Martin, Thanks for the info. I ended up editing out the offending polygons with OpenJump; there were only about 20. They were created by someone's sloppy geoprocessing and we were the lucky beneficiaries.
I guess what is interesting is the self-intersecting and loop self-intersecting polygons were built by ogr2ogr when converting from ArcInfo coverages to PostGIS. I need to think about this to decide whether I'm up to writing some code to try to automate the kind of conversion you mention. Again, thanks; and also to Nicolas who sent me in the right direction in the first place. Martin Davis wrote: > Chris, > > This is a classic self-intersecting polygon, which self-touches at a > single point. These are invalid according to the OGC model. They are > valid in some other spatial models (notably ArcSDE), where they are > referred to as "inverted polygons", and are used to represent polygons > with a hole that touches an edge. In the OGC model, that situation > must be represented as a polygon with a shell ring and a separate hole > ring (which is in turn invalid in ArcSDE). > > You can inter-convert between the two models, but it does require some > fussy bookkeeping. > > I'm a bit surprised by the area discrepancy - even if this shape is > illegal, the area computation should be well-defined. At least, > that's the way it would work in JTS/GEOS. The area code in PostGIS > may be different, however. > > > > Chris Hermansen wrote: >> Martin, all; >> >> Here is the wkt for the enclosing polygon: >> >> 53327 | SRID=1;POLYGON((1224673.7804 499721.97084,1224664.2479 >> 499721.56584,1224639.311 499709.63254,1224658.0919 >> 499675.28084,1224664.2479 499721.56584,1224673.1624 >> 499747.57644,1224676.7094 499739.52814,1224676.916 >> 499740.76654,1224692.459 499812.83074,1224668.6628 >> 499811.81964,1224658.0339 499811.36804,1224630.597 >> 499769.81704,1224627.078 499734.63704,1224622.024 >> 499724.36664,1224616.0171 499712.16004,1224595.8521 >> 499691.28704,1224573.8378 499685.40964,1224561.8108 >> 499682.19864,1224555.2798 499681.92114,1224553.7247 >> 499680.03974,1224549.0699 499678.79694,1224549.2472 >> 499674.62264,1224550.0095 499656.68104,1224534.9948 >> 499656.04304,1224527.759 499655.73564,1224525.1614 >> 499651.95204,1224525.556 499642.66424,1224526.0666 >> 499630.64754,1224527.1273 499605.67644,1224502.1236 >> 499604.61414,1224503.1844 499579.64304,1224478.1807 >> 499578.58064,1224479.2415 499553.60954,1224466.8676 >> 499553.08384,1224458.2316 499552.71694,1224454.6943 >> 499541.79814,1224454.9879 499534.88804,1224450.0691 >> 499527.35404,1224431.6821 499499.19104,1224417.7948 >> 499486.96474,1224416.8351 499484.88504,1224410.5131 >> 499470.72264,1224410.1876 499470.02924,1224409.5706 >> 499468.87504,1224408.8795 499467.76364,1224408.658 >> 499467.43914,1224405.783 499463.31414,1224405.2422 >> 499462.57484,1224404.4119 499461.56314,1224404.1681 >> 499461.29034,1224397.5431 499454.00904,1224397.4215 >> 499453.87654,1224386.8436 499442.45874,1224379.5284 >> 499434.45384,1224369.2823 499422.31714,1224369.0369 >> 499422.03184,1224368.1423 499421.07654,1224367.1869 >> 499420.18194,1224366.6558 499419.73214,1224355.8777 >> 499410.92434,1224341.2698 499396.04914,1224341.1422 >> 499395.92044,1224329.9105 499384.68884,1224324.2636 >> 499378.59734,1224302.9088 499332.37814,1224392.4667 >> 499355.14934,1224393.122 499355.91874,1224487.2901 >> 499450.78514,1224496.2156 499459.77684,1224493.9911 >> 499462.52154,1224506.465 499484.35254,1224564.6899 >> 499528.85044,1224605.2731 499587.61344,1224614.2741 >> 499599.88254,1224625.0241 499614.53564,1224653.1386 >> 499652.85784,1224658.315 499659.91354,1224667.3321 >> 499683.31754,1224673.7804 499721.97084)) >> >> point 0 is the se corner of the bow tie. point 1 is the "knot" of the >> bow-tie, and you will see it reappear as point 4. the triangle made up >> of points 1, 2, 3, 4 is the one I mentioned previously that goes in the >> wrong direction. >> >> I must confess to a slight simplification in my previous e-mails; the >> bow tie is not quite perfectly formed; it's southeast corner has been >> trimmed off so that the eastern "triangle" is actually a >> quadrilateral. Sorry :D >> >> Martin Davis wrote: >> >>> Chris, >>> >>> Can you post the WKT for the problematic feature? (Ascii art is >>> nifty, but not really a substitute for a 1600x1200 pixel display... >>> 8^) >>> >>> Chris Hermansen wrote: >>> >>>> Hi Nicolas, all; >>>> >>>> Nicolas, I understand your comment. But after looking at the data >>>> some >>>> more in OpenJump, that appears to not be my problem. >>>> >>>> If I load the PostGIS theme into OpenJump and run the QA on it, I see >>>> that it is the polygon that contains the bow-tie shaped figure that is >>>> the problem. >>>> >>>> I can represent this schematically as: >>>> >>>> +-----------------+ >>>> | | >>>> | |><| >>>> | | >>>> +-----------------+ >>>> >>>> (again, I hope this works out visually. I have done the figure in >>>> fixed-width text). >>>> >>>> OpenJump's QA tool shows that there is an error in the enclosing >>>> polygon >>>> at the "bow-tie point", and that the enclosing polygon is a bad >>>> feature. >>>> >>>> The triangle polygons themselves are not shown as bad features. There >>>> are points shown at all corners of the triangles, ie there is no real >>>> "bow-tie" type self intersection. >>>> >>>> The enclosing polygon contains all the edges shown in the above >>>> figure, >>>> including those of the triangles. Each triangle is a separate >>>> polygon. >>>> >>>> I've traced the order of the edges of the enclosing polygon and it >>>> appears that it is the ordering of these edges that is creating the >>>> crossing / bow-tie in the enclosing polygon. That is, the enclosing >>>> polygon's points define the edges as follows: >>>> >>>> pt 0: se corner of east triangle >>>> pt 1: bow-tie point >>>> pt 2: nw corner of west triangle >>>> pt 3: sw corner of west triangle >>>> pt 4: bow-tie point >>>> pt 5: ne corner of east triangle >>>> ... and off to the remainder of the enclosing polygon points >>>> >>>> So it appears to me that the edges of the western triangle are >>>> actually >>>> traversed in the wrong order in the enclosing polygon. As well, >>>> clearly >>>> there is a self-intersection in the enclosing polygon. >>>> >>>> I conclude from this that the order of traversal of the western >>>> triangle >>>> edges could be reversed and preserve the clockwise traversal of the >>>> enclosing polygon's edges. However I think the fact that the bow-tie >>>> point is traversed twice in the same polygon is a real problem that >>>> must >>>> be repaired (manually in this case, I guess). >>>> >>>> Does anyone have any comments on this? Thanks in advance! >>>> >>>> Nicolas Ribot wrote: >>>> >>>> >>>>>> Good people; >>>>>> >>>>>> I have this odd problem that I hope others may clarify for me. >>>>>> >>>>>> I have a big ArcInfo polygon coverage produced by a sequence of >>>>>> spatial >>>>>> unions. One of the component polygon coverages was brutalized >>>>>> somewhere >>>>>> along the way and it has some polygons with triangular and bow-tie >>>>>> inclusions. >>>>>> >>>>>> Nevertheless, the coverage is clean and sober as far as ArcInfo is >>>>>> concerned. >>>>>> >>>>>> If you all can picture in your minds a bow-tie polygon |><| >>>>>> where the >>>>>> rightmost | of the bowtie is actually the edge of an enclosing >>>>>> polygon, kind >>>>>> of like: >>>>>> >>>>>> | >>>>>> |><| >>>>>> | >>>>>> >>>>>> (I sure hope that works out visually...) >>>>>> >>>>>> OK, when I use ogr2ogr to import this coverage into postgis 1.3.1 / >>>>>> geos-3.0.0rc4, I see the following: >>>>>> >>>>>> >>>>>> >>>>>> the containing polygon fails at st_isValid() >>>>>> the st_area() of the containing polygon appears to include the area >>>>>> of the >>>>>> bow-tie polygon, ie it is larger than the area copied over from the >>>>>> ArcInfo >>>>>> coverage by about the same amount as the area of the bow-tie >>>>>> polygon When I >>>>>> look at this little mess with OpenJump, sure enough the QA routines >>>>>> find the >>>>>> bow-tie and complain. >>>>>> >>>>>> The long and the short of this is that PostGIS returns slightly >>>>>> more area >>>>>> than ArcInfo does for the same big ugly polygon coverage, because >>>>>> of the >>>>>> apparent loss of these bow ties. >>>>>> >>>>>> What I'm not sure about is what exactly is wrong here. Is the >>>>>> bow-tie >>>>>> shape itself illegal? Or is it the fact that both sides of the >>>>>> bow-tie >>>>>> belong to the same polygon? Or something else I'm missing? >>>>>> >>>>>> Thanks for any light anyone can shed on this. >>>>>> >>>>>> >>>>>> >>>>> Hi, >>>>> I will only give a partial answer: >>>>> yes, a bowtie polygon is invalid for PostGIS (as stated in the OGC >>>>> SFSQL documentation, polygon's boundary cannot cross itself). >>>>> If you want to represent this with valid polygons, you will need 2 >>>>> triangles touching at one point. >>>>> >>>>> HTH >>>>> nicolas >>>>> _______________________________________________ >>>>> postgis-users mailing list >>>>> postgis-users@postgis.refractions.net >>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users >>>>> >>>> >> >> >> > -- Regards, Chris Hermansen · mailto:[EMAIL PROTECTED] tel:+1.604.714.2878 · fax:+1.604.733.0631 Timberline Natural Resource Group · http://www.timberline.ca 401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5 C'est ma façon de parler. _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
