Converting a PostGIS raster back to vector

Question

Is it possible to convert a PostGIS raster back to a line?

I'd like to generalize some vector data by converting it to raster and altering the cellsize to fix 'gaps' in the vector connectivity.

Once I've reached a good raster cellsize, I'd like to convert that raster back to a vector line, connecting all the cell centroids into continuous lines within a given tolerance (so as not to connect too many adjacent cells at intersections).

I figure the query would look something like this, but can't figure out what the outer function would be:

Convert To Vector (line) (
  Convert to Raster(cellsize)
  (
  Original Line (geom)
  )
)

One simple example would be to convert the raster 'line' back to a vector as in the ST_AsRaster documentation SQL:

ST_AsRaster(
        ST_Buffer(
            ST_GeomFromText('LINESTRING(50 50,150 150,150 50)'), 10,'join=bevel'), 
            200,200,ARRAY['8BUI', '8BUI', '8BUI'], ARRAY[118,154,118], ARRAY[0,0,0]);

Answer 1

You would use something like:

-- Dump As Polygons -- http://postgis.net/docs/RT_ST_DumpAsPolygons.html

or

-- ST_Polygon -- http://postgis.net/docs/RT_ST_Polygon.html

To convert your your raster back to a geometry. Except note that since rasters are tiny pixels, you get back an areal geometry.

In PostGIS 2.2.0 -- release is eminent, if you have built with SFCGAL, you can use

the ST_ApprovimateMedialAxis.html

http://postgis.net/docs/manual-dev/ST_ApproximateMedialAxis.html

the ST_StraightSkeleton function,

http://postgis.net/docs/manual-dev/ST_StraightSkeleton.html

I think ST_ApprovimateMedialAxis.html does close to what you want when applied to the polygon you get back from raster. I think others have plpgsql functions lying around that do something similar too if you can't get your hands on SFCGAL enriched PostGIS 2.2