Display R Spatial Objects With Google Maps/Google Earth

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# Generate KML files using three methods: 
# Method 1: KmlPolygon() / KmlLine() functions in Maptools package
# Method 2: ReadOGR() / WriteOGR() functions from rgdal package
# Method 3: ogr2ogr utility from FWTools package, executed using the R system() function                     # Bonus Method: Reading a KML file into an R Spatial object 
# rgdal provides readOGR() and writeOGR()
# maptools/sp provides transformation and kml translators
# PBSmapping provides polygon centroid creation function

  library(rgdal)  # loads 'dependant' sp package
  library(maptools)

# Remember to set the current working directory to the folder into which you unpacked this archive.

#  setwd("C:/Projects/GeospatialUseCasesMar2009/CreateKMLFiles/dl") #example

# Re-project the counties into Geographic (unprojected) space.
# Then, write the new polygons to a NEW shape file.

# Read a polygon shape file, which is in a Albers Equal Area map transformation

  NwCountiesAE <- readOGR("FiveNWStatesCounties.shp","FiveNWStatesCounties")

# Reference: For GoogleMaps overlays, project data into EPSG:3857 (Spherical Mercator projection)
# For this Use Case, we will project all Shape Files for GoogleEarth overlay   
  epsg3857String <- CRS("+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378137 +b=6378137 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs")   
# For GoogleEarth overlays, reproject data into EPSG:4326 (simple Latitude/Longitude 'geographic' projection)

  epsg4326String <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")
  NwCountiesGMap <- spTransform(NwCountiesAE,epsg4326String)
  writeOGR(NwCountiesGMap,".","NwCountiesGMap",driver="ESRI Shapefile")  

# Re-read the (reprojected) Shape File with readOGR to verify the projection information

  NwCountiesGMap <- readOGR("NwCountiesGMap.shp","NwCountiesGMap")

# Read a point shape file containing centroids for all Northwest counties 

  MapPolysCent <- readOGR("NwCountiesCentroids.shp","NwCountiesCentroids")

# We pass ONLY the Polygons component of the SPDF to kmlPolygon, so extract it.
# Create the KML with only Idaho counties (Idaho County FIPS code: 53)
# Extract the Polygons component of the SpatialPointsDataFrame,
# as needed by the kmlToPolygon() function.

# Note: May 2010 - kmlPolygon only writes the first spatial feature
# (with Polygon ID 0) from a Spatial...DataFrame. To capture all of 
# the spatial features in this file, need to 'drill down' to the 'polygons' layer) 
#  CountyPolys <- slot(NwCountiesGP[NwCountiesGP$STATEFIPS %in% c("53"),], "polygons")[[1]]  
#  kmlPolygon(CountyPolys, kmlfile="NWStateCountyPolys.kml", name="NWStateCountyPolys",
# col="blue", lwd=1,
#             border=1, kmlname="R Test",kmldescription="KML File:NW USA State Counties")

   sw  <- NwCountiesGMap[NwCountiesGMap$STATEFIPS %in% c("53"),] # extract desired SPDF subset 
   out <- sapply(slot(sw, "polygons"), function(x) {
                 kmlPolygon(x,
                            name=as(sw, "data.frame")[slot(x, "ID"), "STATEFIPS"],     
                            col="red", lwd=1.5, border='black', 
                             description=paste("ISO3:", slot(x, "ID")))
                                                   }
                 )  # outside of 'sapply' function call
   tf <- "MyNewKMLFILE.kml" 
   kmlFile <- file(tf, "w")

   cat(kmlPolygon(kmlname="R Test", kmldescription="Hello")$header,file=kmlFile, sep="\n")
   cat(unlist(out["style",]), file=kmlFile, sep="\n")
   cat(unlist(out["content",]), file=kmlFile, sep="\n")
   cat(kmlPolygon()$footer, file=kmlFile, sep="\n")
   close(kmlFile)

 # Alternate method: use the GDAL tools writeOGR() to convert and write the shapefile in KML format.  

   writeOGR(NwCountiesGMap, "NWStateCountyPolysOGR.kml", "NWStateCountyPolysOGR", driver="KML")  

Display R Spatial Objects With Google Maps/Google Earth

Project Requirements:

To add R Spatial objects to GoogleMaps / GoogleEarth sessions:

Creating KML files for R Spatial objects: Three techniques

**Technique 1: maptools package methods: kmlLine(), kmlPolygon(), and kmlOverlay()**

Overlay a raster image on a Google Earth background: kmlOverlay()

**Technique 2: Use rgdal package writeOGR() function**:

**Technique 3: Use R system() function to call FWTools toolkit ogr2ogr utility**

Reading KML files into R Spatial objects:

Using RGoogleMaps package to get map images for plot backgrounds:

Learning More:

Display R Spatial Objects With Google Maps/Google Earth

Project Requirements:

To add R Spatial objects to GoogleMaps / GoogleEarth sessions:

Creating KML files for R Spatial objects: Three techniques

Technique 1: maptools package methods: kmlLine(), kmlPolygon(), and kmlOverlay()

Overlay a raster image on a Google Earth background: kmlOverlay()

Technique 2: Use rgdal package writeOGR() function:

Technique 3: Use R system() function to call FWTools toolkit ogr2ogr utility

Reading KML files into R Spatial objects:

Using RGoogleMaps package to get map images for plot backgrounds:

Learning More:

**Technique 1: maptools package methods: kmlLine(), kmlPolygon(), and kmlOverlay()**

**Technique 2: Use rgdal package writeOGR() function**:

**Technique 3: Use R system() function to call FWTools toolkit ogr2ogr utility**