Fixing gaps in osm roads when plotting with ggplot2/ geom_sf

Question

When plotting road data downloaded from osm using osmdata, the resulting plot has gaps in it when using larger values of size in geom_sf (see image).

Here is a reproducible example using a section of road in SW London. How can I remove the white gaps in the line when plotting?

library(tidyverse)
library(sf)
library(osmdata)

# define bounding box for osm data
my_bbox <- 
  matrix(c(-0.2605616, -0.2605616,
           -0.2004485, -0.2004485,
           -0.2605616, 51.4689943,
           51.4288980, 51.4288980,
           51.4689943, 51.4689943),
         ncol = 2)
bbox_sf <- st_geometry(st_polygon(x = list(my_bbox)))
st_crs(bbox_sf) <- 4326

#get osm road data for bounding box
osm_roads_secondary_sf <- 
  opq(bbox = st_bbox(bbox_sf)) %>%
  add_osm_feature(key = 'highway', value = 'secondary') %>%
  osmdata_sf()

ggplot() + 
  geom_sf(data=osm_roads_secondary_sf$osm_lines,size=4)

session info:

R version 3.5.0 (2018-04-23)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.6

other attached packages:
 [1] osmdata_0.0.7        sf_0.6-3             forcats_0.3.0        
stringr_1.3.1       
 [5] dplyr_0.7.5          purrr_0.2.5          readr_1.1.1          
tidyr_0.8.1         
 [9] tibble_1.4.2         ggplot2_3.0.0        tidyverse_1.2.1.9000

Supply a `lineend` parameter. `"round"` would probably give you the nicest joins. — alistaire, Aug 12 '18 at 20:11
@alistaire thanks, `geom_sf` doesn't look to have a `lineend` parameter though — Chris, Aug 12 '18 at 20:18
Yeah, it should pass it through to `geom_line`, but it doesn't look like it's working properly. — alistaire, Aug 12 '18 at 20:21
If you `ggsave` the image with a high resolution, are the gaps still there? — camille, Aug 12 '18 at 21:18
If you don't care about projection, you could pull out the coordinates and use `geom_path` so you can specify `lineend` properly: `osm_roads_secondary_sf$osm_lines %>% st_coordinates() %>% as.data.frame() %>% ggplot(aes(X, Y, group = L1)) + geom_path(size = 5, lineend = "round") + coord_equal()` — alistaire, Aug 12 '18 at 21:52
A more natural approach is to join the lines: `osm_roads_secondary_sf$osm_lines %>% st_union() %>% st_line_merge() %>% ggplot() + geom_sf(size = 5)` It doesn't work for intersections, but the gaps within the segments are gone. — alistaire, Aug 12 '18 at 21:58
@alistaire thanks. Converting to points and using `geom_path` is a nice solution for my purposes. Can you write up as an answer? Would still be nice to see a solution using `geom_sf`! — Chris, Aug 12 '18 at 22:17

alistaire · Accepted Answer · 2018-08-13T02:55:26.370

The ideal solution is to pass lineend = "round" to geom_sf, which should get passed along to geom_path (its underlying layer, really) to round off the end of the lines, causing slight overlaps and a smooth appearance. That sadly doesn't work, though:

ggplot(osm_roads_secondary_sf$osm_lines) + 
    geom_sf(size = 4, lineend = "round")
#> Warning: Ignoring unknown parameters: lineend

I've filed an issue on GitHub, but since ggplot just had a release, any fix won't make it to CRAN for a while yet.

In the mean time, workarounds include extracting the paths from the geometry column with st_coordinates. The resulting matrix, coerced to a data frame, can be plotted with geom_path, which happily accepts a lineend parameter:

osm_roads_secondary_sf$osm_lines %>% 
    st_coordinates() %>% 
    as.data.frame() %>% 
    ggplot(aes(X, Y, group = L1)) + 
    geom_path(size = 4, lineend = "round") + 
    coord_sf(crs = 4326)

Change the color to a suitable shade of gray for a more geom_sf-like appearance.

A more sf-native approach is to merge the line segments into continuous lines, which, of course, have no gaps. st_line_merge does the heavy lifting, but you'll need to aggregate them into multilines beforehand so it has the necessary data:

osm_roads_secondary_sf$osm_lines %>% 
    st_union() %>% 
    st_line_merge() %>% 
    ggplot() + 
    geom_sf(size = 4)

Note that this is mostly, but not entirely better. The gaps within the lines are gone, but st_line_join doesn't know how to fix the three-way intersection, so there's still a tiny gap there. If your real data has lots of such intersections (which is quite possible), this approach won't yield good results.

A last approach is to simply use base sf plotting, which defaults to a round line-end:

plot(osm_roads_secondary_sf$osm_lines$geometry, lwd = 10)

Whether such an approach is practical depends on what else remains to be done with the plot and how facile you are with base plotting.

It should be possible to combine the `geom_path` solution with `coord_sf()` to get a result with proper gratiules and axis tick labels. — Claus Wilke, Aug 13 '18 at 01:44

Fixing gaps in osm roads when plotting with ggplot2/ geom_sf

1 Answers1