Hi Martin, > I just noticed that the code we use for the CombineSelectedFeature plugin > is very close to GeometryFactory.buildGeometry() method. > Hence, I checked buildGeomery code and found that in the particular case > of two adjacent polygons, it will also produce an invalid geometry. And probably also in all the cases of overlapping polygons, which means the suggested change is not just a corner case.
Michaël > > I don't know what would be the cost of an extra test for validity, but I > think > buildGeometry should never return invalid geometries from valid input. > > Best regards, > > Michaël > >> Hi, >>> Is it in OJ possible to check first for valid >>> geometries when >>> "Combine Selected Features" >>> or is this a great problem? >>> >>> If it is a great problem maybe it is better >>> "Combine Selected Features" >>> makes always a geometrycollection? >> There are several points : >> >> 1 - in the step by step process you described, error should >> have been thrown while combining the two polygons, not >> while moving the resulted invalid multipolygon >> >> 2 - I agree that in this case, building a valid GeometryCollection >> is better than building an invalid Multipolygon, even if >> GeometryCollection are much less useful (many operations >> accepting MultiPolygon will fail on GeometryCollection). >> If if no one has any objection, I propose to do the change. >> >> 3 - you surely knows that OpenJUMP has a hidden option to >> accept/refuse invalid geometries during edit operations (in >> this case, it is of no help though) >> >> Regards, >> >> Michaël >>> What do you think? >>> >>> Regards >>> >>> Uwe >>> >>> Am 09.04.2013 03:21, schrieb Martin Davis: >>>> 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 >>>>>>>>>> >>>>>>>>>> ------------------------------------------------------------------------------ >>>>>>>>>> >>>>>>>>>> >>> > > ------------------------------------------------------------------------------ > 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. 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