[postgis-users] Re: Polygon validity with one common point

[Nicolas Ribot]

Sorry about this post, reading an old spec leads to this
misinterpretation of the polygon validity.

I attach anyway the picture describing the polygon.
The common point in the given polygon is ("936.000000 211.000000")


2008/11/28 Nicolas Ribot <nicky666 at gmail.com>:
> Hi all,
>
> I'v got a validity issue with one polygon.
> As you can see in the attached picture, i've got a polygon with a
> linear ring that touch itself at one point :
>
> <gml:Polygon srsName="urn:ogc:def:derivedCRSType:OGC:1.0:image">
>         <gml:exterior>
>           <gml:LinearRing>
>             <gml:posList>33.000000 889.000000 51.000000 894.000000
> 64.000000 912.000000 80.000000 924.000000 119.000000 923.000000
> 125.000000 929.000000 157.000000 928.000000 160.000000 915.000000
> 179.000000 914.000000 186.000000 907.000000 186.000000 901.000000
> 189.000000 901.000000 190.000000 913.000000 211.000000 918.000000
> 211.000000 936.000000 255.500000 938.500000 300.000000 941.000000
> 295.000000 945.000000 249.000000 947.000000 223.000000 973.000000
> 210.000000 977.000000 211.000000 936.000000 184.000000 934.500000
> 157.000000 933.000000 156.000000 929.000000 125.250000 930.500000
> 94.500000 932.000000 63.750000 933.500000 33.000000 935.000000
> 33.000000 889.000000</gml:posList>
>           </gml:LinearRing>
>         </gml:exterior>
> </gml:Polygon>
>
> Or in WKT
>
> MULTIPOLYGON(((889.000000 33.000000,894.000000 51.000000,912.000000
> 64.000000,924.000000 80.000000,923.000000 119.000000,929.000000
> 125.000000,928.000000 157.000000,915.000000 160.000000,914.000000
> 179.000000,907.000000 186.000000,901.000000 186.000000,901.000000
> 189.000000,913.000000 190.000000,918.000000 211.000000,936.000000
> 211.000000,938.500000 255.500000,941.000000 300.000000,945.000000
> 295.000000,947.000000 249.000000,973.000000 223.000000,977.000000
> 210.000000,936.000000 211.000000,934.500000 184.000000,933.000000
> 157.000000,929.000000 156.000000,930.500000 125.250000,932.000000
> 94.500000,933.500000 63.750000,935.000000 33.000000,889.000000
> 33.000000)))
>
> This polygon seems to be not valid for postgis since it detects a
> Self-Intersection
>   select polygon_id from polygon where isvalid(the_geom) = false;
>   INFO: Ring Self-Intersection
>
> The OGC Simple Feature for SQL says  : "A Polygon has no rings that
> cross. The rings in the boundary of a Polygon may intersect at a
> Point, but only as a tangent"
>
> If i well understand this definition, the polygon seems to be correct
> and should be valid for postgis...
> Am i wrong ?
>
> Regards
>
> Jerome Gasperi
>
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