[postgis-users] ST_Difference

[Paul Ramsey]

On the flip side, if the number of "bad points" of ships-on-land is  
small, the stripping operation is probably faster than the positive  
containment test on all the ships-in-water points.

P

On 24-Oct-07, at 2:01 PM, Paul Ramsey wrote:

> 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
>>>>
>>>>
>>>>
>>>>
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>>>>
>>>>
>>>
>>>
>>>
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>>
>> <panda_diff2.csv>
>
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