Plotting lines over maps in R with ggplot2/ggmap

Question

I'm attempting to create a map in R which overlays an artificial geographic boundary (determined by a set of (x, y) points w/ lat and lon values) over a Google Maps image of an area from ggmap. The code I'm using is below.

ggmap(mapgilbert) +
  geom_point(data = df, aes(x = lon, y = lat, fill = "red", alpha = 0.8), size = 1, shape = 21) +
  guides(fill=FALSE, alpha=FALSE, size=FALSE) +
  for (i in 1:3){
      geom_segment(data = coords, aes(x = lon[[i]], y = lat[[i]], xend = lon[[(i+1)]], yend = lat[[(i+1)]], alpha = 0.8))
  }

Note that coords is a data.frame containing the pertinent lat and lon values, and df is a similar data.frame containing point values; they are both structured properly.

With just one iteration, the above code works fine; the area map appears, the three points I would like to plot appear, and the line drawn between two of them appears as well. However, when I try to perform this action iteratively within a for loop, none of the lines print. I read on a similar post that this is because R's auto-print feature doesn't work within loops, so I tried wrapping the relevant statements in print() functions, but that just returns "NULL" for some reason. I have a feeling I'm making some glaringly obvious mistake but I'm unsure what it is. Thank you in advance!

It isn't super clear what you're trying to do without being able to see your data, but a for loop isn't going to work there, in part because you need to add ggplot layers on top of each other, and that isn't happening here. geom_path does what I think you're looking for. — camille

jazzurro jazzurro · Accepted Answer · 2018-03-25T02:51:30

Since there is no reproducible data, I created a random data. Please provide your data from next time. This is a necessary thing for all SO users when they ask some help.

What you need is to create a data frame for geom_segment. You do not have to loop through the data at all. Each row of of mydf is a line. You specify two points for longitude and latitude, respectively using x, y, xend and yend.

library(ggmap)
library(ggplot2)

# Create a data frame for segments.
mydf <- data.frame(id = 1:3, 
                   lat_1 = c(37.78, 37.75, 37.73), 
                   lon_1 = c(-122.41, -122.40, -122.405), 
                   lat_2 = c(37.77, 37.75, 37.72), 
                   lon_2 = c(-122.43, -122.42, -122.415))

# Create randam data points.
set.seed(111)

mydf2 <- data.frame(lon = runif(n = 100, min = -122.49, max = -122.38),
                    lat = runif(n = 100, min = 37.69, max = 37.813))

# Get a map 
map <- get_map(location = c(left = -122.523, bottom = 37.69,
                            right = -122.35, top = 37.8),
               maptype = "roadmap", source = "google", color = "bw")

# Plot the points and draw the segments on the map. 
ggmap(map) + 
geom_point(data = mydf2, 
           aes(x = lon, y = lat), color = "red", alpha = 0.6, size = 2) + 
geom_segment(data = mydf, 
             aes(x = lon_1, y = lat_1, xend = lon_2, yend = lat_2), 
             color = "green", size = 2, alpha = 0.8, lineend = "round")
#> Warning: Removed 34 rows containing missing values (geom_point).

Created on 2018-03-25 by the reprex package (v0.2.0).

Plotting lines over maps in R with ggplot2/ggmap

1 Answers