plot multiple shp file on a graph using spplot in R

Question

I have 3 shp files representing the house, room, and beds of a house respectively. I need to plot them on a graph using R so that they all overlap with each other. I know that in plot function, I can use line to plot new lines on top of the existing plot, is there anything equivalent in spplot? Thanks.

Answer 1

Here's one approach, using the nifty layer() function from the latticeExtra package:

# (1) Load required libraries
library(sp)
library(rgeos)        # For its readWKT() function
library(latticeExtra) # For layer()

# (2) Prepare some example data
sp1 = readWKT("POLYGON((0 0,1 0,1 1,0 1,0 0))")
sp2 = readWKT("POLYGON((0 1,0.5 1.5,1 1,0 1))")
sp3 = readWKT("POLYGON((0.5 0,0.5 0.5,0.75 0.5,0.75 0, 0.5 0))")

# spplot provides "Plot methods for spatial data with attributes",
# so at least the first object plotted needs a (dummy) data.frame attached to it.
spdf1 <- SpatialPolygonsDataFrame(sp1, data=data.frame(1), match.ID=1)

# (3) Plot several layers in a single panel
spplot(spdf1, xlim=c(-0.5, 2), ylim=c(-0.5, 2), 
       col.regions="grey90", colorkey=FALSE) +
layer(sp.polygons(sp2, fill="saddlebrown")) +
layer(sp.polygons(sp3, fill="yellow"))

Alternatively, you can achieve the same result via spplot()'s sp.layout= argument. (Specifying first=FALSE ensures that the 'roof' and 'door' will be plotted after/above the grey square given as spplot()'s first argument.)

spplot(spdf1, xlim=c(-0.5, 2), ylim=c(-0.5, 2), 
       col.regions="grey90", colorkey=FALSE,
       sp.layout = list(list(sp2, fill="saddlebrown", first=FALSE),
                        list(sp3, fill="yellow", first=FALSE)))

Answer 2

You can use the sp.layout argument in spplot. Alternatively, you can use ggplot2. Some example code (untested):

library(ggplot2)
shp1_data.frame = fortify(shp1)
shp1_data.frame$id = "shp1"
shp2_data.frame = fortify(shp2)
shp2_data.frame$id = "shp2"
shp = rbind(shp1_data.frame, shp2_data.frame)

ggplot(aes(x = x, y = y, group = group, col = id), data = shp) + geom_path()

In ggplot2, columns in the data are linked to graphical scales in the plot. In this case x is the x-coordinate, y is the y-coordinate, group is a column in the data.frame shp which specifies to which polygon a point belongs, and col is the color of the polygon. The geometry I used is geom_path, which draws a series of lines based on the polygon input data.frame. An alternative is to use geom_poly, which also supports filling the polygon.