Mapping using ggmap & Stamen maps in R: labelling points and scale

Question

I'm trying to make a map of my study site using ggmap & Stamen maps. I've seen a few similar questions but haven't figured out a way to incorporate the solution into my Stamen map code.

I have two questions regarding this: 1. How can I custom label the points on the map? 2. How can I add a scale to maps in Stamen map? (either as a line indicating distance or something like x cm on map = y km in real life)

Tcoords <- read.csv("Tcoords.csv")

My file looks like this

# trap latitude longitude
1 52.34431 0.5374620
2 52.34281 0.5382080
3 52.34468 0.5406787
4 52.34357 0.5398280
5 52.34431 0.5397050
6 52.34516 0.5406294

In response to the suggestion, I've pasted the results to dput(head(Tcoords)) here:

 structure(list(trap = c("1", "2", "3", "4", "5", "6"), latitude = c(52.344312, 
52.342809, 52.3446849, 52.343572, 52.34431, 52.3451601), longitude = c(0.537462, 
0.538208, 0.5406787, 0.539828, 0.539705, 0.5406294)), row.names = c(NA, 
6L), class = "data.frame")

This the code I'm using to plot my points

center = c(lon = 0.5406294, lat = 52.3451601)
qmap(center, zoom = 16, source = "stamen", maptype = "watercolor")+ 
      geom_point(aes(x = longitude, y = latitude), size = 4, shape = 21, 
                 fill = "dark green", data = Tcoords)

But somehow trap isn't being recognised as an object. It's probably something elementary but I'm not really sure what I've missed (new to R). I've saved "trap" as a text object here.

Thanks for your help!

Answer 1

I would like to suggest tmap as an alternative to ggmap. This is one of many others possible packages for creating maps CRAN Task View: Spatial but I found the scale bar that tmap generates pretty nice and the code simple.

The code to generate the final plot requires the following packages

# To create the map
library(tmap)
# To create the layer with the points given in Tcoords.csv
library(sf)
# To read the background map
library(tmaptools)
library(OpenStreetMap)

Then, we read the coordinates of the six points to be mapped and turn them into an sf object

# Read coordinates
Tcoords = dget("Tcoords.R")

# create an sf object for the six points in the file
coordinates = matrix(c(Tcoords$longitude, Tcoords$latitude), 6, 2)
tcoords_sfc = lapply(1:6, function(k) st_point(coordinates[k, ])) %>%
  st_sfc(crs = 4326)
tcoords_sf = st_sf(trap = Tcoords$trap, geometry = tcoords_sfc)

Next, we find the limits of the six points (bounding box) and extend them by a factor 2.5. You can play with this factor to get maps with other scales.

bb_new = bb(tcoords_sf, ext = 2.5)

Finally we read the background map

map_osm = read_osm(bb_new, zoom = 15, type = "stamen-watercolor")

and draw the final map

with the following code

tmap_mode("plot")
tm_shape(map_osm, projection = 4326, unit = "m") +
  tm_rgb() +
  tm_shape(tcoords_sf) + 
  tm_symbols(col = "darkgreen", shape = 21, size = 2) +
  tm_text(text = "trap", col = "white") +
  tm_scale_bar(breaks = c(0, 50, 100, 150, 200), text.size = 0.6) +
  tm_compass(position = c("left", "top"))

To get a dynamic map is even simpler as you do not have read first the basemap (read_osm) and then draw the map.

tmap_mode("view")
tm_shape(tcoords_sf, bbox = bb_new, unit = "m") + 
  tm_symbols(col = "darkgreen", shape = 21, size = 3) +
  tm_text(text = "trap", col = "white") +
  tm_basemap("Stamen.Watercolor") +
  tm_scale_bar()

In the static plot, colors, text and breaks in the scale can be personalized. Note the parameter unit = "m" in the tm_shape in order to get the scale in meters, if not it will be in kilometers.

Hope you will find this alternative worth mentioning.

Answer 2

Getting the labels onto the map was just a matter of redefining the data source in the geom_text() function.
In order to get the scale to print on the map, it was a matter of following the solution in this question: Is there a way to add a scale bar (for linear distances) to ggmap?

#get base map
map.base <- get_map(location = center, zoom = 16, source = "stamen", maptype = "watercolor") # could also use zoom = "auto"
#get extent of base map
bb <- attr(map.base,"bb")

#define the location and length of scale bar
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)))

#Calculate distance in meters
library(geosphere)
sbar$distance = distGeo(c(sbar$lon.start,sbar$lat.start), c(sbar$lon.end,sbar$lat.end))

map.scale <- ggmap(map.base, extent="device")   +
   geom_point(aes(x = longitude, y = latitude), size = 4, shape = 21, fill = "dark green", data = Tcoords) +
   geom_text(data=Tcoords, aes(label=trap, x = longitude, y = latitude), nudge_x = 0.0001, nudge_y = 0.0001, color="black") +

   geom_segment(data = sbar,  aes(x = lon.start, xend = lon.end, y = lat.start, yend = lat.end)) +
   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(distance,   digits = 4, nsmall = 2), 'm')),
             hjust = 0.5, vjust = 0)  
map.scale

May need to adjust the nudge_x & _y in the geom_text() function for proper label placement.