[postgis-users] ST_Buffer returning multipolygons

[marquz74]

Stephen Woodbridge wrote
> On 1/18/2013 11:10 AM, marquz74 wrote:
>> Francois Hugues wrote
>>> We don't know what are your original geometries or what kind of buffer
>>> you apply (since you work with linestrings we can assume it is a
>>> positive buffer). Maybe some of your linestrings are multilinestrings
>>> with disjoint components.
>>
>> the query:
>>
>> SELECT GeometryType(the_geom)
>> FROM streams
>> WHERE GeometryType(the_geom)  <> 'LINESTRING';
>>
>> returns 0 records.
> 
> If you have a linestring that follows a stream or rivier then you likely 
> will have an ox bow like geometry, think of the letter Omega. If you add 
> enough buffer to this you will end up intersecting your buffer with 
> another part of the buffer. This creates polygon or multipolygon.
> 
> You could post you command and geometry so others can look at it.
> 
> select gid, astext(the_geom) from streams where 
> GeometryType(st_buffer(the_geom, 
> <distance>
> )) = 'MULTIPOLYGON';

Ok. I have a totale of 8554 streams (LINESTRINGs).
With the query

SELECT gid, AsText(the_geom) FROM streams 
WHERE GeometryType(st_buffer(the_geom, 100)) = 'MULTIPOLYGON';

I get 11 records as MULTIPOLYGONs

The first record is:

 674 | LINESTRING(640770.332537465 216785.425146015,640770.356311913
216785.560172686,640770.475488952 216786.226162019,640770.647396495
216787.800152365,640770.658978918 216789.268376902,640770.447752096
216792.399898366,640769.98018796 216795.624688816,640769.110287021
216798.527305023,640767.648768098 216801.891389348,640766.02448565
216804.805892778,640764.74005588 216807.134569434,640763.337363474
216809.560477487,640761.650901703 216812.246990061,640759.955600712
216815.327305019,640758.482499366 216817.868732504,640757.383693167
216819.548488661,640756.238862077 216821.226720821,640754.01503683
216824.434442434,640751.366929134 216828.435255234,640749.204368809
216832.450088865,640747.25516891 216836.227787618,640746.014020829
216838.801219168,640745.393751588 216840.360274284,640744.315976633
216842.854457673,640743.400050801 216844.65704848,640742.582575565
216846.277673322,640741.52491745 216848.478638524,640740.503835408
216850.705816576,640739.787553975 216852.270967708,640739.053289307
216853.851968469,640738.173025147 216855.546355058,640737.55488951
216856.568961104,640736.817272035 216857.822910812,640735.910795021
216859.35697228,640734.77510795 216861.236982439,640733.832969266
216862.938074642,640732.814325629 216865.039674844,640731.225095251
216869.012141189,640729.979984761 216871.879095724,640729.445974092
216873.02148841,640729.002794006 216873.679857725,640728.952197105
216873.745389857,640728.676962154 216874.089814544)

I isolated the stream and the related buffer, you can see large and small
zoom at this links:
https://docs.google.com/file/d/0B0MkYF-Ljs34MlNkTkJwc0dEbG8/edit
https://docs.google.com/file/d/0B0MkYF-Ljs34c1owdkc1U0Q4OFk/edit

Thank you for your patience :-)

Marco



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