Raster Images
Two-dimensional RasterLayer objects (from the raster
package) can be turned into images and added to Leaflet maps using
the addRasterImage function.
The addRasterImage function works by projecting the
RasterLayer object to EPSG:3857 and
encoding each cell to an RGBA color, to produce a PNG image. That image
is then embedded in the map widget.
It’s important that the RasterLayer object is tagged
with a proper coordinate reference system. Many raster files contain
this information, but some do not. Here is how you’d tag a raster layer
object “r” which contains WGS84 data:
crs(r) <- sp::CRS("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")Large Raster Warning
Because the addRasterImage function embeds the image in
the map widget, it will increase the size of the generated HTML
proportionally. In order to avoid unacceptable download times and memory
usage, addRasterImage will error when the PNG is beyond the
size indicated by the maxBytes argument (defaults to 4
megabytes).
If you have a large raster layer, you can provide a larger number of
bytes and see how it goes, or use raster::resample or
raster::aggregate to decrease the number of cells.
Projection Performance
The addRasterImage function projects using
raster::projectRaster, which can take a while on all but
the smallest rasters. To improve performance, the first thing to do is
install a new version of raster; version 2.4 includes
optimizations that speed up bilinear projection by about 10X. This
version has not yet been released to CRAN at the time of this writing
(June 17, 2015) but can be installed directly from R-Forge:
install.packages('raster', repos = 'http://r-forge.r-project.org/', type = 'source')If you have a large raster layer or expect to call
addRasterImage on the same raster layer many times, you can
perform the EPSG:3857
projection yourself (either using
leaflet::projectRasterForLeaflet or using another GIS
library or program) and call addRasterImage with
project = FALSE.
Be sure that your pre-projected raster layer is tagged with an accurate extent and CRS, as these values are still needed to place the image in the proper position on the map.
Coloring
In order to render the RasterLayer as an image, each
cell value must be converted to an RGB(A) color. You can specify the
color scale using the colors argument, which accepts a
variety of color specifications:
- The name of a Color Brewer 2
palette. If no
colorsargument is provided, then"Spectral"is the default. - A vector that represents the ordered list of colors to map to the
data. Any color specification that is accepted by
grDevices::col2rgbcan be used, including"#RRGGBB"and"#RRGGBBAA"forms. Example:colors = c("#E0F3DB", "#A8DDB5", "#43A2CA"). - A color scaling function, like those detailed in the Colors topic. For example:
colors = colorBin("Greens", domain = NULL, bins = 5, na.color = "transparent").
Example
library(raster)
r <- raster("nc/oisst-sst.nc")
pal <- colorNumeric(c("#0C2C84", "#41B6C4", "#FFFFCC"), values(r),
na.color = "transparent")
leaflet() %>% addTiles() %>%
addRasterImage(r, colors = pal, opacity = 0.8) %>%
addLegend(pal = pal, values = values(r),
title = "Surface temp")