Compressing Google Earth files with is not well documented on the Web, so I thought I should share my approach. I am working on my presentation for the upcoming Minnesota Assessment Conference. I will be presenting geographic maps of 2010 Minnesota assessment results. To minimize clutter in the maps and enhance the experience of stakeholders, I am creating two versions of each map: a static PDF map with no school district labels and a Web-based, interactive map. Each PDF will link to a keyhole markup language (KML) file through Google Maps, much like this proficiency map described here.
The KML file sizes exceed Google Maps’ interface limit, so I had to find a way to compress them into KMZ files. I could compress each KML file using Google Earth or 7-Zip, but since I am already creating the maps with , I decided to find a way to compress the KML files via code. Gunzipping is easy in (e.g., see gzfile), but Google Maps will not accept gunzipped KML files. Thanks to Duncan Temple Lang, the zip() function in library(Rcompression) will create KMZ files with standard ZIP compression. Here’s a reproducible example:
sw <- slot(wrld_simpl[wrld_simpl$NAME=="South Africa",], "polygons")[]
tf <- tempfile()
kmlPolygon(sw, kmlfile=paste(tf, ".kml", sep=""), name="South Africa", col="#df0000aa", lwd=5, border=4, kmlname="KMZ test", kmldescription="<i>KMZ</i> file created with <a href='http://www.r-project.org'>R</a>.")
zip(zipfile=paste(tf, ".kmz", sep=""), files=paste(tf, ".kml", sep=""))
paste(tf, ".kmz", sep="") #path and filename