[postgis-users] How to get the centerline of a polygon?

[Mark Fredrickson]

If you could break your rope into a series of smaller polygons such
that each new polygon has two opposite edges that are from the
original polygon, you could take the centroids of all these points and
create a linestring. If you could make these polygons infinitely
small, you'd have the "true" center line I think.

Still begs the question of how to cut up the big polygon, but perhaps
more doable in the end.

-Mark


On Jan 18, 2008 9:54 AM, Eric Yu <ericyu at gmail.com> wrote:
> Thanks for your comments. The questions you asked are exactly what I faced
> while I was searching for answers. It is easy to draw a centerline by human,
> but I don't even know where to start. My initial idea was like this:
>
> (Imagine we have a polygon, shaped like a rope, lying on the plane)
> 1. First we need to find the two ends of the rope
> 2. From the two "central points" of the two ends (if we can determine),
>     we can split the exterior ring of the rope into two line strings.
> 3. Along the two line strings, we can locate the points at 0.01, 0.02, 0.03, ...
>     with this method, maybe we can ignore the unmatched points on the two edges.
>
> However, the first step is hard  (just as what you said about where to
> "start" and "end").
> I can't figure out a good way to do this.
>
> In fact, the problem comes from here: I have a highway road network
> (up/down line,
> and ramp branching off, etc.) and I need to draw a simplified line
> following the road.
> So I first use ST_Buffer() and then union the buffers into a
> rope-shaped polygon.
> And there comes the centerline problem.
>
> Maybe there is some other way to achieve this?
>
> Thanks!
>
> Eric
>
>
> On Jan 18, 2008 11:11 PM, Stephen Woodbridge <woodbri at swoodbridge.com> wrote:
> >
> > Eric Yu wrote:
> > > Hi,
> > >
> > > Suppose I have a long shaped polygon, like river area or the road,
> > > is it possible to get the centerline of the area using PostGIS?
> > > Or with other GIS library/software? I searched and tried but failed to
> > > get any solution.
> >
> > I think the problem to start with is how are you going to define what
> > the center line of the polygon in algorithmic terms? Does the center
> > line change if you rotate it 90 degrees? Assuming you had a center line
> > of a polygon, how would the points of the centerline relate to the polygon.
> >
> > I assume you want some set of points that approximates a path between
> > the two adjacent polygon virtual "edges", but polygons are made of
> > multiple points and hence multiple edges so what points belong to which
> > "edge". Also if the number of points does not match on the opposite
> > "edge" how should that be handled? How does one determine what points
> > belong to the "start" and "end" "edge of the polygon that the center
> > line needs to "start" on and "end" on?
> >
> > If you can answer these questions clearly then it is possible to write
> > an algorithm to do what you want. But just looking at the polygon and
> > intuiting that there should be a centerline that follows some visual
> > path the your mind perceives is kind of hard to deal with.
> >
> > -Steve W
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