contourLines {grDevices}  R Documentation 
Calculate contour lines for a given set of data.
contourLines(x = seq(0, 1, length.out = nrow(z)),
y = seq(0, 1, length.out = ncol(z)),
z, nlevels = 10,
levels = pretty(range(z, na.rm = TRUE), nlevels))
x, y 
locations of grid lines at which the values in 
z 
a matrix containing the values to be plotted ( 
nlevels 
number of contour levels desired iff

levels 
numeric vector of levels at which to draw contour lines. 
contourLines
draws nothing, but returns a set of contour lines.
There is currently no documentation about the algorithm. The source code is in ‘R_HOME/src/main/plot3d.c’.
A list
of contours, each itself a list
with elements:
level 
The contour level. 
x 
The xcoordinates of the contour. 
y 
The ycoordinates of the contour. 
options("max.contour.segments")
for the maximal
complexity of a single contour line.
contour
: Its ‘Examples’ demonstrate how
contourLines()
can be drawn and are the same (as those from
contour()
).
x < 10*1:nrow(volcano)
y < 10*1:ncol(volcano)
cl < contourLines(x, y, volcano)
## summarize the sizes of each the contour lines :
cbind(lev = vapply(cl, `[[`, .5, "level"),
n = vapply(cl, function(l) length(l$x), 1))
z < outer(9:25, 9:25)
pretty(range(z), 10) # 300 200 ... 600 700
utils::str(c2 < contourLines(z))
# no segments for {300, 700};
# 2 segments for {200, 100, 0}
# 1 segment for 100:600