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I would like to simplify some polygons that represent rivers into linestrings. I assume that holes in polygons can be omitted. Anybody has some good idea how to simply do it?
It would be nice, if it was possible to be done directly in postgis or by another open source tool, but brief algorithm description will be sufficient.
What you need is called "skeletisation algorithm".
Have a look at this article:
Some of these algorithms are developped in the CGAL library.
In PostGIS 2.2 with SFCGAL, this can be done with ST_StraightSkeleton or ST_ApproximateMedialAxis, depending on your criteria.
the JUMP skeletonizer plugin is available as part of openjump http://www.openjump.org
Just as a warning, I've never tried this before. Your success will depend on the quality of the data you currently have.
So, think of the polygon as a strip of triangles. Each triangle connects to a vertex on the opposite side of the corridor with an edge. Now, for each of the edges in the corridor, simply place a vertex right smack in the center. These new vertices are the points of your linestring. You can probably fiddle with how you select the middle point on the edge for greater accuracy.
Once again, ET GeoWizards might be the answer. Try the Create Centerlines from Polygons tool.
