diff --git a/tests/testthat/test-prep_coords.R b/tests/testthat/test-prep_coords.R
new file mode 100644
index 0000000..e55df2e
--- /dev/null
+++ b/tests/testthat/test-prep_coords.R
@@ -0,0 +1,319 @@
+
+testthat::test_that("correct_flipped_geo_coords works correctly", {
+  # Test data
+  data <- data.frame(
+    id = c("{0CDA1C45-9529-4188-BE2F-863E287EDA71}",
+           "{387991C9-EA43-4C8C-8C96-E7F69F068F16}"),
+    lat = c(40.7128, -74.0060),
+    lon = c(-74.0060, 40.7128)
+  )
+  
+  custom_shapefile <- data.frame(
+    region_id = c("{0CDA1C45-9529-4188-BE2F-863E287EDA71}",
+                  "{387991C9-EA43-4C8C-8C96-E7F69F068F16}"),
+    correct_lat = c(40.7128, 41.8781),
+    correct_lon = c(-74.0060, -87.6298)
+  )
+  
+  suppressMessages(
+    # Test without shapefile
+    result <- correct_flipped_geo_coords(
+      data,
+      join_key_a = "id",
+      lat_col = "lat",
+      lon_col = "lon"
+    )
+  )
+  
+  # Check if coordinates are corrected
+  testthat::expect_equal(
+    round(result$lat), c(41, 41), tolerance = 1e-4)
+  testthat::expect_equal(
+    round(result$lon), c(-74, -74), tolerance = 1e-4)
+  
+  suppressMessages(
+    # Test with custom shapefile
+    result2 <- correct_flipped_geo_coords(
+      data,
+      shapefile_data = custom_shapefile,
+      join_key_a = "id",
+      join_key_b = "region_id",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat",
+      correct_lon_col = "correct_lon"
+    )
+  )
+  
+  # Check if coordinates are corrected
+  testthat::expect_equal(
+    round(result2$lat), c(41, 41), tolerance = 1e-4)
+  testthat::expect_equal(
+    round(result2$lon), c(-74, -74), tolerance = 1e-4)
+  
+  # Check if flipped column is correct
+  testthat::expect_equal(result2$potentially_flipped, c(FALSE, TRUE))
+  
+  # Test error messages
+  testthat::expect_error(
+    correct_flipped_geo_coords(
+      data,
+      shapefile_data = custom_shapefile,
+      join_key_a = "non_existent_key",
+      join_key_b = "region_id",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat",
+      correct_lon_col = "correct_lon"
+    ),
+    "The join key 'non_existent_key' is not present in the data dataframe."
+  )
+  
+  testthat::expect_error(
+    correct_flipped_geo_coords(
+      data,
+      shapefile_data = custom_shapefile,
+      join_key_a = "id",
+      join_key_b = "non_existent_key",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat",
+      correct_lon_col = "correct_lon"
+    ),
+    paste0("The join key 'non_existent_key' is not ",
+           "present in the shapefile_data dataframe.")
+  )
+  
+  testthat::expect_error(
+    correct_flipped_geo_coords(
+      data,
+      shapefile_data = custom_shapefile,
+      join_key_a = "id",
+      join_key_b = "region_id",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat2",
+      correct_lon_col = "correct_lon"
+    ),
+    paste0("The column 'correct_lat2' is not present in ",
+           "the shapefile_data dataframe.")
+  )
+  
+  testthat::expect_error(
+    correct_flipped_geo_coords(
+      data,
+      shapefile_data = custom_shapefile,
+      join_key_a = "id",
+      join_key_b = "region_id",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat",
+      correct_lon_col = "correct_lon2"
+    ),
+    paste0("The column 'correct_lon2' is not present in ",
+           "the shapefile_data dataframe.")
+  )
+  
+  
+  # Test data2
+  data2 <- data.frame(
+    id = c("{0CDA1C45-9529-4188-BE2F-863E287EDA71}",
+           "{387991C9-EA43-4C8C-8C96-E7F69F068F16}"),
+    lat =  c(40.7128, 41.8781),
+    lon = c(-74.0060, -87.6298)
+  )
+  
+  
+  suppressMessages(
+    res <- correct_flipped_geo_coords(
+      data2,
+      shapefile_data = custom_shapefile,
+      join_key_a = "id",
+      join_key_b = "region_id",
+      lat_col = "lat",
+      lon_col = "lon",
+      correct_lat_col = "correct_lat",
+      correct_lon_col = "correct_lon"
+    ))
+  
+  # Check if coordinates are corrected
+  testthat::expect_equal(
+    round(res$lat), c(41, 42), tolerance = 1e-4)
+  testthat::expect_equal(
+    round(res$lon), c(-74, -88), tolerance = 1e-4)
+})
+
+testthat::test_that("check_land_water function works correctly", {
+  # Test known locations
+  test_coords <- data.frame(
+    lon = c(-74.1613, -70.7210, 32.9333, 22.8465, 
+            -0.6796, -0.0420, NA, 45.5017, -180, 180),
+    lat = c(40.7772, 40.2987, -1.1466, 1.7656, 
+            4.6282, 5.5880, NA, NA, 90, -90)
+  )
+  
+  expected_results <- c(
+    "Land",         # New York City (Land)
+    "Ocean",        # Atlantic Ocean
+    "Inland water", # Lake Victoria
+    "Land",         # Central African Republic (Land)
+    "Ocean",        # Gulf of Guinea
+    "Ocean",        # Gulf of Guinea (close to coast)
+    NA,             # Missing longitude
+    NA,             # Missing latitude
+    "Ocean",        # North Pole
+    "Ocean"         # South Pole
+  )
+  
+  results <- check_land_water(test_coords$lon, test_coords$lat)
+  
+  testthat::expect_equal(results, expected_results)
+  
+  # # Test edge cases
+  # edge_cases <- data.frame(
+  #   lon = c(0, 0, 180, -180, 179.99, -179.99),
+  #   lat = c(0, 90, 0, 0, 89.99, -89.99)
+  # )
+  # 
+  # suppressWarnings(
+  # edge_results <- check_land_water(edge_cases$lon, edge_cases$lon))
+  # 
+  # testthat::expect_true(all(!is.na(edge_results)))
+  # 
+  # # Test invalid coordinates
+  # invalid_coords <- data.frame(
+  #   lon = c(181, -181, 0, 0),
+  #   lat = c(0, 0, 91, -91)
+  # )
+  # 
+  # invalid_results <- check_land_water(invalid_coords$lon, invalid_coords$lat)
+  # testthat::expect_true(all(is.na(invalid_results)))
+  # 
+  # Test empty input
+  # testthat::expect_equal(length(check_land_water(numeric(0), numeric(0))), 0)
+  
+  # Test large number of coordinates
+  set.seed(123)
+  large_coords <- data.frame(
+    lon = runif(10000, -180, 180),
+    lat = runif(10000, -90, 90)
+  )
+  
+  large_results <- check_land_water(large_coords$lon, large_coords$lat)
+  testthat::expect_equal(length(large_results), 10000)
+  testthat::expect_true(all(large_results %in% c(
+    "Land", "Ocean", "Inland water")))
+  
+  # Test coordinates on country borders
+  border_coords <- data.frame(
+    lon = c(6.1466, -3.0053, 14.5962),
+    lat = c(49.8153, 16.9742, 35.9025)
+  )
+  
+  border_results <- check_land_water(border_coords$lon, border_coords$lat)
+  testthat::expect_equal(border_results, c("Land", "Land", "Ocean"))
+  
+  # Test coordinates in small islands
+  island_coords <- data.frame(
+    lon = c(-157.8583, 55.4515, 166.6186),
+    lat = c(21.3069, -20.9043, -0.5228)
+  )
+  
+  island_results <- check_land_water(
+    island_coords$lon, island_coords$lat)
+  testthat::expect_equal(island_results, c("Land", "Land", "Ocean"))
+})
+
+testthat::test_that("check_land_water handles NA values correctly", {
+  na_coords <- data.frame(
+    lon = c(NA, 0, NA, 100),
+    lat = c(0, NA, NA, 50)
+  )
+  
+  na_results <- check_land_water(na_coords$lon, na_coords$lat)
+  testthat::expect_equal(na_results, c(NA, NA, NA, "Land"))
+})
+
+testthat::test_that("check_land_water is consistent for repeated calls", {
+  set.seed(456)
+  test_coords <- data.frame(
+    lon = runif(100, -180, 180),
+    lat = runif(100, -90, 90)
+  )
+  
+  results1 <- check_land_water(test_coords$lon, test_coords$lat)
+  results2 <- check_land_water(test_coords$lon, test_coords$lat)
+  
+  testthat::expect_equal(results1, results2)
+})
+
+# Mock data for testing
+mock_data <- data.frame(
+  id = c("A1", "B2", "C3", "D4", "E5"),
+  lat = c(40.7128, -74.0060, NA, 51.5074, 48.8566),
+  lon = c(-74.0060, 40.7128, -0.1278, NA, 2.3522),
+  country = c("USA", "USA", "UK", "UK", "France")
+)
+
+testthat::test_that("check_coords performs all checks correctly", {
+  
+  suppressMessages(
+    result <- check_coords(mock_data, "lat", "lon", "id")
+  )
+  
+  testthat::expect_true("missing_coords" %in% names(result))
+  testthat::expect_true("potentially_flipped" %in% names(result))
+  testthat::expect_true("on_water" %in% names(result))
+  
+  testthat::expect_equal(sum(result$missing_coords, na.rm = TRUE), 2)
+  testthat::expect_false(any(result$potentially_flipped, na.rm = TRUE))
+  testthat::expect_true(any(result$on_water != "Land", na.rm = TRUE))
+})
+
+testthat::test_that("check_coords respects the checks parameter", {
+  suppressMessages(
+    result_flip <- check_coords(mock_data, "lat", "lon", "id", checks = "flip"))
+  
+  testthat::expect_true("potentially_flipped" %in% names(result_flip))
+  testthat::expect_false("missing_coords" %in% names(result_flip))
+  testthat::expect_false("on_water" %in% names(result_flip))
+  
+  suppressMessages(
+    result_missing <- check_coords(mock_data, "lat", "lon", "id", 
+                                   checks = "missing"))
+  
+  testthat::expect_false("potentially_flipped" %in% names(result_missing))
+  testthat::expect_true("missing_coords" %in% names(result_missing))
+  testthat::expect_false("on_water" %in% names(result_missing))
+})
+
+testthat::test_that("check_coords handles aggregate_by correctly", {
+  
+  suppressMessages(
+    result <- check_coords(mock_data, "lat", "lon", "id", 
+                           aggregate_by = "country", summary_table = TRUE))
+  
+  testthat::expect_equal(nrow(result), 3)  # 3 unique countries
+  testthat::expect_true("country" %in% names(result))
+  testthat::expect_true(all(c(
+    "total_coords",
+    "missing_coords", "potentially_flipped", 
+    "on_water") %in% names(result)))
+})
+
+testthat::test_that("check_coords produces correct summary table", {
+  
+  
+  suppressMessages(
+    result <- check_coords(mock_data, "lat", "lon", "id", 
+                           summary_table = TRUE))
+  
+  testthat::expect_true(is.data.frame(result))
+  testthat::expect_equal(nrow(result), 1)  # One summary row
+  testthat::expect_true(
+    all(c(
+      "total_coords",
+      "missing_coords", 
+      "potentially_flipped", "on_water") %in% names(result)))
+})
+