pacman::p_load(sf, tmap, tidyverse)Hands-on Exercise 7b: Visualising Geospatial Point Data
1 Overview
Proportional symbol maps (also known as graduate symbol maps) are a class of maps that use the visual variable of size to represent differences in the magnitude of a discrete, abruptly changing phenomenon, e.g. counts of people. Like choropleth maps, you can create classed or unclassed versions of these maps. The classed ones are known as range-graded or graduated symbols, and the unclassed are called proportional symbols, where the area of the symbols are proportional to the values of the attribute being mapped. In this hands-on exercise, we learn how to create a proportional symbol map showing the number of wins by Singapore Pools’ outlets using tmap.
2 Getting Started
In this exercise, we will be using the following packages:
| Package | Description |
|---|---|
| tmap | For choropleth mapping. |
| tidyverse | readr: For importing delimited text file. tidyr: For tidying data. dplyr: For data wrangling. |
| sf | For handling geospatial data. |
| knitr | For html tables |
3 Geospatial Data Wrangling
3.1 The Data
The data set use for this hands-on exercise is called SGPools_svy21. The data is in csv file format.
It consists of seven columns. The XCOORD and YCOORD columns are the x-coordinates and y-coordinates of SingPools outlets and branches. They are in Singapore SVY21 Projected Coordinates System.
3.2 Data Import and Preparation
| Function | Package | Description | Output |
|---|---|---|---|
read_csv() |
readr | Import SGPools_svy21.csv into R | Tibble data frame |
sgpools <- read_csv("data/aspatial/SGPools_svy21.csv")Let’s examine if the datafile has been imported correctly:
[[1]]
# A tibble: 5 × 7
NAME ADDRESS POSTCODE XCOORD YCOORD `OUTLET TYPE` `Gp1Gp2 Winnings`
<chr> <chr> <dbl> <dbl> <dbl> <chr> <dbl>
1 Livewire (Mari… 2 Bayf… 18972 30842. 29599. Branch 5
2 Livewire (Reso… 26 Sen… 98138 26704. 26526. Branch 11
3 SportsBuzz (Kr… Lotus … 738078 20118. 44888. Branch 0
4 SportsBuzz (Po… 1 Sele… 188306 29777. 31382. Branch 44
5 Prime Serangoo… Blk 54… 552542 32239. 39519. Branch 03.3 Creating a sf data frame from an aspatial data frame
| Function | Package | Description | Output |
|---|---|---|---|
st_as_sf() |
sf | Converts sgpools data frame into a simple feature data frame | Simple feature |
sgpools_sf <- st_as_sf(sgpools,
coords = c("XCOORD", "YCOORD"),
crs= 3414)
list(sgpools_sf)[[1]]
Simple feature collection with 306 features and 5 fields
Geometry type: POINT
Dimension: XY
Bounding box: xmin: 7844.194 ymin: 26525.7 xmax: 45176.57 ymax: 47987.13
Projected CRS: SVY21 / Singapore TM
# A tibble: 306 × 6
NAME ADDRESS POSTCODE `OUTLET TYPE` `Gp1Gp2 Winnings`
* <chr> <chr> <dbl> <chr> <dbl>
1 Livewire (Marina Bay Sands) 2 Bayf… 18972 Branch 5
2 Livewire (Resorts World Sen… 26 Sen… 98138 Branch 11
3 SportsBuzz (Kranji) Lotus … 738078 Branch 0
4 SportsBuzz (PoMo) 1 Sele… 188306 Branch 44
5 Prime Serangoon North Blk 54… 552542 Branch 0
6 Singapore Pools Woodlands C… 1A Woo… 731001 Branch 3
7 Singapore Pools 64 Circuit … Blk 64… 370064 Branch 17
8 Singapore Pools 88 Circuit … Blk 88… 370088 Branch 16
9 Singapore Pools Anchorvale … Blk 30… 540308 Branch 21
10 Singapore Pools Ang Mo Kio … Blk 20… 560202 Branch 25
# ℹ 296 more rows
# ℹ 1 more variable: geometry <POINT [m]>Things to learn from the arguments above:
The coords argument requires you to provide the column name of the x-coordinates first then followed by the column name of the y-coordinates.
The crs argument required you to provide the coordinates system in epsg format. EPSG: 3414 is Singapore SVY21 Projected Coordinate System. You can search for other country's epsg code by refering to epsg.io.
Quick Comparison of the 2 datasets:
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4 Drawing Proportional Symbol Map
To create an interactive proportional symbol map in R, the view mode of tmap will be used.
The code chunk below will turn on the interactive mode of tmap.
tmap_mode("view")4.1 Interactive Point Symbol Map
tm_basemap("OpenStreetMap") +
tm_shape(sgpools_sf)+
tm_bubbles(col = "salmon",
size = 1,
border.col = "black",
border.lwd = 1,
alpha = 0.7)4.2 Proportional Symbols
To draw a proportional symbol map, we need to assign a numerical variable to the size visual attribute. The code chunks below show that the variable Gp1Gp2Winnings is assigned to size visual attribute.
tm_basemap("OpenStreetMap") +
tm_shape(sgpools_sf)+
tm_bubbles(col = "salmon",
size = "Gp1Gp2 Winnings",
border.col = "black",
border.lwd = 1,
alpha = 0.7)4.3 Qualitative Colours
A continuous variable is mapped to size while a categorical variable is mapped to colour.
tm_basemap("OpenStreetMap") +
tm_shape(sgpools_sf)+
tm_bubbles(col = "OUTLET TYPE",
size = "Gp1Gp2 Winnings",
border.col = "black",
border.lwd = 1,
alpha = 0.7)4.4 Coordinated Facet
The argument sync in tm_facets() can be used to produce multiple maps with synchronised zoom and pan settings.
tm_basemap("OpenStreetMap") +
tm_shape(sgpools_sf) +
tm_bubbles(col = "OUTLET TYPE",
size = "Gp1Gp2 Winnings",
border.col = "black",
border.lwd = 1,
id = "Name",
popup.vars=c("Name: " = "NAME",
"Address: " = "ADDRESS",
"Winnings: " = "Gp1Gp2 Winnings")
) +
tm_facets(by= "OUTLET TYPE",
nrow = 1,
sync = TRUE)Switch tmap’s Viewer back to plot mode to revert settings back to default.
tmap_mode("plot")7 Reference
- Kam, T.S. (2023). Visualising Geospatial Point Data.