[postgis-users] valid MULTIPOLYGONs and order of vertices?

[Hubert Fröhlich]

> You can use simplify(geom, 0) as a mean to remove useless vertexes
> (in [0 0, 0 5, 0 10] 0 5 is useless)
thanks!

> 
> I haven't read the specs about OGC validity of the shapes,
> anyway JTS gives the same valid result (postgis/jts is a killer
> GEOS debugger!!).


I think that shape is only "near" OGC SFS, but the "standard" (i.e. it 
is   a de-facto standard, "commonly used")  was developed by ESRI and 
the last update was 1998 . The current SFS came out in 1999 . Some 
concepts are the same, but in detail - hmmm, I read 
http://archives.postgresql.org/pgsql-general/2002-10/msg00561.php (page 8)

"Because this specification does not forbid consecutive points with 
identical coordinates, shapefile readers must handle such cases. On the 
other hand, the degenerate, zero length or zero area parts that might 
result are not allowed." According to this, the Postgis way of Shape 
should be OK and my statement (see below: " does not conform to the 
specs") is somewhat wrong. However, we have customers who claim that 
some ESRI products report this behavior as an error.

So, the dilemma of the quasi-standard
  * A correct Shape is what is defined by the specs (or is it just  a 
white paper?)
vs.
  * A correct Shape is (more or less) what can be treated by ESRI 
software without errors

will remain forever...

Greetings,
Hubert

> 
> --strk;
> 
> On Tue, Apr 26, 2005 at 01:13:22PM +0200, Hubert Fröhlich wrote:
> 
>>Hi list,
>>I am using Postgres 8.0.0, Postgis 1.0.0 RC1 and geos 2.1.1 under SuSE
>>Linux 9.2.
>>
>>
>>Say I have a polygon like
>>
>>
>>
>> select isValid('MULTIPOLYGON(((4457484.31 5403325.86, 4457473.8 
>>5403324.77, 4457473.29 5403329.76, 4457472.55 5403330.04, 4457471.9 
>>5403330.68, 4457471.64 5403331.45, 4457471.26 5403335.14, 4457471.42 
>>5403335.87, 4457471.88 5403336.47, 4457472.53 5403336.79, 4457472.08 
>>5403341.31, 4457472.04 5403341.71, 4457472.03 5403341.83, 4457513.210 
>>5403346.130, 4457513.219 5403346.129, 4457513.266 5403346.120, 
>>4457513.314 5403346.109, 4457513.360 5403346.096, 4457513.406 
>>5403346.081, 4457513.452 5403346.063, 4457513.496 5403346.044, 
>>4457513.539 5403346.022, 4457513.582 5403345.998, 4457513.623 
>>5403345.973, 4457513.663 5403345.945, 4457513.670 5403345.940, 
>>4457513.680 5403345.933, 4457513.718 5403345.903, 4457513.754 
>>5403345.871, 4457513.789 5403345.837, 4457513.823 5403345.802, 
>>4457513.854 5403345.765, 4457513.884 5403345.727, 4457513.912 
>>5403345.687, 4457513.938 5403345.646, 4457513.963 5403345.604, 
>>4457513.985 5403345.561, 4457513.99 5403345.55, 4457514.61 5403339.69, 
>>4457515.27 5403339.78, 4457515.76 5403335.67, 4457515.04 5403335.56, 
>>4457515.660 5403329.770, 4457515.660 5403329.769, 4457515.643 
>>5403329.718, 4457515.624 5403329.668, 4457515.602 5403329.619, 
>>4457515.578 5403329.571, 4457515.552 5403329.524, 4457515.524 
>>5403329.478, 4457515.494 5403329.434, 4457515.461 5403329.391, 
>>4457515.427 5403329.350,
>><i mean this>
>>4457515.390 5403329.310, 4457515.390 5403329.310,
>></i mean this>
>>4457515.378 5403329.298, 4457515.339 5403329.261, 4457515.299 
>>5403329.225, 4457515.257 5403329.192, 4457515.213 5403329.161, 
>>4457515.168 5403329.131, 4457515.122 5403329.104, 4457515.074 
>>5403329.079, 4457515.026 5403329.057, 4457515.01 5403329.05, 4457514.91 
>>5403329.03, 4457507.42 5403328.25, 4457484.31 5403325.86)))' );
>> isvalid
>>---------
>> t
>>
>>
>>Postgis will check it as valid despite it has two identic points 
>>following each other.
>>
>>a) Is this correct?
>>b) When I produce a shape from this with pgsql2shp, both points are 
>>written to a shape, producing a shape that does not conform to the specs.
>>c) How to avoid that? I.e. is there a way to suppress subsequent 
>>identical points?
>>-- 
>>Mit freundlichen Grüßen / With kind regards
>>
>>Hubert Fröhlich
>>
>>-------------------------------------------------------------------------------
>>Dr.-Ing. Hubert Fröhlich			
>>Bezirksfinanzdirektion München 			
>>Alexandrastr. 3, D-80538 München, GERMANY
>>Tel. :+49 (0)89 / 2190 - 2980
>>Fax  :+49 (0)89 / 2190 - 2997
>>hubert dot froehlich at bvv dot bayern dot de
>>_______________________________________________
>>postgis-users mailing list
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>>http://postgis.refractions.net/mailman/listinfo/postgis-users
> 
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-- 
Mit freundlichen Grüßen / With kind regards

Hubert Fröhlich

-------------------------------------------------------------------------------
Dr.-Ing. Hubert Fröhlich			
Bezirksfinanzdirektion München 			
Alexandrastr. 3, D-80538 München, GERMANY
Tel. :+49 (0)89 / 2190 - 2980
Fax  :+49 (0)89 / 2190 - 2997
hubert dot froehlich at bvv dot bayern dot de