No subject
Tue Oct 7 10:08:30 PDT 2008
elevation point.
e.g.
POINT (2 2)
MULTILINSTRING((0 0, 3 3, 6 6), (3 3, 3 4, 4 5))
Returns would be
LINESTRING(0 0, 3 3)
So if I write it in pseudo code, function prototype should look like :
LINESTRING get_matching_segment(MULTILINESTRING source, POINT needle)
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.
Any tips about detailed existing functions' documentation and the way to
create custom SQL functions would be greatly appreciated.
Thank you :-).
Best regards.
Nicolas
Gillet
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<p class=3DMsoNormal><span lang=3DEN-US>Hello,<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>I am new in postgis environment =
and start
to discover this powerful tool.<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>I am facing problem with the =
data I have
and need to transform them.<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>The behavior I would reach =
is<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>From one multilinestring extract =
the line segment
that intersects my elevation point.<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>e.g.<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>POINT (2 =
2)<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>MULTILINSTRING((0 0, 3 3, 6 6), =
(3 3, 3 4,
4 5))<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>Returns would =
be<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>LINESTRING(0 0, 3 =
3)<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>So if I write it in pseudo code, =
function
prototype should look like :<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>LINESTRING
get_matching_segment(MULTILINESTRING source, POINT =
needle)<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-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=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>Thank you =
:-).<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>Best =
regards.<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US><o:p> </o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>Nicolas<o:p></o:p></span></p>
<p class=3DMsoNormal><span lang=3DEN-US>Gillet<o:p></o:p></span></p>
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