Week 1: Discussion Section - Answer Key

Create a Map of Easter Island with tmap

library(here)
library(tidyverse)
library(stars)
library(sf)
library(tmap)

# import all data files for Easter Island (3 vectors, 1 raster)
ei_points <- sf::read_sf(here::here("data", "easter_island", "ei_points.gpkg"))
volcanoes <- subset(ei_points, type == "volcano")
ei_elev <- stars::read_stars(here::here("data", "easter_island", "ei_elev.tif"))
ei_borders <- sf::read_sf(here::here("data", "easter_island", "ei_border.gpkg"))
ei_roads <- sf::read_sf(here::here("data", "easter_island", "ei_roads.gpkg"))

# first plot the elevation raster
tm_shape(ei_elev) +
  tm_raster(style = "cont",                # continuous values for this layer
            palette = "-RdYlGn",           # reversed red-yellow-green continuous palette
            title = "Elevation (m asl)") + # legend title with units
  # add vector: Easter Island vector in default gray
  tm_shape(ei_borders) +
    tm_borders() +
  # add vector: road lines in default black
  tm_shape(ei_roads) +
    tm_lines(lwd = "strokelwd", # line width depends on attribute value 
           legend.lwd.show = FALSE) +
  # add vector: volcano points in default gray
  tm_shape(volcanoes) +
    tm_symbols(shape = 24,                       # triangle
             size = "elevation",                 # symbol size depends on attribute value 
             title.size = "Volcanoes (m asl)") + # legend title with units
  # general map layout options
  tm_layout(main.title = "Easter Island",
            bg.color = "lightblue") +            # background color for map (ocean)
  tm_scale_bar(position = c("left", "bottom")) +
  tm_compass(show.labels = 1)                    # only show North label on compass

Create an Interactive Map of Easter Island with tmap

# same code as static map above
ei_map <- tm_shape(ei_elev) +
  tm_raster(style = "cont",
            palette = "-RdYlGn",
            title = "Elevation (m asl)") +
  tm_shape(ei_borders) + 
  tm_borders() +
  tm_shape(ei_roads) + 
  tm_lines(lwd = "strokelwd", 
           legend.lwd.show = FALSE) +
  tm_shape(volcanoes) +
  tm_symbols(shape = 24, 
             size = "elevation",
             title.size = "Volcanoes (m asl)") +
  tm_layout(main.title = "Easter Island",
            bg.color = "lightblue") +
  tm_scale_bar(position = c("left", "bottom")) +
  tm_compass(show.labels = 1)

# convert default static map to interactive map 
tmap_mode("view")

ei_map

Create a Map of Easter Island with ggplot2

# extract lat & long from geom column 
volcanoes_point <- volcanoes %>%
  mutate(lon = unlist(map(volcanoes$geom,1)), # longitude = first component (x)
         lat = unlist(map(volcanoes$geom,2))) # latitude = second component (y)

ggplot() +
  # first add Easter Isand borders
  geom_sf(data = ei_borders, color = "#212529") +
  # add elevation raster
  geom_stars(data = ei_elev) +
  scale_fill_distiller(name = "Elevation (m asl)", # legend title with units
                       palette = "RdYlGn",         # red-yellow-green palette (no need to reverse)
                       na.value = "lightblue") +  # set NA color (background ocean)
  # add road vector
  geom_sf(data = ei_roads, color = "#343a40") +
  # add volcano vector
  geom_point(data = volcanoes_point, 
             aes(x = lon, y = lat, 
                 size = elevation), # point size depends on attribute value
             shape = 17,            # triangle
             color = "#22577a") +
  scale_size_continuous(name = "Volcanoes (m asl)") +       # legend title with units
  ggspatial::annotation_north_arrow(location = "br",        # bottom right
                                    which_north = "true") + # point to north pole
  ggspatial::annotation_scale(location = "bl",              # bottom left
                              width_hint = 0.5) +           # proportion of map area the scalebar should occupy
  labs(title = "Easter Island") +
  theme_minimal()