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

[strk at refractions.net]

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

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!!).

--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
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