R plot grid value on maps

Question

I have several file with the following format: lat,lon,value. I would like to plot a colored grid map with such value for each one of the lat,lon point and overlapping it over the EU map.

Thanks for support.

Answer 1

There are roughly two options: one using the lattice and sp package and the other using the ggplot2 package:

lattice / sp

First you have to transform your data to a SpatialPixelsDataFrame (provided by the sp-package), assuming your data is called df:

require(sp)
gridded(df) = c("lat","lon")

and then plotting:

spplot(df, "value")

overlaying additional information such as country boundaries can be done using the sp.layout argument:

spplot(df, "value", sp.layout = list("sp.lines", cntry_boundaries))

where cntry_boundaries is a SpatialLines(DataFrame), or SpatialPolygons(DataFrame). Reading a polygonset into R can be done using the rgdal package, or the maptools package (readShapeLines).

ggplot2

I personally prefer using ggplot2 over spplot. ggplot2 is more flexible and has a more clear syntax. Note that ggplot2 works with normal data.frame's, and not with the spatial objects of the sp-package.

A minimal example would look something like:

ggplot(aes(x = lat, y = lon), data = df) + geom_tile(aes(fill = value)) + 
    geom_path(data = cntry_boundaries)

For more information see these earlier answers of mine:

• Plotting interpolated data on map
• Average values of a point dataset to a grid dataset