Is there a way to add a scale bar (for linear distances) to ggmap?

Question

Not that it\'s critical to my question, but here is my plot example, on top of which I\'d like to add a scale bar.

ggmap(get_map(location = \"Kinston, NC\",          


        

Answer 1

I've reworked @Andy Clifton's code to add a more precise measure of the distance, and to allow for the scale bar to be of a desired length, as opposed to depending on the positioning of the bar.

Andy's code got me 99 percent of the way, but the Haversine formula used in his code is not validated with results from other sources, although I can't find the error myself.

This first part is copied from Andy Clifton's answer above just for completeness of the code:

sites.data = data.frame(lon = c(-77.61198, -77.57306, -77.543),
                        lat = c(35.227792, 35.30288, 35.196),
                        label = c("PP Site","NOAA", "CRONOS Data"),
                        colour = c("red","blue","blue"))
map.base <- get_map(location = c(lon = mean(sites.data$lon),
                                   lat = mean(sites.data$lat)),
                      zoom = 10)
bb <- attr(map.base,"bb")
sbar <- data.frame(lon.start = c(bb$ll.lon + 0.1*(bb$ur.lon - bb$ll.lon)),
                     lon.end = c(bb$ll.lon + 0.25*(bb$ur.lon - bb$ll.lon)),
                     lat.start = c(bb$ll.lat + 0.1*(bb$ur.lat - bb$ll.lat)),
                     lat.end = c(bb$ll.lat + 0.1*(bb$ur.lat - bb$ll.lat)))

The next two steps are different:

First use the distVincentyEllipsoid function from the geosphere package to calculate the distance even more preciseley than the Haversine formula:

sbar$distance <- geosphere::distVincentyEllipsoid(c(sbar$lon.start,sbar$lat.start),
 c(sbar$lon.end,sbar$lat.end))

Then correct the scale bar so that is a standard length - depending on the scale of your map. In this example 20km seems like a nice reasonable choice, i.e. 20,000 meters:

scalebar.length <- 20
sbar$lon.end <- sbar$lon.start +
((sbar$lon.end-sbar$lon.start)/sbar$distance)*scalebar.length*1000

Again using Andy's code, I've only added the arrows to the geom_segment because I think it looks nicer

ptspermm <- 2.83464567  # need this because geom_text uses mm, and themes use pts. Urgh.

map.scale <- ggmap(map.base,
                   extent = "normal", 
                   maprange = FALSE) %+% sites.data +
  geom_point(aes(x = lon,
                 y = lat,
                 colour = colour)) +
  geom_text(aes(x = lon,
                y = lat,
                label = label),
            hjust = 0,
            vjust = 0.5,
            size = 8/ptspermm) +    
  geom_segment(data = sbar,
               aes(x = lon.start,
                   xend = lon.end,
                   y = lat.start,
                   yend = lat.end),
               arrow=arrow(angle = 90, length = unit(0.1, "cm"),
                           ends = "both", type = "open")) +
  geom_text(data = sbar,
            aes(x = (lon.start + lon.end)/2,
                y = lat.start + 0.025*(bb$ur.lat - bb$ll.lat),
                label = paste(format(scalebar.length),
                              'km')),
            hjust = 0.5,
            vjust = 0,
            size = 8/ptspermm)  +
  coord_map(projection = "mercator",
            xlim=c(bb$ll.lon, bb$ur.lon),
            ylim=c(bb$ll.lat, bb$ur.lat))  

# Fix presentation ----
map.out <- map.scale +  
  theme_bw(base_size = 8) +
  theme(legend.justification = c(1,1), 
        legend.position = c(1,1)) 

ggsave(filename ="map.png", 
       plot = map.out,
       dpi = 300,
       width = 4, 
       height = 3,
       units = c("in"))