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

[Stephen Woodbridge]

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