(This article was first published on

**theBioBucket***, and kindly contributed to R-bloggers)I’m posting a recent project’s spin-off, which is a custom line-generalizer which I used for huge KML-paths. Anyone with a less clumpsy approach?

## 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)))

To

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