Map Making

This is a blog post that is my journey on how to make maps.

Let’s Make Some Maps

tmap basics

This is a blog post about my process navigating chapter 8 in GeoComputation with R for EDS 223. I don’t know where this journey will take me, but I figured that I’d make a blog post about my process as I make some cool maps and learn how to use tmap.

Making a basic map of New Zealand with tmap. The New Zealand object comes from the sf data

tm_polygons() combines both tm_fill() and tm_borders() so map_nz1 and map_nz2 below are the same.

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map_nz1 <- tm_shape(nz) +
  tm_fill() + 
  tm_borders() 

map_nz2 <- tm_shape(nz) + tm_polygons()
map_nz2

Combining multiple layers

tm_raster() plots a raster layer and the argument alpha is the transparency of that layer

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map_nz3 <- map_nz2 + tm_shape(nz_elev) +
  tm_raster(alpha = 0.7) 
map_nz3

New Zealand water addition

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nz_water <- st_union(nz) %>% #returns a single geometry with resolved boundaries
  st_buffer(22200) %>% #computes a buffer around this geometry
  st_cast(to = "LINESTRING") #cast geometry to another type

map_nz4 <- map_nz3 + 
  tm_shape(nz_water) + tm_lines()
map_nz4

Adding the height element to the map of New Zealand

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map_nz5 <- map_nz4 + 
  tm_shape(nz_height) + tm_dots()

map_nz5

To combine different maps using tmap in a similar way as you would use patchwork in ggplot you do the following:

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tmap_arrange(map_nz3, map_nz4, map_nz5)

tmap aesthetics

The common aesthetics for fill and border layers are color col , transparency alpha , line width lwd and line typelty.

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map1 <- tm_shape(nz) + tm_fill(col = "green")
map2 <- tm_shape(nz) + tm_fill(col = "red", alpha = 0.5)
map3 <- tm_shape(nz) + tm_borders(col = "darkgrey")
map4 <- tm_shape(nz) + tm_borders(lwd = 2)
map5 <- tm_shape(nz) + tm_borders(lty = 3)
map6 <- tm_shape(nz) + tm_fill(col = "green", alpha = 0.5) +
  tm_borders(col = "darkgrey", lwd = 2, lty = 3)
tmap_arrange(map1, map2, map3, map4, map5, map6)

You cannot use a column name in tmap like you can in R to set the aesthetic of a variable like color, but instead you have to put it in quotations. You can treat plot the same way as you would R though. Hopefully, I am understanding this correctly.

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# tm_shape(nz) + tm_fill(col = nz$Land_area) This line fails
plot(st_geometry(nz), col = nz$Land_area) # This works because plot has the same functionality as R

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tm_shape(nz) + tm_fill(col = "Land_area") #The color aesthetic has to be in quotes here 

To add a title in the legend of the aesthetic you are using, you define this in the same place as you define the aesthetic. expression() is used in the commented seciton below to add the special script for the squared kilometers. If you do not need fancy text, you can just put your title in quotes.

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#legend_title = expression("Area (km"^2*")") 
legend_title = "Area in Kilometers Squared"
map_nza = tm_shape(nz) +
  tm_fill(col = "Land_area", title = legend_title) + tm_borders(lwd = 1.5)
map_nza

Color Settings

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tm_shape(nz) + tm_polygons(col = "Median_income")

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breaks = c(0, 3, 4, 5) * 10000
tm_shape(nz) + tm_polygons(col = "Median_income", breaks = breaks)

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tm_shape(nz) + tm_polygons(col = "Median_income", n = 10)

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tm_shape(nz) + tm_polygons(col = "Median_income", palette = "viridis")

This is all the map work I’ve done for the day! Stay tuned, maybe there will be more.