[postgis-users] Strange behavior with a polygon and line that claim to intersect ....

[Nicklas Avén]

Well, I looked at it in QGIS and the line seems to cross the border of
the polygon. I have attached the part of the polygon where it crosses,
with the line in violet and the intersecting part in green.

/Nicklas

On Tue, 2011-01-25 at 15:00 -0500, Bryan Montgomery wrote:
> Hello,
> Is there a way to debug a problem I have with a polygon and line? I
> was running a query that used st_intersects and st_intersection to get
> the length of lines that are within the polygon. I was spot checking
> some of the data and came up with a particularly strange example.
> 
> Visually, the polygon looks as it should, the line clearly appears to
> be completely contained within the polygon however st_crosses returns
> true and intersection returns a subset of the line. I've tried this on
> postgis version 1.3 and 1.4 and both behave the same. I'm inclined to
> believe there is a stray point but given the polygon 2,700+ points I'm
> not sure how I find it! I suppose it's possible that there is a bug -
> or maybe I'm just completely missing something.
> 
> I tried finding the closest point on the polygon to the line - but
> when I tried to find a way to do this it seemed that the suggestions
> were for finding the closest point when the other point was outside
> the polygon?
> 
> Appreciate any help,
> 
> Thanks - Bryan.
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