Identify a linear feature on a raster map and return a linear shape object using R

Question

I would like to identify linear features, such as roads and rivers, on raster maps and convert them to a linear spatial object (SpatialLines class) using R.

<

Answer 1

Thanks to @klewis at gis.stackexchange.com for linking to this elegant algorithm for finding the centre line (in response to a related question I asked there).

The process requires finding the coordinates on the edge of a polygon describing the linear feature and performing a Voronoi tessellation of those points. The coordinates of the Voronoi tiles that fall within the polygon of the linear feature fall on the centre line. Turn these points into a line.

Voronoi tessellation is done really efficiently in R using the deldir package, and intersections of polygons and points with the rgeos package.

## Find points on boundary of rPoly (see question)
rPolyPts <-  coordinates(as(as(rPoly, "SpatialLinesDataFrame"),
                "SpatialPointsDataFrame"))

## Perform Voronoi tessellation of those points and extract coordinates of tiles
library(deldir)
rVoronoi <- tile.list(deldir(rPolyPts[, 1], rPolyPts[,2]))
rVoronoiPts <- SpatialPoints(do.call(rbind, 
                 lapply(rVoronoi, function(x) cbind(x$x, x$y))))

## Find the points on the Voronoi tiles that fall inside 
## the linear feature polygon
## N.B. That the width parameter may need to be adjusted if coordinate
## system is fractional (i.e. if longlat), but must be negative, and less
## than the dimension of a cell on the original raster.
library(rgeos)
rLinePts <- gIntersection(gBuffer(rPoly, width=-1), rVoronoiPts)

## Create SpatialLines object
rLine <- SpatialLines(list(Lines(Line(rLinePts), ID="1")))

The resulting SpatialLines object: