Get contour lines — get_contourlines • anem

Get contour lines, using a wrapper for contourLines

get_contourlines(df = NULL, nlevels = 10, drop_outer = TRUE,
  levels = NULL, ..., type = "data.frame", crs = 4326)

Arguments

df	A data.frame containing x, y, and z columns. x and y must form a raster, meaning every x must be represented for each y, and vice versa.
nlevels	An integer containing number of levels to return. Used IFF levels not supplied.
drop_outer	If `TRUE`, `nlevels+2` will be calculated, dropping the outer 2 values. Used IFF levels not supplied.
levels	A vector of values at which contours should be plotted. If used, nlevels is ignored.
...	Should contain x, y, and z, and then df will be ignored.
type	Should be either "data.frame" or "sf"
crs	The target crs for "sf" output. Used as input in `sf::st_as_sf`

Examples

# Example 1
df <- expand.grid(x=-10:10,y=-10:10)
df$z <- df$x^2
cl <- get_contourlines(df,nlevels=5)
unique(cl$level)
#> [1] 16.66667 33.33333 50.00000 66.66667 83.33333

library(ggplot2)
ggplot() +
  geom_raster(data=df,aes(x,y,fill=z)) +
  geom_path(data=cl,aes(x,y,group=line))

cl <- get_contourlines(df,levels=c(15,20,60))
unique(cl$level)
#> [1] 15 20 60

# Example 2, with "sf" object
library(sf)
#> Warning: package ‘sf’ was built under R version 3.6.2
#> Linking to GEOS 3.7.2, GDAL 2.4.2, PROJ 5.2.0
df <- expand.grid(x=seq(-5,5,length.out=20),y=seq(-5,5,length.out=20))
df$z <- sqrt(df$x^2+df$y^2)

cl <- get_contourlines(df,levels=seq(1,10,by=1.5), type="sf")
ggplot() +
  geom_raster(data=df,aes(x,y,fill=z)) +
  geom_sf(data=cl,aes())