[postgis-users] bow-tie polygons and the like

[Martin Davis]

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
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>>   
>>     
>
>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022