How to read a dataframe and show it on leaflet

Question

I am trying to show the data that I have in a data frame on a map by using leaflet library, but not sure how.

geom <- c('LINESTRING(174.7698950000 -41.3435950000,174.7698040000 -41.3434660000,174.7714690000 -41.3395070000)')
description <- c('labelA')
df <- data.frame(geom, description)
leaflet(df) %>%
  addTiles() %>%
  addPolylines(stroke = FALSE, smoothFactor = 0.3, fillOpacity = 1,
               fillColor = factor('description'),
               label = ~paste0('description', ": ", formatC(description, big.mark = ","))) 

Error I am getting:

Error in polygonData.default(data) : 
 Don't know how to get path data from object of class data.frame

Please let me know how I can improve the question instead of down voting

Answer 1

You are only missing one step. What you have is a standard data frame, which Leaflet cannot interpret as a spatial object.

You first need to convert your df to a spatial object: the sf package is excellent for that. Your leaflet code was masking the stroke so the line was not showing: I changed that and set the color to red.

library(leaflet)
library(sf)

geom <- c('LINESTRING(174.7698950000 -41.3435950000,174.7698040000 -41.3434660000,174.7714690000 -41.3395070000)')
description <- c('labelA')


df <- data.frame(geom, description) # ordinary data frame

df <- st_as_sf(df, wkt = "geom") # convert to spatial data frame

leaflet(df) %>%
   addTiles() %>%
   addPolylines(stroke = TRUE, 
                smoothFactor = 0.3,
                fillOpacity = 1,
                color = "red",
                label = ~paste0('description', ": ", formatC(description, big.mark = ",")))

Note that you may have to set the CRS of your object so that it is projected correctly by Leaflet and shows in the right place. Leaflet uses WGS84 (EPSG: 4326) but your data might be recorded in something else. You can adapt this:

df <- st_set_crs(df, 27700)  # this would set the CRS to the British National Grid  
df <- st_transform(df, 4326) # reproject in WGS84

You can search for EPSG codes here.