[postgis-users] query multipolygon

Tue Jun 2 16:44:50 PDT 2009

For small misalignments.  ST_SnapToGrid sometimes does the trick.  Though I
guess depends.

Doing a negative buffer in theory should be fine, so you may just want to
upgrade your GEOS to 3.1.1 (when that comes out which hmm should be any day
now) or GEOS 3.2.

Paul knows more about the enhancements and fixes.  I just know from scanning
the commits that a lot of fixes and speed enhancements have been done to
ST_Buffer between GEOS 3.0.3 and the later versions and upgrading has solved
some peoples buffer issues.

I'm not sure how many of those patches have been backported to the upcoming
GEOS 3.0.4 (which should also be out any day), but certainly probably not
any of the speed improvements.

Hope that helps,
Regina 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of sgrocho2
Sent: Tuesday, June 02, 2009 7:07 PM
To: postgis-users at postgis.refractions.net
Subject: RE: [postgis-users] query multipolygon

I did not do the st_isvalid check on the ST_Buffer, however when I removed
it, the query ran fast (468ms) and gave the correct answer.  We use the
buffer of -1 in order to minimize error in data creation when trying to snap
things up between layers.  For example Zoning is generally derived from the
parcels but at times get out of alignment when parcels are adjusted.  The -1
buffer takes care of the slight amount of movement created by the
adjustment.  In this case with the fish & wildlife data the buffer really
serves no purpose, we can do without it since it is not parcel derived.  Any
ideas on how to best deal with ST_Buffer? 

Paragon Corporation-2 wrote:
> 
> Did you do an ST_IsValid check on the multipolygons or the ST_Buffer. 
> Could
> be the ST_buffer that is the problem. 
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
> sgrocho2
> Sent: Tuesday, June 02, 2009 5:08 PM
> To: postgis-users at postgis.refractions.net
> Subject: Re: [postgis-users] query multipolygon
> 
> 
> Okay, I did run ST_IsValid() and everything from each of the layers 
> returned a t.  When visually checking the query results with 167 rows 
> it appears that the query is only returning rows that intersect the 
> first polygon it processes.  Leaving out those rows that only 
> intersect the other 5 polygons.
> 
> The following image is an example of what is being missed.  The 
> polygons outlined in blue are not selected by the query since they 
> only touch one of the parcel polygons(green).
> http://www.nabble.com/file/p23840915/missed_features.jpg
> 
> 
> 
> 
> Paul Ramsey-4 wrote:
>> 
>> Slow-vs-fast I can understand... the spatial index can more 
>> effectively parcel out the problem for processing when the units of 
>> processing are smaller.
>> 
>> Wrong-vs-right I have a problem with. Are your multi-polygons valid?
>> Run ST_IsValid() on them. It's possible that they are valid taken 
>> separately but invalid when combined (multipolygons are not allowed to 
>> have overlapping components).
>> 
>> Paul
>> 
>> On Tue, Jun 2, 2009 at 11:05 AM, sgrocho2 <sgrocho at gmail.com> wrote:
>>>
>>> I am having trouble querying a multipolygon feature intersection with 
>>> another multipolygon feature.  When I execute the query it takes 45 
>>> seconds and returns and incomplete answer, 167 rows.  If I explode 
>>> the parcel polygon into its 6 parts and then run the same query it 
>>> takes 5 seconds and returns
>>> 209 rows which is the correct answer.  Is there a way to write this 
>>> query to return the all the rows without having to first manipulate 
>>> the data?  The same query works fine in SQL Server but we want to use 
>>> PostGIS. Below is the code for each of the Parcel tables they include 
>>> only one parcel - one with 1 row, the other with 6 rows.
>>>
>>> THIS IS THE SINGLE FEATURE MULTIPOLYGON CODE: 45sec 167 rows SELECT 
>>> DISTINCT g1.objectid As taxpar_id, g1.tax_parcel_num As 
>>> parcel_number, g2.objectid As fw_id, g2.wdfw_phs, g2.wdfw_occur_pt, 
>>> g2.wdfw_haulout, g2.pc_for_fish, g2.wdfw_fish_dist, g2.pc_oaks, 
>>> g2.pc_salmon, g2.wdfw_occur_pl, g2.doh_sf, g2.wdfw_owl, 
>>> g2.pc_eelgrass, g2.pc_hydro, g2.wdfw_sasi
>>>        FROM loader.single_parcel As g1, 
>>> loader.potential_fw_habitat_cons_areas_pg
>>> As g2
>>>    WHERE (g1.tax_parcel_num = '0618161001' AND 
>>> ST_Intersects(ST_Buffer(g1.Shape,-1), g2.Shape))
>>>
>>> THIS IS THE EXPLODED 6 FEATURES CODE: 5sec 209 rows SELECT DISTINCT 
>>> g1.objectid As taxpar_id, g1.tax_parcel_num As parcel_number, 
>>> g2.objectid As fw_id, g2.wdfw_phs, g2.wdfw_occur_pt, g2.wdfw_haulout, 
>>> g2.pc_for_fish, g2.wdfw_fish_dist, g2.pc_oaks, g2.pc_salmon, 
>>> g2.wdfw_occur_pl, g2.doh_sf, g2.wdfw_owl, g2.pc_eelgrass, 
>>> g2.pc_hydro, g2.wdfw_sasi
>>>        FROM loader.multi_parcel As g1, 
>>> loader.potential_fw_habitat_cons_areas_pg
>>> As g2
>>>    WHERE (g1.tax_parcel_num = '0618161001' AND 
>>> ST_Intersects(ST_Buffer(g1.Shape,-1), g2.Shape))
>>>
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