Quick Logistical Things

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Coding Workshop!

Geospatial Operations 1: Unary Operations

Getting the Geometries

Using rnaturalearth with mapview

Code

set.seed(6805)
library(tidyverse)

── Attaching core tidyverse packages ──────────────────────── tidyverse 2.0.0 ──
✔ dplyr     1.2.1     ✔ readr     2.2.0
✔ forcats   1.0.1     ✔ stringr   1.6.0
✔ lubridate 1.9.5     ✔ tibble    3.3.1
✔ purrr     1.2.1     ✔ tidyr     1.3.2
── Conflicts ────────────────────────────────────────── tidyverse_conflicts() ──
✖ dplyr::filter() masks stats::filter()
✖ dplyr::lag()    masks stats::lag()
ℹ Use the conflicted package (<http://conflicted.r-lib.org/>) to force all conflicts to become errors

Code

library(sf)

Linking to GEOS 3.13.0, GDAL 3.8.5, PROJ 9.5.1; sf_use_s2() is TRUE

Code

library(rnaturalearth)
library(mapview)
france_sf <- ne_countries(country = "France", scale = 50)
(france_map <- mapview(france_sf, label = "geounit", legend = FALSE))

Centroid of France

Code

france_cent_sf <- sf::st_centroid(france_sf)

Warning: st_centroid assumes attributes are constant over geometries

Code

france_map + mapview(france_cent_sf, label = "Centroid", legend = FALSE)

One We Already Saw: Union

Computing the union of all geometries in the sf via sf::st_union()

Code

library(leaflet.extras2)

Loading required package: leaflet

Code

africa_sf <- ne_countries(continent = "Africa", scale = 50)
africa_union_sf <- sf::st_union(africa_sf)
africa_map <- mapview(africa_sf, label="geounit", legend=FALSE)
africa_union_map <- mapview(africa_union_sf, label="st_union(africa)", legend=FALSE)
africa_map | africa_union_map

Helpful for Rasterizing: BBox

Code

africa_bbox_sf <- sf::st_bbox(africa_sf)
africa_bbox_map <- mapview(africa_bbox_sf, label="st_bbox(africa)", legend=FALSE)
africa_map | africa_bbox_map

Convex Hulls by Country

Code

africa_countries_cvx <- sf::st_convex_hull(africa_sf)
africa_countries_cvx_map <- mapview(africa_countries_cvx, label="geounit", legend=FALSE)
africa_map | africa_countries_cvx_map

Convex Hull of Continent

Use st_union() first:

Code

africa_cvx <- africa_sf |> st_union() |> st_convex_hull()
africa_cvx_map <- mapview(africa_cvx, label="geounit", legend=FALSE)
africa_map | africa_cvx_map

One We Already Saw: Centroids

Computing the centroid of all geometries in the sf via sf::st_centroid()

Code

africa_cents_sf <- sf::st_centroid(africa_sf)

Warning: st_centroid assumes attributes are constant over geometries

Code

africa_cents_map <- mapview(africa_cents_sf, label="geounit", legend=FALSE)
africa_map | africa_cents_map

Coordinate Reference Systems

First things first… what are coordinates?
We need a way to unambiguously specify where things are on the earth

Latitude and Longitude are Angles!

Angular Distance vs. Travel Distance

The Earth’s “width” is slightly greater than its “length” 😰

Projections

Smooshing 3D into 2D

Avoid Getting Lost in the Sauce

How To Avoid Getting Lost in the Sauce

**Tissot Circles**: Imagine infinitely small ellipses placed at regular intervals on the curved surface of the earth. Imagine these ellipses being projected along with the earth’s surface. When scaled up, changes in the ellipses show the location and quality of distortions on the projected map.

Your “Escape Rope” When Lost in the Sauce

https://epsg.io/4326

https://epsg.io/3857

Choropleths for Good and Evil

Crucial crucial aspect of GIS: Having Domain Knowledge of a Region

The US’s Ur-Choropleth #1: Population

Kieran Healy, *“America’s Ur-Choropleths”*

The US’s Ur-Choropleth #2: Race

Crime in Mongolia

Population of Mongolia

Exhibit A