[postgis-users] ST_Difference

[Paul Ramsey]

Oh, yeah, oops, I know. It's the join logic, its finding the things  
not contained for each world feature, not for the full world set.

select pd.* from pandatestdata pd,
(select p.id
from pandatestdata p, world2_12nm w
where st_contains(w.the_geom, p.the_geom)) as wd where pd.id <> wd.id;

Ugly, but probably effective: find all the things contained, then  
just strip those out.

The trouble is that disjointness is not an easily spatially indexable  
operation. If you're going to take this into operation with large  
data volumes, a more effective data set would be ocean polygons, cut  
up into smallish regular grid squares, then you can test containment  
with relatively good index selectivity.

P.

On 24-Oct-07, at 1:53 PM, Richard Heimann [C] wrote:

> Thanks Paul for the response. I hadn’t thought of this logic, it  
> didn’t work
> however. Strangely, it increased the number of features by eight  
> times. Any
> thoughts?
>
> Vr
> Rich
>
>
>
> -----Original Message-----
> From: Paul Ramsey [mailto:pramsey at refractions.net]
> Sent: Wednesday, October 24, 2007 4:07 PM
> To: Richard.Heimann at nrl.navy.mil; PostGIS Users Discussion
> Subject: Re: [postgis-users] ST_Difference
>
> select p.*
> from pandatestdata p, world2_12nm w
> where not st_contains(w.the_geom, p.the_geom);
>
> On 24-Oct-07, at 11:45 AM, Richard Heimann [C] wrote:
>
>> Thanks for your response W. I should have posted the sql query
>> earlier.
>> Anyway...its below.
>>
>> My goal is to drop all features (points) that fall within my
>> polygon. In
>> this case, I have ship track data and want to filter it with world
>> country
>> buffers.
>>
>> World2_12nm - my world country 12nm buffered polygon
>> Pandatestdata - track data (point)
>> Panda_diff2 - new table
>>
>>
>> CREATE TABLE panda_diff2 AS
>> SELECT AsText(Difference(world2_12nm.the_geom,
>> pandatestdata.the_geom)) FROM
>> world2_12nm, pandatestdata
>>
>> Also attached are the astext versions of my data as well as a csv
>> of the
>> output from the above sql. Please debunk the mystery...
>>
>> Thanks again
>> Rich
>>
>> -----Original Message-----
>> From: Webb Sprague [mailto:webb.sprague at gmail.com]
>> Sent: Wednesday, October 24, 2007 12:00 PM
>> To: Richard.Heimann at nrl.navy.mil; PostGIS Users Discussion
>> Subject: Re: [postgis-users] ST_Difference
>>
>> You might get more useful help if you post an "astext()" version of
>> your data, the query, the result, and  a *desired* result.
>>
>> I know the difference functions can be tricky and are sometiimes
>> defined in ways you might not expect.
>>
>> Thx
>> W
>>
>> On 10/24/07, Richard Heimann [C] <heimann at ait.nrl.navy.mil> wrote:
>>>
>>>
>>>
>>>
>>> All,
>>>
>>>
>>>
>>> I would appreciate help in constructing an SQL statement that
>>> performs a
>>> difference function on two datasets. My intersect function works but
>> cannot
>>> work out the kinks with st_difference. Thanks for your help.Im  
>>> new to
>>> Postgis.
>>>
>>>
>>>
>>> Rich
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
>>>
>>
>>
>>
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>
> <panda_diff2.csv>