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This algorithm will work if you can guarantee that there will be no
overlapping line segments on your linestrings. If overlaps are
possible, you will get a mixture of 4 line segment intersections and
pairs of 3 line segment intersections with overlap between them.<br>
<br>
To robustly determine if you have a transverse intersection between
2 linestrings:<br>
<br>
- create a circle centered at the intersection point (4 segment
case) or at the start and end of the overlap interval (3 segment
case)<br>
- decompose each circle into two circular arcs where one line string
intersects the circle<br>
<br>
You have a transverse intersection if the incoming line segment of
the other linestring intersects a different circular arc than the
outgoing line segment.<br>
<br>
On 30/05/2012 5:26 PM, Martin Davis wrote:
<blockquote cite="mid:4FC6828C.9060500@telus.net" type="cite">
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The DE-9IM model isn't designed to be able to distinguish between
these two cases, so there's no way to compute this by using any of
the named predicates, or even a custom relate matrix. <br>
<br>
You could do this via some fairly straightforward custom code.
The algorithm is:<br>
- identify vertices in each linestring which are coincident<br>
- build a simple "local topology" for the 4 incident line segments
at each intersection<br>
- examine the intersection to see if the segments alternate or are
adjacent (you can do this by using the angles of the segments)<br>
<br>
On 5/30/2012 3:40 AM, Jasmin FORMONT wrote:
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<p class="MsoNormal"><span lang="EN-US">Hi everyone<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I can’t find a way to
tell if two linestrings “really” cross each other.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">For instance if I have
this linestring :<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Line1 : LINESTRING(0
0, 1 1, 2 0)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">How can I
differentiate the relation of line 1 with one of the
following :<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Line2 : LINESTRING(0
2, 1 1, 2 2) -> no “real” cross, just a contact
point<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Line3 : LINESTRING(0
2, 1 1, 1 0) -> lines “really” cross each other<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I can’t find a good
combination of crosses, touches, etc… to differentiate
these cases.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<br>
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