Union of circles and polygon in leaflet

Question

I am adding two circles and a polygon to a leaflet map. Here\'s my code which plots these three shapes. Is there any way I can get a union of these three shapes?

Answer 1

As far as I know you can't. Whenever you add something on a leaflet map its considered as a separate layer, with separate properties and data:

• The idea is to be able to hide/show those layers using interactive legends (so you want to keep your layers separated)
• I don't see any simple way to access each circle's points' coordinates

If you want to display something that's composed of several shapes, you'll have to create a complex SpatialPolygon yourself with points coordinates, sp package and this kind of code:

require(sp)
require(leaflet)

#Used for sp polygon creation
createPolygon <- function(latitude, longitude, name = "polygon"){

  return(Polygons(list(Polygon(cbind(longitude, latitude))), ID = name))

}

#Will give "res" points coordinates for a circle centered on x0 y0
#with a radius r
CreateCircle <- function(x0, y0, r, res = 50){

  theta = seq(0, 2*pi, length.out = res+1)

  x = r*cos(theta) + x0
  y = r*sin(theta) + y0

  return(data.frame(x, y))

}

#Computes two circles points'
circleA <- CreateCircle (0, 0, 2, res = 200)
circleB <- CreateCircle (10, 10, 4, res = 6)

#Add them to polygons
A = createPolygon(circleA$x, circleA$y, "A")
B = createPolygon(circleB$x, circleB$y, "B")
C = SpatialPolygons(list(A, B))

#Create and display the leaflet map
m = leaflet() %>% addTiles() %>%
  addPolygons(data = C, fillColor = topo.colors(10, alpha = NULL), stroke = TRUE)
m

If it can't display the result you want (e.g. because you want the circles to have different colors than your rectangles), you will have to use several layers and group them.

Answer 2

I'd recommend moving away from spatial objects from the sp package and instead look at simple feature objects from the sf package.

Simple Features are the 'new' spatial class for R (and are made by the same guy who did sp).

So, to get the union of your circles, you can use

library(rgeos)
library(sf)


## A dataframe of your points
df <- data.frame(lon = c(ptTop1[1], ptTop2[1], ptBottom1[1], ptBottom2[1]),
                 lat = c(ptTop1[2], ptTop2[2], ptBottom1[2], ptBottom2[2]))

## convert them into a simple features data.frame
sf_df <- st_as_sf(df, coords = c("lon", "lat"))

## convert into circles
sf_circles <- st_buffer(sf_df, dist = 0.04)

## find the union of all the circles
sf_combined <- st_union(sf_circles)


## now sf_combined is a single polygon
sf_combined
# Geometry set for 1 feature 
# geometry type:  POLYGON
# dimension:      XY
# bbox:           xmin: 72.79573 ymin: 20.38366 xmax: 72.88561 ymax: 20.47837
# epsg (SRID):    NA
# proj4string:    NA
# POLYGON((72.8745460445306 20.4072956786729, 72....

As for plotting, leaflet can handle sf objects (except for MULTIPOINT), so you can plot it directly

library(leaflet)

sp <- as(sf_combined, 'Spatial')

sf_combined %>%
    leaflet() %>%
    addTiles() %>%
    addPolygons()