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<p class=MsoNormal><span lang=EN-US>Hello,<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 am new in postgis environment and start
to discover this powerful tool.<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 am facing problem with the data I have
and need to transform them.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>I use a library to render maps based on “painter’s
algorithm”. (We are especially focused on roads transportation elements).<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>So to be more accurate I need to represent
road crossing properly with bridges above tunnels and not the opposite …<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 match elevation points and road
elements being able to see an “above / below” information but there
is a big problem :<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>One road element can match many elevation
points because a long road element can be once above and, a few meters further,
below another road. It becomes impossible to order them.<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>My only solution is to split road elements
into pieces which can have one and only one possible elevation.<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 think I will need a function in order to
do this and .. I have never written any SQL function :’(<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>The behavior I would reach is<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>From one multilinestring extract the line segment
that intersects my elevation point.<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>e.g.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>POINT (2 2)<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>MULTILINSTRING((0 0, 3 3, 6 6), (3 3, 3 4,
4 5))<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>Returns would be<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>LINESTRING(0 0, 3 3)<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>So if I write it in pseudo code, function
prototype should look like :<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>LINESTRING
get_matching_segment(MULTILINESTRING source, POINT needle)<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 found something about how to split a
MULTI* geometry (GeometryN(geometry, index)), getting points inside a geometry
(PointN(geometry, index) or getting a part of a geometry (Line_SubString(geometry,
double, double)), … but I can’t find a way to put them together to
get what I want.<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>Any tips about detailed existing functions’
documentation and the way to create custom SQL functions would be greatly
appreciated.<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>Thank you :-).<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>Best regards.<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>Nicolas<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US>Gillet<o:p></o:p></span></p>
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