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!

Answer 1

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).