diff --git a/icedyno/preprocess/geolocation.py b/icedyno/preprocess/geolocation.py
index cecdca3..0e5ba63 100644
--- a/icedyno/preprocess/geolocation.py
+++ b/icedyno/preprocess/geolocation.py
@@ -128,10 +128,10 @@ def polar_lonlat_to_xy(
     return x, y
 
 
-def find_closest_index_in_grid(target: float, coordinates: np.array) -> int:
+def find_closest_index_in_grid(target: float) -> int:
     """
-    For IMS 1km data, given a target coordinate in projected coordinates (x or y) and the list of coordinates,
-    return the index of the closest number in the coordinates. Assumes a 1km grid.
+    For IMS 1km data, given a target coordinate in projected coordinates (x or y),
+    return the index of the closest number in the IMS coordinates. Assumes a 1km grid.
     """
     start = -12287500.0
 
@@ -141,6 +141,4 @@ def find_closest_index_in_grid(target: float, coordinates: np.array) -> int:
     # Calculate the index of the closest number
     index = int((target - start) // grid_resolution)
 
-    # If the solution is correct, the target and the found value should never be more than the grid_resolution apart.
-    # assert abs(coordinates[index] - target) < grid_resolution
     return index
diff --git a/tests/test_geolocation.py b/tests/test_geolocation.py
index 1630843..c2c9ce5 100644
--- a/tests/test_geolocation.py
+++ b/tests/test_geolocation.py
@@ -17,3 +17,21 @@ def test_xy_to_latlon_inversion():
 
         reverted_lat_lon = geolocation.polar_xy_to_lonlat(*xy_converted_coordinates)
         assert np.allclose((lon, lat), reverted_lat_lon)
+
+
+def test_find_closest_index_in_grid():
+    # Grid parameters for IMS data
+    start = -12287500.0
+    stop = 12287500.0
+    grid_resolution = 1000.0
+    coordinates = np.arange(start=start, stop=stop + 1000.0, step=grid_resolution)
+
+    test_val = -1400.0
+    index = geolocation.find_closest_index_in_grid(test_val)
+    assert np.isclose(coordinates[index], -1500.0)
+
+    test_vals = np.random.uniform(start, stop, 10)
+    for test_val in test_vals:
+        index = geolocation.find_closest_index_in_grid(test_val)
+        # If the solution is correct, the target and the found value should never be more than the grid_resolution apart.
+        assert abs(coordinates[index] - test_val) < grid_resolution