what I had hoped is that the query to create clipped2 would return
a table that results in the following graphic:
but_want_this.GIF
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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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