Getting bounding box from leaflet in R

Question

I am using R, RStudio and the leaflet package to visualise a map.

I would like to get the the min and max lat-longs of of the bounding box of a leaflet object. I think this can be done using Shiny (by using something like input$mapobj_bounds) but is there a non-shiny method to do this.

m <- leaflet(width=500,height=400) %>% 
   addTiles() %>% 
   setView(lng = -0.106831, lat = 51.515328, zoom = 18) %>%
   addCircleMarkers(lng = -0.106831, lat = 51.515328)

What i need is a function to get the bounding box using the input argument m.

Can this be done?

Also, the parameter values when looking into the object m look incorrect.

e.g.

> m$x$limits
$lat
[1] 51.51533 51.51533

$lng
[1] -0.106831 -0.106831

EDIT

I think the javascript function map.getBounds() may be of help here...as suggested here (Get the bounding box of the visible leaflet map?), but do not know how to apply this to our problem. Any help on this would be much appreciated.

Answer 1

If you adapt Jeremys original answer a bit you can actually do it without javascript:

Reproducible example:

library(magrittr)
library(leaflet)

m <- leaflet(width = 500,height = 400) %>% 
  addTiles() %>% 
  setView(lng = -0.106831, lat = 51.515328, zoom = 18) %>%
  addCircleMarkers(lng = -0.106831, lat = 51.515328)
m
getBox <- function(m){
  view <- m$x$setView
  lat <- view[[1]][1]
  lng <- view[[1]][2]
  zoom <- view[[2]]
  zoom_width <- 360 / 2^zoom
  lng_width <- m$width / 256 * zoom_width
  lat_height <- m$height / 256 * zoom_width
  return(c(lng - lng_width/2, lng + lng_width/2, lat - lat_height/2, lat + lat_height/2))
}
getBox(m)

In shiny you can simply you use: input$MAPID_bounds

Reproducible example:

library(shiny)
library(leaflet)
library(magrittr)

app <- shinyApp(

  ui = fluidPage(leafletOutput('myMap')),

  server = function(input, output) {

    output$myMap = renderLeaflet({
      leaflet() %>% 
        addTiles() %>% 
        setView(
          lng = 50, 
          lat = 10, 
          zoom = 17
        )
    })

    observeEvent(input$myMap_bounds, {
      print(input$myMap_bounds)
    })

  }
)

for more info see here: https://rstudio.github.io/leaflet/shiny.html.

Here a javscript version (initial workaround). For the better version, see above.

  leaflet() %>% addTiles()  %>% 
  setView(lng = -0.106831, lat = 51.515328, zoom = 18) %>%
  addEasyButton(easyButton(
    states = list(
      easyButtonState(
        stateName="unfrozen-markers",
        icon="ion-toggle",
        title="Get Bounding box",
        onClick = JS("
                     function(btn, map) {
                        alert(map.getBounds().getEast());
                        alert(map.getBounds().getWest());
                        alert(map.getBounds().getNorth());
                        alert(map.getBounds().getSouth());
                     }")
      )
    )
  )
)

Answer 2

Thanks to @BigDataScientist's answer for pointing out, that width & height are available!

It is possible to calculate the bounding boxes, as long as you know the leaflet widget's dimensions. See leafletjs.com/examples/zoom-levels

Given that this is specified with leaflet(width=500,height=400), this will work.

if (is.null(m$width) | is.null(m$height)) {
    print("Leaflet width and height must be speciied")
} else {
       width <- m$width 
       height <- m$height 
       zoom <- m$x$setView[[2]]
       lng <- m$x$setView[[1]][2]
       lat <- m$x$setView[[1]][1]
       lng_width <- 360 * width / 2^(zoom + 8)
       lng_east <- lng - lng_width/2
       lng_west <- lng + lng_width/2
       lat_height <- 360 * height * cos(lat/180 * pi) / 2^(zoom + 8)
       lat_north <- lat + lat_height/2
       lat_south <- lat - lat_height/2
}

> lng_east
[1] -0.1081721
> lng_west
[1] -0.1054899
> lat_north
[1] 51.516
> lat_south
[1] 51.51466

Comparing to @BigDataScientist, this gives the same answer as map.getBounds to 3 decimal places.

EDIT I based my answer on the documentation from leaflet referenced. It would seem that this is a simplification. I have added the cos(lat/180 * pi) term which improves accuracy. For example, this now gives north-boundary of 51.516, which is only a difference of 0.0000029 from leaflet's 51.51599707.

I have tested this at a few different latitudes and zooms. Accuracy decreases at lower zoom levels.

Answer 3

This is an old question but it recently [29 Apr 2022] assisted me in finding a solution to a problem I was faced with. To say thanks I combined the approaches of @TonioLiebrand and @JeremyVoisey into a single self-contained (neatly rounded-off) function as follows:

# Self-contained function to calculate 'Leaflet Map Widget' Bounding Box co-ordinates ...
"f.calc.leaflet.bounding.box.coords" <- function(objLeafletMap=NULL) {
  FUNC_ID_SHORT <- "fCLBBC"; FUNC_ID_FULL <- "f.calc.leaflet.bounding.box.coords";
  boundNorth_ <- NULL; boundWest_ <- NULL; boundSouth_ <- NULL; boundEast_ <- NULL;
  if (is.null(objLeafletMap) || is.null(objLeafletMap$width) || is.null(objLeafletMap$height)) {
    base::message(base::paste0(" --> ", FUNC_ID_SHORT, " - Line 4 ", "| LEAFLET MAP WIDTH & HEIGHT NOT SPECIFIED ( FUNC ID: '", FUNC_ID_FULL, "' )."))
    base::stop(base::paste0(" --> ", FUNC_ID_SHORT, " - Line 5 ", " | Function Execution Terminated [ reason: REQUIRED PARAMS are NULL ] !!"))
  } else {        
    # Extract Leaflet Map Widget 'x' property list values ( i.e. [center 'lat' & 
    # 'lon'], 'zoom' and 'options' widget property values )...
    view_ <- objLeafletMap$x$setView; zoom_ <- view_[[2]]; 
    lon_ <- view_[[1]][2]; lat_ <- view_[[1]][1];
    
    # Extract Leaflet Map Widget 'width' and 'height' values ...
    width_ <- objLeafletMap$width; height_ <- objLeafletMap$height;
    
    # Calculate Leaflet Map Widget peripheral extent in co-ordinate dimensions ...
    lon_width_ <- 360 * width_ / 2 ^ (zoom_ + 8);
    lat_height_ <- 360 * height_ * cos(lat_ / 180 * base::pi) / 2 ^ (zoom_ + 8);
    
    # Calculate Leaflet Map Widget peripheral extent ( i.e. Bounding Box ) in co-ordinate values ...
    boundEast_ <- lon_ + lon_width_ / 2; boundWest_ <- lon_ - lon_width_ / 2;
    boundNorth_ <- lat_ + lat_height_ / 2; boundSouth_ <- lat_ - lat_height_ / 2;
  }
  return(list(top=boundNorth_, right=boundEast_, bottom=boundSouth_, left=boundWest_, zoom=zoom_))
}

This might be a bit of an overkill for some, but might also be a boon for others looking for a quick [time-critical] solution (as I did). Simply copy & paste the function into your R script, and once the function is read into your R Session Memory extract your Leaflet Map Widget bounding co-ordinates as follows:

# Simply call the function from wherever you need in your R script ...
mapBounds <- f.calc.leaflet.bounding.box.coords(m);   # <- 'm' == Leaflet Map Widget (with 'width' and 'height' defined) !!


# ... and extract the results as follows:
mapBounds$top
> -5.83050217387398

mapBounds$right
> 38.25046875 

mapBounds$bottom
> -40.209497826126 

mapBounds$left
> -9.21046875

I also added the zoom value as an output (because it seems wasteful to compute it inside the function but not return it [as a result] to the function call). So now you can easily easily get the zoom value by calling ...

mapBounds$zoom
> 5

... after every map zoom change - if you really, really need to do that.

Lastly, I concur with @JeremyVoisey, there is an accuracy issue with the results from this approach - but this code snippet was sufficient in resolving the problem I had (and I was a bit time-pressed) ... so I didn't look into fixing the accuracy issue at the time.