FAQs

Table of Contents

How to define the breaks

Use the at argument to define the breaks. 

f <- system.file("external/test.grd", package="raster")
r <- raster(f)

my.at <- seq(100, 1850, by = 250)
levelplot(r, at=my.at)

FAQ_at.png

Moreover, if you want to define where the labels are located define the components of colorkey: 

myColorkey <- list(at=my.at, ## where the colors change
                   labels=list(
                       at=my.at ## where to print labels
                     ))
levelplot(r, at=my.at, colorkey=myColorkey)

FAQ_at2.png

Last, you can even define the contents of the labels: 

myColorkey <- list(at=my.at, ## where the colors change
                   labels=list(
                     labels=letters[seq_along(my.at)], ## labels
                     at=my.at ## where to print labels
                     ))
levelplot(r, at=my.at, colorkey=myColorkey)

FAQ_at3.png

How to suppress axis labels and ticks

Use xlab=NULL, ylab=NULL and draw=FALSE inside scale. 

  r <- raster()
  r <- init(r, runif)
  
  levelplot(r, xlab=NULL, ylab=NULL, scales=list(draw=FALSE))

FAQ_axis_labels.png 

  levelplot(r, xlab=NULL, scales=list(x=list(draw=FALSE)))

FAQ_axis_labelsX.png 

  levelplot(r, ylab=NULL, scales=list(y=list(draw=FALSE)))

FAQ_axis_labelsY.png

How to rotate the axis label or use a different color

Define main, xlab and ylab as vectors or lists with arguments to be passed to grid::gpar. 

  r <- raster(nrow=10, ncol=10)
  r[] <- runif(100)
  
  levelplot(r, margin=FALSE,
            main=list('My plot', col='red'),
            xlab=c('Longitude', 'X-Axis'),
            ylab=list('Latitude', rot=30, fontface='bold')
            )

FAQ_label_color.png

How to add an arrow

Use sp::layout.north.arrow with latticeExtra::layer. 

library(sp)

f <- system.file("external/test.grd", package="raster")
r <- raster(f)

levelplot(r, margin=FALSE, auto.key=FALSE, scales=list(draw=FALSE)) + 
  layer({
    SpatialPolygonsRescale(layout.north.arrow(),
                           offset = c(179000,332500),
                           scale = 400)
  })

FAQ_arrow.png

How to add an scale

Use grid.rect, grid.text and latticeExtra::layer. 

  f <- system.file("external/test.grd", package="raster")
  r <- raster(f)

  levelplot(r) +
    layer({
      xs <- seq(181000, 181400, by=100)
      grid.rect(x=xs, y=330500,
                    width=100, height=30,
                    gp=gpar(fill=rep(c('transparent', 'black'), 2)),
                    default.units='native')
      grid.text(x= xs - 50, y=330560, seq(0, 400, by=100),
                gp=gpar(cex=0.5), rot=30,
                default.units='native')
      })

FAQ_scale.png

How to overlay a shapefile

Several layers can be combined using the +.trellis and layer functions from the latticeExtra package (which is automatically loaded with rasterVis).

Let’s build some data to play. You can skip this part if you already have a raster file and a shapefile. 

library(maps)
library(mapdata)
library(maptools)

## raster
myRaster <- raster(xmn=-100, xmx=100, ymn=-60, ymx=60)
myRaster <- init(myRaster, runif)

## polygon shapefile
ext <- as.vector(extent(myRaster))

boundaries <- map('worldHires', fill=TRUE,
    xlim=ext[1:2], ylim=ext[3:4],
    plot=FALSE)

## read the map2SpatialPolygons help page for details
IDs <- sapply(strsplit(boundaries$names, ":"), function(x) x[1])
bPols <- map2SpatialPolygons(boundaries, IDs=IDs,
                              proj4string=CRS(projection(myRaster)))

The polygons are printed with sp::sp.polygons using +.trellis and layer to display them over the result of levelplot. 

levelplot(myRaster) + layer(sp.polygons(bPols))

FAQ_overlay.png

Read the help page of sp.polygons for detailed information about its options.

How to use +.layer inside a function

The help page of latticeExtra::layer explains that:

the evaluation used in layer is non-standard, and can be confusing at first: you typically refer to variables as if inside the panel function (x, y, etc); you can usually refer to objects which exist in the global environment (workspace), but it is safer to pass them in by name in the data argument to layer.

When using layer inside a function, you can embed your object in a list and pass it in the data argument: 

myplot <- function(rast, pol) {
  rastplot <- levelplot(rast, margin = FALSE)
  polyplot <- layer(sp.polygons(x), 
                    data = list(x = pol))
  print(rastplot + polyplot)
}

Now the function produces the desired result: 

myplot(myRaster, pol = bPols)

How to overlay graticules (longitude - latitude lines)

The graticule package defines and draws graticules (longitude and latitude lines). The result can be combined with the levelplot output as shown in this gist.

How to print several Rasters with different legends in the same page

Use print.trellis and its argument split.

For two different RasterLayer. 

  r <- r2 <- raster()
  r[] <- runif(ncell(r))
  r2[] <- 1:10
  
  p <- levelplot(r, par.settings=RdBuTheme(), margin=FALSE)
  p2 <- levelplot(r2, par.settings=BTCTheme(), margin=FALSE)
  
  
  print(p, split=c(1, 1, 1, 2), more=TRUE)
  print(p2, split=c(1, 2, 1, 2))

FAQ_print_split.png

A more sophisticated solution to print more than two layers. 

  s <- stack(r, r2, 10*r+r2, 10*r-r2)
  nl <- nlayers(s)
  m <- matrix(1:nl, nrow=2)
  themes <- list(RdBuTheme(), BTCTheme(), GrTheme(), PuOrTheme())
  for (i in 1:nl){
      p <- levelplot(s, layers=i,
                     par.settings=themes[[i]],
                     margin=FALSE)
      print(p, split=c(col(m)[i], row(m)[i], ncol(m), nrow(m)), more=(i<nl))
  }

FAQ_print_split4.png

How to label each panel with a different label from the layer name

Use names.attr. 

  r <- raster(nrow=10, ncol=10)
  r[] <- runif(100)
  s <- stack(lapply(1:8, function(i)r))
  
  levelplot(s, scales=list(draw=FALSE), colorkey=FALSE,
            par.settings=GrTheme(),
            names.attr=paste0('MNF-', 1:8))

FAQ_namesattr.png

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