Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

The subtitle of this post can be “How to plot multiple elements on interactive web maps in R“.
In this experiment I will show how to include multiple elements in interactive maps created using both plotGoogleMaps and leafletR. To complete the work presented here you would need the following packages: sp, raster, plotGoogleMaps and leafletR.

In particular I downloaded the shapefile with the stores, the one with the tourist attractions and the polyline shapefile with all the roads in London. I will assume that you want to spend a day or two walking around London, and for this you would need the location of some hotels and the locations of all the Greggs in the area, for lunch. You need to create a web map that you can take with you when you walk around the city with all these customized elements, that’s how you create it.

Once you have downloaded the shapefile from weogeo.com you can open them and assign the correct projection with the following code:

stores <- shapefile("weogeo_j117529/data/shop_point.shp")
projection(stores)=CRS("+init=epsg:3857")

tourism <- shapefile("weogeo_j117529/data/tourism_point.shp")
projection(tourism)=CRS("+init=epsg:3857")

To extract only the data we would need to the map we can use these lines:

Greggs <- stores[stores$NAME %in% c("Gregg's","greggs","Greggs"),] Hotel <- tourism[tourism$TOURISM=="hotel",]
Hotel <- Hotel[sample(1:nrow(Hotel),10),]

Footpaths <- roads[roads\$ROUTE=="foot",]

I created three objects, two are points (Greggs and Hotel) and the last is of class SpatialLinesDataFrame. We already saw how to plot Spatial objects with plotGoogleMaps, here the only difference is that we need to create several maps and then link them together.
Let's take a look at the following code:

Greggs.google <- plotGoogleMaps(Greggs,iconMarker=rep("http://local-insiders.com/wp-content/themes/localinsiders/includes/img/tag_icon_food.png",nrow(Greggs)),mapTypeId="ROADMAP",add=T,flat=T,legend=F,layerName="Gregg's",fitBounds=F,zoom=13)

plotGoogleMaps(Footpaths,col="dark green",mapTypeId="ROADMAP",filename="Multiple_Objects_GoogleMaps.html",legend=F,previousMap=Hotel.google,layerName="Footpaths",strokeWeight=2)

As you can see I first create two objects using the same function and then I call again the same function to draw and save the map. I can link the three maps together using the option add=T and previousMap.
We need to be careful here though, because the use of the option add is different from the standard plot function. In plot I can call the first and then if I want to add a second I call again the function with the option add=T. Here this option needs to go in the first and second calls, not in the last. Basically in this case we are saying to R not to close the plot because later on we are going to add elements to it. In the last line we do not put add=T, thus saying to R to go ahead and close the plot.

Another important option is previousMap, which is used starting from the second plot to link the various elements. This option is used referencing the previous object, meaning that I reference the map in Hotel.google to the map map to Greggs.google, while in the last call I reference it to the previous Hotel.google, not the very first.

The zoom level, if you want to set it, goes only in the first plot.

Another thing I changed compared to the last example is the addition of custom icons to the plot, using the option iconMarker. This takes a vector of icons, not just one, with the same length of the SpatialObject to be plotted. That is why I use the function rep, to create a vector with the same URL repeated for a number of times equal to the length of the object.
The icon can be whatever image you like. You can find a collection of free icons from this website: http://kml4earth.appspot.com/icons.html

The result is the map below, available here: Multiple_Objects_GoogleMaps.html

leafletR
We can do the same thing using leafletR. We first need to create GeoJSON files for each element of the map using the following lines:

Greggs.geojson <- toGeoJSON(Greggs)
Hotel.geojson <- toGeoJSON(Hotel)
Footpaths.geojson <- toGeoJSON(Footpaths)

Now we need to set the style for each element. For this task we are going to use the function styleSingle, which basically defines a single style for all the elements of the GeoJSON. This differ from the map in a previous post in which we used the function styleGrad to create graduated colors depending of certain features of the dataset.
We can change the icons of the elements in leafletR using the following code:

Greggs.style <- styleSingle(marker=c("fast-food", "red", "s"))
Hotel.style <- styleSingle(marker=c("lodging", "blue", "s"))
Footpaths.style <- styleSingle(col="darkred",lwd=4)

As you can see we have the option marker that takes a vector with the name of the icon, its color and its size (between "s" for small, "m" for medium and "l" for large). The names of the icons can be found here: https://www.mapbox.com/maki/, where you have a series of icons and if you hover the mouse over them you would see some info, among which there is the name to use here, as the very last name. The style of the lines is set using the two options col and lwd, for line width.

Then we can simply use the function leaflet to set the various elements and styles of the map:

leaflet(c(Greggs.geojson,Hotel.geojson,Footpaths.geojson),style=list(Greggs.style,Hotel.style,Footpaths.style),popup=list(c("NAME"),c("NAME"),c("OPERATOR")),base.map="osm")

The result is the image below and the map available here: http://www.fabioveronesi.net/Blog/map.html

R code snippets created by Pretty R at inside-R.org