Add a raster image as a layer

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Description

Create an image overlay from a RasterLayer object. This is only suitable for small to medium sized rasters, as the entire image will be embedded into the HTML page (or passed over the websocket in a Shiny context).

Usage

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addRasterImage(map, x, colors = "Spectral", opacity = 1, attribution = NULL, 
    layerId = NULL, group = NULL, project = TRUE, maxBytes = 4 * 1024 * 1024)

projectRasterForLeaflet(x)

Arguments

map

a map widget object

x

a RasterLayer object–see raster

colors

the color palette (see colorNumeric) or function to use to color the raster values (hint: if providing a function, set na.color to "#00000000" to make NA areas transparent)

opacity

the base opacity of the raster, expressed from 0 to 1

attribution

the HTML string to show as the attribution for this layer

layerId

the layer id

group

the name of the group this raster image should belong to (see the same parameter under addTiles)

project

if TRUE, automatically project x to the map projection expected by Leaflet (EPSG:3857); if FALSE, it's the caller's responsibility to ensure that x is already projected, and that extent(x) is expressed in WGS84 latitude/longitude coordinates

maxBytes

the maximum number of bytes to allow for the projected image (before base64 encoding); defaults to 4MB.

Details

The maxBytes parameter serves to prevent you from accidentally embedding an excessively large amount of data into your htmlwidget. This value is compared to the size of the final compressed image (after the raster has been projected, colored, and PNG encoded, but before base64 encoding is applied). Set maxBytes to Inf to disable this check, but be aware that very large rasters may not only make your map a large download but also may cause the browser to become slow or unresponsive.

By default, the addRasterImage function will project the RasterLayer x to EPSG:3857 using the raster package's projectRaster function. This can be a time-consuming operation for even moderately sized rasters. Upgrading the raster package to 2.4 or later will provide a large speedup versus previous versions. If you are repeatedly adding a particular raster to your Leaflet maps, you can perform the projection ahead of time using projectRasterForLeaflet(), and call addRasterImage with project=FALSE.

Examples

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library(raster)

r <- raster(xmn = -2.8, xmx = -2.79, ymn = 54.04, ymx = 54.05, nrows = 30, ncols = 30)
values(r) <- matrix(1:900, nrow(r), ncol(r), byrow = TRUE)
crs(r) <- CRS("+init=epsg:4326")

leaflet() %>% addTiles() %>% addRasterImage(r, colors = "Spectral", opacity = 0.8)