R GIS: Generalizer for KML Paths

## line generalizing function: takes two vectors of with x/ycoords 
## and return ids of x/y elements which distance to its next element
## is shorter than the average distance between consecutive vertices
## multiplied by 'fac'
check_dist <- function(x, y, fac) {
    dm <- as.matrix(dist(cbind(x, y)))
    
    ## supradiagonal holds distance from 1st to 2nd, 2nd to 3rd, etc. element
    d <- diag(dm[-1, -ncol(dm)])
    mean_dist <- mean(d)
    keep <- logical()
    
    ## allways keep first..
    keep[1] <- T
    for (i in 1:(length(x) - 2)) {
        keep[i + 1] < - (d[i] > mean_dist * fac)
        message(paste0("Distance from item ", i, " to item ", i + 1, " is: ", d[i]))
    }
    message(paste0("Treshold is: ", mean_dist * fac))
    cat("--\n")
    ## .. and always keep last
    keep[length(x)] <- T
    return(keep)
}

## Testing function check_dist:
x <- rnorm(5)
y <- rnorm(5)
(keep <- check_dist(x, y, 1.2))

plot(x, y)
lines(x[keep], y[keep], lwd = 4, col = "green")
lines(x, y, lwd = 1, col = "red")
text(x, y + 0.1, labels = c(1:length(x)))


## exclude vertices by generalization rule. coordinate-nodes with low number of vertices, 
## segments with less than 'min_for_gen' vertices will not be simplified, in any case coordinates will be
## rounded to 5-th decimal place

generalize_kml_contour_node <- function(node, min_for_gen, fac) {
    
    require(XML)
    
    LineString <- xmlValue(node, trim = T)
    
    LineStrSplit <- strsplit(unlist(strsplit(LineString, "\\s")), ",")
    
    # filter out empty LineStrings which result from strsplit on '\\s'
    LineStrSplit < - LineStrSplit[sapply(LineStrSplit, length) > 0]
    
    # all 3 values are required, in case of error see for missing z-values:
    x <- round(as.numeric(sapply(LineStrSplit, "[[", 1, simplify = T)), 5)
    y <- round(as.numeric(sapply(LineStrSplit, "[[", 2, simplify = T)), 5)
    z <- round(as.numeric(sapply(LineStrSplit, "[[", 3, simplify = T)), 5)
    
    # for lines longer than 'min_for_gen' vertices, generalize LineStrings
    if (length(x) >= min_for_gen) {
        keep <- check_dist(x, y, fac)
        x <- x[keep]
        y <- y[keep]
        z <- z[keep]
        xmlValue(node) <- paste(paste(x, y, z, sep = ","), collapse = " ")
        
        # for all other cases, insert rounded values
    } else {
        xmlValue(node) <- paste(paste(x, y, z, sep = ","), collapse = " ")
    }
}

## mind to use the appropiate namespace definition: alternatively use: 
## c(kml ='http://opengis.net/kml/2.2')
kml_generalize <- function(kml_file, min_for_gen, fac) {
    doc <- xmlInternalTreeParse(kml_file)
    nodes <- getNodeSet(doc, "//kml:LineString//kml:coordinates", c(kml = "http://earth.google.com/kml/2.0"))
    mapply(generalize_kml_contour_node, nodes, min_for_gen, fac)
    saveXML(doc, paste0(dirname(kml_file), "/simpl_", basename(kml_file)))
}

## get KML-files and generalize them
kml_file <- tempfile(fileext = ".kml")
download.file("http://dev.openlayers.org/releases/OpenLayers-2.13.1/examples/kml/lines.kml", 
              kml_file, mode = "wb")
kml_generalize(kml_file, 5, 0.9)
shell.exec(kml_file)
shell.exec(paste0(dirname(kml_file), "/simpl_", basename(kml_file)))