leaflet.extras: measure distance in metres

Question

I would like to create a map where I can interactively measure the distance between 2 points. Luckily, leaflet.extras has exactly what I need, however, I'm struggling to get it to produce the outputs in metres (or kilometres) as opposed to feet.

Consider the below piece of code:

library(leaflet)
library(leaflet.extras)
leaflet() %>% 
  addTiles() %>%
  addDrawToolbar(
    editOptions=editToolbarOptions(selectedPathOptions=selectedPathOptions())
  )

It creates the following map:

However, this example (chunk 3) effectively the same code to create the same measuring tool (polyline), except it works in KM, whereas my example works in feet.

If you have any tips that can help me switch to metres as opposed to feet, I would really appreciate it.

Answer 1

The drawPolylineOptions function does not allow to set the option feet=FALSE.
Hence, I suggest to modify drawPolylineOptions as follows:

library(leaflet)
library(leaflet.extras)

mydrawPolylineOptions <- function (allowIntersection = TRUE, 
    drawError = list(color = "#b00b00", timeout = 2500), 
    guidelineDistance = 20, metric = TRUE, feet = FALSE, zIndexOffset = 2000, 
    shapeOptions = drawShapeOptions(fill = FALSE), repeatMode = FALSE) {
    leaflet::filterNULL(list(allowIntersection = allowIntersection, 
        drawError = drawError, guidelineDistance = guidelineDistance, 
        metric = metric, feet = feet, zIndexOffset = zIndexOffset,
        shapeOptions = shapeOptions,  repeatMode = repeatMode)) }

leaflet() %>% setView(10.975342,45.421588,9) %>%
  addTiles() %>%
  addProviderTiles(providers$OpenStreetMap.Mapnik) %>%
  addDrawToolbar(
    polylineOptions = mydrawPolylineOptions(metric=TRUE, feet=FALSE),
    editOptions=editToolbarOptions(selectedPathOptions=selectedPathOptions())
  ) 

Otherwise, using addMeasures you can add to your map a useful tool to measure distances (see the icon at the top right corner of the map).
It is possibile to specify units used to display length results by the primaryLengthUnit option.

leaflet() %>% setView(10.975342,45.421588,9) %>%
  addTiles() %>%
  addProviderTiles(providers$CartoDB.Positron) %>%
  addDrawToolbar(
    editOptions=editToolbarOptions(selectedPathOptions=selectedPathOptions())
  ) %>% 
  addMeasure(primaryLengthUnit="kilometers", secondaryLengthUnit="kilometers")