Using APIs

Coursework Example
R
Author

Lily Cook

Published

May 19, 2025

Libraries

library(tidyverse)
library(httr)
library(tidyjson)
library(jsonlite)
library(leaflet)

Reading in capitals

capitals <- read_table("state_capitals_ll.txt", col_names = F)

capital_names <- read_table("state_capitals_name.txt", col_names = F)

colnames(capitals) <- c("state", "lat", "long") #add names
colnames(capital_names) <- c("state", "capital")

capital_names <- capital_names %>% mutate(capital =  str_remove_all(capital, '"')) # remove unnecessary quote

capitals <- capitals %>% 
  filter(state != "US") %>% # filter out random coords
  filter(state != "AK") %>% # filter out alaska, doesn't pass by
  left_join(capital_names, by = "state")

API functions

# function to get pass times for a single capital

get_single_location_passes <- function(lat, long) {
  api_base_url <- "https://api.g7vrd.co.uk/v1/satellite-passes/25544"
  request_url <- paste0(api_base_url, "/", lat, "/", long, ".json")
  
  # get request
  response <- GET(request_url)
  
  # check if the request successful, if success get content
  if (status_code(response) == 200) {
    content <- content(response, "text", encoding = "UTF-8")
    pass_data <- fromJSON(content)
    
    # extract time 
    if (length(pass_data$passes) > 0 && "tca" %in% names(pass_data$passes)) {
      # get first 3 TCA timestamps
      return(head(pass_data$passes$tca, 3))
    } else {
      return(NULL) # if no passes found or tca field missing
    }
  } else {
    warning(paste("API request failed for lat:", lat, "long:", long, "with status:", status_code(response)))
    return(NULL) # if request failed
  }
}

# function to get pass times for capitals and create df
get_all_capitals_passes <- function(df) {
  # empty list to store results
  all_passes_list <- list()
  
  # loop through each capital
  for (i in 1:nrow(df)) {
    capital_info <- df[i, ]
    
    pass_times_vector <- get_single_location_passes(lat = capital_info$lat, long = capital_info$long)
    
    # make tibble for the capital pass times
    # three time columns, NA if less than 3 
    current_passes_df <- tibble(
      state_abbr = capital_info$state,
      capital_name = capital_info$capital,
      lat = capital_info$lat,
      long = capital_info$long,
      time1 = if (length(pass_times_vector) >= 1) pass_times_vector[1] else NA_character_,
      time2 = if (length(pass_times_vector) >= 2) pass_times_vector[2] else NA_character_,
      time3 = if (length(pass_times_vector) >= 3) pass_times_vector[3] else NA_character_
    )
    
    all_passes_list[[i]] <- current_passes_df
    
    # wait for 1 second between requests
    Sys.sleep(1) 
  }
  
  # combine all tibbles
  final_df <- bind_rows(all_passes_list)
  return(final_df)
}

Pass Times Data

# raw pass times for all capitals
iss_pass_data_raw <- get_all_capitals_passes(capitals)

# convert times and sort
iss_pass_data_processed <- iss_pass_data_raw |>
  # convert UTC to datetime - help from AI
  mutate(
    time1_dt = ymd_hms(time1, tz = "UTC"),
    time2_dt = ymd_hms(time2, tz = "UTC"),
    time3_dt = ymd_hms(time3, tz = "UTC")
  ) |>
  # arrange by first pass time
  arrange(time1_dt) |>
  # remove rows where time 1 is NA
  filter(!is.na(time1_dt))

2-5. Mapping the Data and Drawing the ISS Route

Define Custom Icon

satellite_icon <- makeIcon(
  iconUrl = "https://png.pngtree.com/png-clipart/20230111/original/pngtree-rocket-icon-vector-png-image_8902705.png",
  iconWidth = 25, 
  iconHeight = 25,
  iconAnchorX = 12, 
  iconAnchorY = 12  
)

Create the Leaflet Map

We construct the map layer by layer.

# ensure there is data for plot
if (nrow(iss_pass_data_processed) > 0) {
  
  # format times (local time zone for readability) for labels etc
  
  map_data <- iss_pass_data_processed |>
    mutate(
      time1_display = format(time1_dt, "%Y-%m-%d %H:%M:%S UTC"),
      time2_display = format(time2_dt, "%Y-%m-%d %H:%M:%S UTC"),
      time3_display = format(time3_dt, "%Y-%m-%d %H:%M:%S UTC"),
      
      # handle NA times in display strings
      time2_display = ifelse(is.na(time2_dt), "N/A", time2_display),
      time3_display = ifelse(is.na(time3_dt), "N/A", time3_display)
    )

  # hover labels
  hover_labels <- paste0(
    "<strong>Capital:</strong> ", map_data$capital_name, "<br>",
    "<strong>State:</strong> ", map_data$state_abbr, "<br>",
    "<strong>Soonest Pass:</strong> ", map_data$time1_display
  ) |> lapply(htmltools::HTML) # lapply with HTML for proper rendering

  # click popups
  click_popups <- paste0(
    "<strong>Capital:</strong> ", map_data$capital_name, " (", map_data$state_abbr, ")<br><br>",
    "<strong>Predicted Pass Times (UTC):</strong><br>",
    "1. ", map_data$time1_display, "<br>",
    "2. ", map_data$time2_display, "<br>",
    "3. ", map_data$time3_display
  ) |> lapply(htmltools::HTML)

  # create the map
  iss_map <- leaflet(data = map_data) |>
    addTiles(group = "OSM (Default)") |> # add default OpenStreetMap map tiles
    addProviderTiles(providers$CartoDB.Positron, group = "CartoDB Positron") |>
    addProviderTiles(providers$Esri.WorldImagery, group = "Esri World Imagery") |>
    
    # markers for each state capital
    addMarkers(
      lng = ~long, 
      lat = ~lat,
      icon = satellite_icon,
      label = hover_labels,
      popup = click_popups,
      group = "State Capitals"
    ) |>
    
    #  polylines connecting capitals in order of first pass time
    addPolylines(
      lng = ~long,
      lat = ~lat,
      color = "#E6007E", 
      weight = 3,
      opacity = 0.8,
      dashArray = "5, 5", # dashed line
      group = "ISS Pass Order Path"
    ) |>
    
    #  layer controls to toggle layers
    addLayersControl(
      baseGroups = c("OSM (Default)", "CartoDB Positron", "Esri World Imagery"),
      overlayGroups = c("State Capitals", "ISS Pass Order Path"),
      options = layersControlOptions(collapsed = FALSE)
    ) |>
    
    # legend for the polyline
    addLegend(
        position = "bottomright",
        colors = "#E6007E",
        labels = "ISS Pass Order Path",
        title = "Map Features"
    )

  # display the map
  iss_map

} else {
  print("No ISS pass data available to map. Check API calls or data processing steps.")
}