[postgis-users] How to simplify and complex (i.e. slow) geometry

[Martin Davis]

Well, don't go by what the documentation says...  8^)  AFAIK (and on 
good authority) buffer should work fine for *any* geometry type.  In 
particular, it will work fine for a MultiPolygon (even one that's 
invalid) - which is what you should have.

The error you're getting is due to the constraint on the table limiting 
it to POLYGON geometries, which was generated by the AddGeometryColumn 
function.  Probably the simplest thing to do is to simply drop this 
constraint.  (You could also change to using MULTIPOLYGON - but 
apparently this will then prevent storing POLYGONs, which may or may not 
be an issue).

Let me know how it goes...

Mike Dvorak wrote:
> Hi Martin/PostGIS users,
>
> Thanks for your response Martin.  Although I tried your suggestion, I
> met some problems.  For one, the documentation for ST_Buffer says that
> you *cannot* use ST_Buffer on a collection.  Isn't that exactly what I'm
> doing by trying to buffer something from ST_Collect ?  I got an error
> when I tried your suggestion (see below):
>
> Here's what I tried:
>
> CREATE TABLE buffered_mask_10( key SERIAL PRIMARY KEY );
>
> SELECT AddGeometryColumn('buffered_mask_10', 'the_geom', 3310,
> 'POLYGON', 2);
>
> INSERT INTO buffered_mask_10 (the_geom) SELECT
> ST_Buffer(Transform(ST_Buffer(ST_Collect(the_geom), 0.0), 3310), 2500)
> AS the_geom FROM usgs_bathy_mm5_20_50_200m
>
> Exception in thread "main" org.postgresql.util.PSQLException: ERROR: new
> row for relation "buffered_mask_10" violates check constraint
> "enforce_geotype_the_geom"
>
> Any more ideas?
>
> Thanks!
> -Mike
>
> Martin Davis wrote:
>   
>> Mike,
>>
>> If I understand your query, it looks like you are running buffer(0) on
>> *each* polygon in your query mask.  This is my bad - what I should have
>> said was that you need to ST_Collect all your query polys into a single
>> MultiPolygon, and then run buffer(0) on that to get your final query
>> geometry.  (The MultiPoly you create will be invalid, but buffer()
>> doesn't care).
>> Actually, I'm told that ST_Union can act as an aggregator itself (when
>> given a single argument).  However, I suspect that it is not using an
>> efficient method of unioning the polygons.
>>
>> Once you have your query polygon, another thing you might try is
>> simplifying it using a small tolerance.  This depends on your need for
>> accuracy, but it can reduce the number of points (and hence speed things
>> up) significantly.
>>
>>
>> Mike Dvorak wrote:
>>     
>>> Thanks for the response Martin.  Unfortunately, that produces about the
>>> same results, but maybe I'm misinterpreting your advice.  Here's what I
>>> tried most recently:
>>>
>>> CREATE TABLE buffered_mask_5( key SERIAL PRIMARY KEY );
>>>
>>> SELECT AddGeometryColumn('buffered_mask_5', 'the_geom', 3310, 'POLYGON',
>>> 2);
>>>
>>> INSERT INTO buffered_mask_5 (the_geom) SELECT
>>> ST_Buffer(Transform(ST_Buffer(the_geom, 0.0), 3310), 2500) AS the_geom
>>> FROM usgs_bathy_mm5_20_50_200m
>>>
>>> Any more advice?
>>>
>>> Cheers,
>>> Mike
>>>
>>>
>>> Martin Davis wrote:
>>>  
>>>       
>>>> Try using ST_Buffer(mask, 0.0).  That should union all the geometries
>>>> into a single (Multi)polygon.
>>>>
>>>> You could also help us to find some funding to support porting the new
>>>> JTS optimized predicates to GEOS  ;^)
>>>>
>>>> Martin
>>>>
>>>>
>>>>
>>>> Mike Dvorak wrote:
>>>>    
>>>>         
>>>>> Dear PostGIS users,
>>>>>
>>>>> First off, I must say that I think the PostGIS project is absolutely
>>>>> great.  I've been using PostGIS daily for my research for about 6
>>>>> months
>>>>> now and have come to love it.  I deal with rather larger geospatial
>>>>> databases (as large as 200 GB), so I think I have been really pushing
>>>>> PostGIS (and Postgres) to its limits but having mostly success in
>>>>> doing so.
>>>>>
>>>>> Here's my current problem.  I have a complex 2D polygon geometry that
>>>>> I'm using as a mask, to limit what areas of wind fields from a weather
>>>>> model I insert into my database.  You can find an example mask of the
>>>>> "shallow" ocean waters near the San Francisco Bay here:
>>>>> http://www.stanford.edu/~dvorak/tmp/how-to-simplify-geom-example.png
>>>>>
>>>>> The problem is that this 2D polygon, contains a lot of redundancy for a
>>>>> mask and it takes forever (i.e. tens of hours) to create the
>>>>> intersection of the 2D mask polygon and the grid of model points that I
>>>>> want to mask (approximately 200 x 240 points).
>>>>>
>>>>> I tried using ST_ConvexHull and ST_Union (with the mask for both
>>>>> arguments) without luck to simplify this example mask in the URL.  Is
>>>>> there a PostGIS function that will get rid of all redundant mask
>>>>> objects
>>>>> i.e. objects that are completely contained within another object, or do
>>>>> I have to program this myself?
>>>>>
>>>>> Thanks for any advice!
>>>>>
>>>>> Cheers,
>>>>> Mike
>>>>>
>>>>>
>>>>>         
>>>>>           
>>>   
>>>       
>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022