As Michael and Stefan point out, Polygons in a MultiPolygon must be edge-disjoint (which another way of stating the formal definition "must only touch at a finite number of points". If they touched along an edge, that would cause an infinite number of points to be coincident).
Another way of looking at this is that MultiPolygons are in a sense the canonical description of a given area of the plane. If edge touches or overlaps were allowed then there would be an infinite number of geometries which described the same area. Also, from the point of view of computing polygon overlay and spatial relationships this is a nice rule to have, since it reduces the number of cases which need to be checked for. This makes the code simpler and more performant. The cost is that it is necessary to ensure that MultiPolygons are valid at creation time. This is a reasonable tradeoff, since in general geometries are queried more often than they are created. GeometryCollections on the other hand have no particular semantics - they are just "bags" of geometries. This makes them useful for holding arbitrary sets of geometries, but makes them more complex (and sometimes slower) to process. On 4/8/2013 11:04 AM, Michaël Michaud wrote: > Hi Uwe, Stefan, > > OpenJUMP (JTS) is right, this MultiPolygon is not OGC conform > > Here is the citation : > Multipolygon > 2. The Boundaries of any 2 Polygons that are elements of a MultiPolygon > may not ‘cross’ and may touch > at only a finite number of points. (Note that crossing is prevented by > assertion 1 above). > > Don't ask me why, I've always thought it is strange that lines can > share their boundaries in a MultiLineString and polygons cannot > share an edge in a MultiPolygon, but it's a well established rule > that JTS follows. > > Michaël > > >> Hi Uwe, >> >> I am not sure I would call it a bug. OJ, should (try to) create data >> that are OGC conform, but in this case it doesn't. Which means, the case >> needs special treatment, but this is not implemented. >> >> That the multi-polygon causes an error is with the OGS SF specification >> = correct. However, that the geometry collection does not cause an error >> should be correct aw well, because there is, I believe, nothing said >> about geometry collections and their validity. Geometry collections >> should be allowed to have any type of geometries in what ever way they >> are drawn - like a "container". If we would check geometry collections >> for their validity it may be that people cannot store anymore the data >> they have. Hence, checking should be optional. >> >> But I guess here, Michael M. knows probably more about OGC conformance? >> I'll also cc to JTS list. >> >> cheers, >> stefan >> >> PS: the Multi-polygon: >> >> MULTIPOLYGON ((( >> 80 125, >> 80 241, >> 175 241, >> 175 125, >> 80 125 >> )), (( >> 175 125, >> 175 241, >> 263 241, >> 263 125, >> 175 125 >> ))) >> >> Am 08.04.13 09:48, schrieb Uwe Dalluege: >>> Hi Stefan, >>> >>> I am afraid I do not understand :-( >>> Do you think this is a bug in OJ? >>> The multipolygon causes an error >>> the geometrycollection not. >>> >>> Is this behaviour OGC-conform (simpel features...)? >>> What do you think? >>> >>> uwe >>> >>> Am 08.04.2013 16:36, schrieb Stefan Steiniger: >>>> Hi, >>>> >>>> so - well the situation is not so nice, as it should be valid. However, >>>> the JTS TestBuilder says the Multi-Polgyon is invalid because of >>>> "Self-intersection at or near point (175.0, 125.0, NaN)" >>>> >>>> Same message appears when you try to add it as a new feature. >>>> >>>> maybe you can make it valid before by running a zero-buffer over it? >>>> >>>> stefan >>>> >>>> Am 08.04.13 07:30, schrieb Uwe Dalluege: >>>>> Hi, >>>>> >>>>> if you put a third geometrie to the two polygons, >>>>> for instance a linestring, and combine them >>>>> you will receive a geometrycollection >>>>> and not a multipolygon. >>>>> >>>>> The geometrycollection causes *no* errors >>>>> but the multipolygon. >>>>> >>>>> Uwe >>>>> >>>>> >>>>> Am 08.04.2013 12:05, schrieb Uwe Dalluege: >>>>>> Hi, >>>>>> >>>>>> I get the error: >>>>>> >>>>>> "The new geometry is invalid. Cancelled." >>>>>> and I am not shure whether this error is correct. >>>>>> >>>>>> 1. Switch "Snap to vertices" option. >>>>>> >>>>>> 2. Draw a rectangle. >>>>>> >>>>>> 3. Draw a second rectangle with two identical >>>>>> vertices from the first rectangle (with snap). >>>>>> >>>>>> 4. Select the two rectangles and >>>>>> "Combine Selected features" >>>>>> >>>>>> 5. Try to move this multipolygon with >>>>>> "Move Selected Items Tool". >>>>>> >>>>>> 6. The error appears >>>>>> "The new geometry is invalid. Cancelled." >>>>>> >>>>>> 7. The QA>Validate Selected Layers... >>>>>> causes a self-intersection in *one* point. >>>>>> >>>>>> Is this a bug in OJ (a precision-problem) >>>>>> or is the new geometry really invalide? >>>>>> >>>>>> Uwe >>>>>> >>>>>> ------------------------------------------------------------------------------ >>>>>> >>>>>> Minimize network downtime and maximize team effectiveness. >>>>>> Reduce network management and security costs.Learn how to hire >>>>>> the most talented Cisco Certified professionals. Visit the >>>>>> Employer Resources Portal >>>>>> http://www.cisco.com/web/learning/employer_resources/index.html >>>>>> _______________________________________________ >>>>>> Jump-pilot-devel mailing list >>>>>> Jump-pilot-devel@lists.sourceforge.net >>>>>> https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >>>>>> >>>>> ------------------------------------------------------------------------------ >>>>> >>>>> Minimize network downtime and maximize team effectiveness. >>>>> Reduce network management and security costs.Learn how to hire >>>>> the most talented Cisco Certified professionals. Visit the >>>>> Employer Resources Portal >>>>> http://www.cisco.com/web/learning/employer_resources/index.html >>>>> _______________________________________________ >>>>> Jump-pilot-devel mailing list >>>>> Jump-pilot-devel@lists.sourceforge.net >>>>> https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >>>>> >>>> ------------------------------------------------------------------------------ >>>> >>>> Minimize network downtime and maximize team effectiveness. >>>> Reduce network management and security costs.Learn how to hire >>>> the most talented Cisco Certified professionals. Visit the >>>> Employer Resources Portal >>>> http://www.cisco.com/web/learning/employer_resources/index.html >>>> _______________________________________________ >>>> Jump-pilot-devel mailing list >>>> Jump-pilot-devel@lists.sourceforge.net >>>> https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >>>> >> ------------------------------------------------------------------------------ >> Minimize network downtime and maximize team effectiveness. >> Reduce network management and security costs.Learn how to hire >> the most talented Cisco Certified professionals. Visit the >> Employer Resources Portal >> http://www.cisco.com/web/learning/employer_resources/index.html >> _______________________________________________ >> Jump-pilot-devel mailing list >> Jump-pilot-devel@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel >> >> > > ------------------------------------------------------------------------------ > Minimize network downtime and maximize team effectiveness. > Reduce network management and security costs.Learn how to hire > the most talented Cisco Certified professionals. Visit the > Employer Resources Portal > http://www.cisco.com/web/learning/employer_resources/index.html > _______________________________________________ > Jump-pilot-devel mailing list > Jump-pilot-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel > > ------------------------------------------------------------------------------ Precog is a next-generation analytics platform capable of advanced analytics on semi-structured data. The platform includes APIs for building apps and a phenomenal toolset for data science. Developers can use our toolset for easy data analysis & visualization. Get a free account! http://www2.precog.com/precogplatform/slashdotnewsletter _______________________________________________ Jump-pilot-devel mailing list Jump-pilot-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/jump-pilot-devel
