How to add relevant scale bars on inset maps using tmap - viewport

Question

I used tmap to create the plot attached. However, I would like to add a scale bar to the inset map, but I haven't been able to figured out how to do that. Can someone please help me?
Here are the codes that I used to create the attached map:
main_map <- tmap::tm_shape(main_map_df) +
tmap::tm_polygons(
col = "var.q5",
palette = c("#CCCCCC", "#999999", "#666666", "#333333", "#000000"),
#alpha = 0.7,
lwd = 0.5,
title = "") +
tmap::tm_layout(
frame = FALSE,
legend.outside = TRUE,
legend.hist.width = 5,
legend.text.size = 0.5,
fontfamily = "Verdana") +
tmap::tm_scale_bar(
position = c("LEFT", "BOTTOM"),
breaks = c(0, 10, 20),
text.size = 0.5
) +
tmap::tm_compass(position = c("LEFT", "TOP"))
inset_map <- tmap::tm_shape(inset_map_df) +
tmap::tm_polygons() +
tmap::tm_shape(main_map_df) +
tm_fill("grey50") +
tmap::tm_scale_bar(
position = c("LEFT", "BOTTOM"),
breaks = c(0, 10, 20),
text.size = 0.5
)
# Combine crude rate map (inset + main) =====
tiff(
"main_map_w_iset.tiff",
height = 1200,
width = 1100,
compression = "lzw",
res = 300
)
main_map
print(
inset_map,
vp = viewport(
x = 0.7,
y = 0.18,
width = 0.3,
height = 0.3,
clip = "off")
)
dev.off()
Thank you!

Answer 1

Here's a simple example using the World data set:
library(tidyverse)
library(tmap)
library(grid)
data("World")
# main map
tm_main <- World %>%
filter(name == "Australia") %>%
tm_shape() +
tm_polygons(col = "red",
alpha = .5) +
tm_scale_bar()
# inset map
tm_inset <- tm_shape(World) +
tm_polygons(col = "gray",
alpha = .5) +
tm_scale_bar()
vp <- viewport(x = .615, y = .5, width = .6, height = .6, just = c("right", "top"))
# final map
tmap_save(tm_main, filename = "test_inset.png", insets_tm = tm_inset, insets_vp = vp,
height = 200, width = 200, units = "mm")