ggplot not plotting linestring points from sf object

Question

Given the following data:

library(tidyverse)
library(sf)
df <- structure(list(geometry = c("LINESTRING (-85.76 38.34, -85.72 38.38)", 
                                  "LINESTRING (-85.46 38.76, -85.42 38.76)", 
                                  "LINESTRING (-85.89 38.31, -85.89 38.31)"
), var1 = c(4, 5, 6
), var2 = c(1, 2, 3
)), row.names = c(NA, -3L), class = c("tbl_df", "tbl", "data.frame"
))
df
df_sf <- sf::st_as_sf( df, wkt = "geometry" )
# Simple feature collection with 3 features and 2 fields
# geometry type:  LINESTRING
# dimension:      XY
# bbox:           xmin: -85.89 ymin: 38.31 xmax: -85.42 ymax: 38.76
# CRS:            NA
# # A tibble: 3 x 3
#                       geometry  var1  var2
#                   <LINESTRING> <dbl> <dbl>
# 1 (-85.76 38.34, -85.72 38.38)     4     1
# 2 (-85.46 38.76, -85.42 38.76)     5     2
# 3 (-85.89 38.31, -85.89 38.31)     6     3

We can use plot to plot the data including the LINESTRING that has two points at the same location (row = 3):

plot(st_geometry(df_sf), lwd = 10)

giving:

but when we plot it using ggplot the point is dropped:

ggplot() +
  geom_sf(data = df_sf, lwd = 8)

Without manually extracting locations that only contain a point, is there a quick way to tell ggplot to plot these? I can see that these points are technically not a line as theres no distance between them but plot is able to pick them up. This question seems related but slightly different, my LINESTRINGs are already created.

thanks

also helpful but doesnt answer why ggplot default is not like plot stackoverflow.com/questions/49887283/… — user63230
– user63230, Commented May 26, 2020 at 8:08

Z.Lin · Accepted Answer · 2020-07-01 10:04:55Z

2

This is one of those situations where the headdesk emoji might come in handy:

ggplot() +
  geom_sf(data = df_sf, lwd = 8, lineend = "round")

From the package's vignette on aesthetic specs, the default lineend is "butt", which stops precisely at the end point of a line (so a line of 0 length won't have anything to show), while the "round" alternative extends beyond the end point:

TL;DR: The 0-length linestring wasn't dropped. We just couldn't see it.

answered Jul 1, 2020 at 10:04

Z.Lin

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1 Comment

user63230 Over a year ago

exactly what I was looking for, makes you think that round should be the default for this particular reason, thanks anyway!

agila · Accepted Answer · 2020-05-25 18:00:31Z

1

I think that you can solve that problem if you modify the LINESTRING geometries whose length is equal to 0 and cast them as POINTS. For example:

# packages
library(ggplot2)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1

# data
df <- structure(
  list(
    geometry = c(
      "LINESTRING (-85.76 38.34, -85.72 38.38)",
      "LINESTRING (-85.46 38.76, -85.42 38.76)", 
      "LINESTRING (-85.89 38.31, -85.89 38.31)"
      ), 
    var1 = c(4, 5, 6), 
    var2 = c(1, 2, 3)
    ), 
  row.names = c(NA, -3L), 
  class = c("tbl_df", "tbl", "data.frame")
)
df_sf <- st_as_sf( df, wkt = "geometry" )

# Rebuild the geometry column in such a way that the zero-length LINESTRINGS are
# actually POINTS:
new_df_sf_geometry <- st_geometry(df_sf)
idx <- which(st_length(new_df_sf_geometry) == 0)
for (i in idx) {
  new_df_sf_geometry[i] <- unique(st_cast(new_df_sf_geometry[i], "POINT"))
}

# This is the result
new_df_sf_geometry
#> Geometry set for 3 features 
#> geometry type:  GEOMETRY
#> dimension:      XY
#> bbox:           xmin: -85.89 ymin: 38.31 xmax: -85.42 ymax: 38.76
#> CRS:            NA
#> LINESTRING (-85.76 38.34, -85.72 38.38)
#> LINESTRING (-85.46 38.76, -85.42 38.76)
#> POINT (-85.89 38.31)

# Replace the geometry
st_geometry(df_sf) <- new_df_sf_geometry

# Plot
ggplot(df_sf) + 
  geom_sf(size = 3)

^{Created on 2020-05-25 by the reprex package (v0.3.0)}

If you need, you can also adopt more sophisticated approaches than a for-loop such as purrr::map_if.

answered May 25, 2020 at 18:00

agila

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3 Comments

user63230 Over a year ago

thanks but I was looking for something that didn't go down this route (I would have thought there would be an option to specify plot points). I think it makes more sense for this to be the default like plot? The user would need to know beforehand that there is LINSTRING data of length 0 within their dataset which I feel is presumptuous!

agila Over a year ago

Hi! I'm sorry but I cannot help you further since I don't understand the internals of ggplot2 or geom_sf. Maybe you can create a new issue on their github page.

user63230 Over a year ago

the solution i was after is above...you may be interested

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