R, Leaflet polygons add black borders

Question

I would like to add a black border around my polygons. See the image below what I have got so far.

I used this code. I dont know how I could include a zip file so its reproducible.

library("rgdal")
library("leaflet")

 amsterdam.districts <- shapefile("~/R/Shiny/Shapefiles/sd2010zw_region.shp")
 amsterdam.districts <- spTransform(amsterdam.districts, CRS("+init=epsg:4326"))
 amsterdam.districts$STADSDEELN<- c("Centrum","Westpoort","West","Nieuw West","Zuid","Oost","Noord","Zuidoost")

 amsterdam.districts$SDNUMMER=as.factor(amsterdam.districts$SDNUMMER) 
 factpal=colorFactor(brewer.pal(n = 11, name ="Spectral") , amsterdam.districts$SDNUMMER) 

   leaflet() %>%
     addPolygons(
       data =amsterdam.districts, 
       stroke = FALSE, fillOpacity = 0.5, smoothFactor = 0.5,
       color = "grey",
       fillColor = ~factpal(SDNUMMER),
       layerId = amsterdam.districts@data$STADSDEELN)%>%
     addProviderTiles("CartoDB.Positron", options= providerTileOptions(opacity = 1))

Thanks for reading!

Martijn

Answer 1

Here's a reproducible example:

library(mapview)
library(RColorBrewer)

factpal <- colorFactor(brewer.pal(n = 11, name ="Spectral") , gadmCHE$NAME_1) 

leaflet() %>%
  addPolygons(
    data = gadmCHE, 
    stroke = TRUE, fillOpacity = 0.5, smoothFactor = 0.5,
    color = "black", opacity = 1,
    fillColor = ~factpal(gadmCHE$NAME_1)) %>%
  addProviderTiles("CartoDB.Positron", options = providerTileOptions(opacity = 1))

You simply need to set stroke = TRUE and color = "black" to get the borders. Note that I also set opacity = 1 to prevent semitransparent borders.