Figures 2.6 Slivers and gaps in a polygon layer

code

extra

Here is how slivers and gaps can happen…

Read in some polygons.

Code

library(sf)
library(tmap)
library(dplyr)

polygons <- st_read("some-polygons.gpkg")

Simplify them without due care and attention, and voila!

Code

m1 <- tm_shape(polygons) +
  tm_fill(col = "name", alpha = 0.5) +
  tm_layout(
    title = "Source polygons", frame = FALSE,
    legend.show = FALSE)

m2 <- tm_shape(polygons %>% 
                 st_simplify(dTolerance = 100)) +
  tm_fill(col = "name", alpha = 0.5) +
  tm_layout(
    title = "Source polygons simplified", frame = FALSE,
    legend.show = FALSE)

tmap_arrange(m1, m2, ncol = 2)

By contrast if we use the simplification function from rmapshaper topology is preserved, even with pretty dramatic simplification (here retaining only 25% of polygon vertices.)

Code

library(rmapshaper)

m3 <- tm_shape(polygons %>% ms_simplify(keep = 0.25)) +
  tm_fill(col = "name", alpha = 0.5) +
  tm_layout(
    title = "Topology preserved", frame = FALSE,
    legend.show = FALSE)

tmap_arrange(m1, m3, ncol = 2)

The rmapshaper::topojson_write function also allows you to write your polygons out to the topology preserving TopoJSON format.

rmapshaper is associated with the excellent MapShaper tool.

Code

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