Great circles with sf

great-circles.R

library("tidyverse")
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library("sf")
#> Linking to GEOS 3.6.1, GDAL 2.1.3, proj.4 4.9.3
library("leaflet")

## Original dataset
journey_data <- tribble(
  ~start.long, ~start.lat, ~end.long, ~end.lat, ~year, ~name,
  -0.118092,    51.509865, -119.698189, 34.420830, 1999, "London to Santa Barbara",
  31.496773,    30.026300, 24.753574, 59.436962, 2000, "New Cairo to Tallinn",
  126.633333, 45.75,    46.738586, 24.774265, 2001, "Charlie"
)

start_locs <- journey_data %>%
  select(contains("start")) %>%
  rename(long = start.long,
         lat = start.lat) %>%
  mutate(journey.id = row_number())
end_locs <- journey_data %>%
  select(contains("end")) %>%
  rename(long = end.long,
         lat = end.lat) %>%
  mutate(journey.id = row_number())

## Generate a new sf object with LINESTRINGs between the start and end location
journey_data_linestrings <- start_locs %>%
  bind_rows(end_locs) %>%
  sf::st_as_sf(coords = c("long","lat")) %>%
  group_by(journey.id) %>%
  arrange(journey.id) %>%
  summarise() %>%
  sf::st_cast("LINESTRING")

## Add these LINESTRINGs to the tibble!
st_geometry(journey_data) <- st_geometry(journey_data_linestrings)

## We need to add a coordinate system to the sf object, let's go with the canonical standard EPSG:4326
journey_data <- st_set_crs(journey_data, st_crs(4326))

## Make a map
journey_data %>%
  st_segmentize(units::set_units(100, km)) %>%
  leaflet() %>%
  addTiles() %>%
  addPolylines()