R：在轮廓图上添加一条线

Question

I would like to plot a function f(x, y) using filled.contour() in R and add a line/curve to identify the points where the value of the function is 0. In order to make the example repricable, let's say that the values for my function are those in the volcano dataset and instead of looking for f(x, y) = 0 we want to add a line/curve to identify where the volcano has value 500. How can I do this? The following code correctly adds a point at point X=500 and Y=500.But how can I add a line so that only points where volcano=500 are joint by a line? I would like to use only base graphics.

x <- 10*1:nrow(volcano)
y <- 10*1:ncol(volcano)
filled.contour(x, y, volcano, color = terrain.colors, plot.axes = { axis(1); axis(2); points(500, 500)})

Answer 1

One solution, which is not perfect:

#create a new plot
contour(x,y,volcano)
#use .filled.colour instead of filled.contour
.filled.contour(x, y, volcano,levels=seq(90,200,1),col=terrain.colors(109))
# add a contour plot with specific levels on top of the filled contour
contour(x,y,volcano, level=130,add=T)

you can specify several contour lines by passing a vectoor to the argument level: level = c(130,150) will draw the contours for height 130 and 150

The problem is that filled.contour ads legend bar so that simply calling filled.contour and then contour makes that the two pictures does not match (see note in help(filled.contour)).

However, with this solution, you loose the legend bar.