Add multiple layers in leaflet widget with R

A simple loop to add multiple layers

Library leaflet with R is well integrated with pipe writing (%>%). Thus, it is really easy to add multiple layers and options in a leaflet widget.

If you want to add a lot of layers in your widget, it is not necessary to provide a “addPolygons” for each of these layers, you can simply use a loop. You will need to previously standardize the names of your layers to make a clear code. Moreover, this also allows to use different group names for different view options.

The leaflet widget below shows an example of multiple layers viewing, added with a loop. The R-code is available on my github.

Discrepancies between fishing quotas and scientific advices areas

The example below is a widget realised for AFH (French Association of fisheries scientists). This shows different cases for the discrepancies between fishing quotas areas defined by the European commission and areas delimited by scientists to give their advices concerning fish resources. The complete article can be directly downloaded on the AFH website (in French).

R-code to loop multiple layers in a leaflet widget

library(leaflet)
library(rgdal)

# Working directory ----
wd <- ""

# Read multiple shapefiles with standardised name ----
species <- c("Sole", "Maquerel", "Langoustine", "Merlan")
Groupnames <- c("Cas 1 : Sole", "Cas 2 : Maquereau", "Cas 3 : Langoustine",
                "Cas 4 : Merlan")

for (sp in species) {
  for (type in c("Avis", "TAC")) {
    files.sp <- readOGR(dsn = wd, layer = paste(sp, type, sep = "_"),
                        verbose = FALSE)
    assign(paste(sp, type, sep = "_"), files.sp)
  }
}

# Create leaflet widget --------------------------------------------------------
m <- leaflet() %>%
  addTiles(group = "OSM (default)")

# Add multiple layers with a loop ----------------------------------------------
for (sp.N in 1:length(species)) {
  sp <- species[sp.N]
  for (type in c("Avis", "TAC")) {
    tmp <- get(paste(sp, type, sep = "_"))

    # Define different colors depending on type of data
    if (type == "Avis") {
      myPal <- colorRampPalette(c("blue", "skyblue", "navyblue"))
      factpal.Div <- colorFactor(myPal(length(tmp$id)), tmp$id)

      m <- m %>%
        addPolygons(data = tmp,
                    fillColor = ~factpal.Div(tmp$id),
                    color = "#000000",
                    opacity = 1,
                    fillOpacity = 0.8,
                    stroke = TRUE,
                    weight = 1.5,
                    smoothFactor = 0.2,
                    popup = paste0("Zone d'avis : ", tmp$id),
                    group = Groupnames[sp.N]
        )
    } else {
      myPal <- colorRampPalette(c("red", "orange", "brown"))
      factpal.Div <- colorFactor(myPal(length(tmp$id)), tmp$id)

      m <- m %>%
        addPolygons(data = tmp,
                    fillColor = "#FF0000",
                    color = ~factpal.Div(tmp$id),
                    opacity = 0.6,
                    fillOpacity = 0.05,
                    stroke = TRUE,
                    weight = 5,
                    smoothFactor = 0.2,
                    popup = paste0("Zone de TAC : ", tmp$id),
                    group = Groupnames[sp.N]
        )
    }
  }
} # end of species

# Additional leaflet options ---------------------------------------------------
m <- m %>%
  # Add layers controls
  addLayersControl(
    baseGroups = Groupnames,
    options = layersControlOptions(collapsed = FALSE)
  ) %>%
  # Add common legend
  addLegend(colors = c("#4367F4", "#FF0000"),
            labels = c("Zones d'avis", "Zones de TAC"),
            opacity = c(0.8, 0.5))

# Print the map ----------------------------------------------------------------
m