diff --git a/ctapipe/coordinates/ground_frames.py b/ctapipe/coordinates/ground_frames.py
index c1c83b2e708..079267f4c0c 100644
--- a/ctapipe/coordinates/ground_frames.py
+++ b/ctapipe/coordinates/ground_frames.py
@@ -98,10 +98,10 @@ def _get_shower_trans_matrix(azimuth, altitude, inverse=False):
 
     Parameters
     ----------
-    azimuth: u.Quantity[angle]
+    azimuth: float or ndarray
         Azimuth angle in radians of the tilted system used
-    zenith: u.Quantity[angle]
-        Zenith angle in radiuan of the tilted system used
+    altitude: float or ndarray
+        Altitude angle in radiuan of the tilted system used
 
     Returns
     -------
@@ -232,7 +232,7 @@ def project_to_ground(tilt_system):
 
     trans = _get_shower_trans_matrix(
         tilt_system.pointing_direction.az.to_value(u.rad),
-        np.pi / 2 - tilt_system.pointing_direction.alt.to_value(u.rad),
+        tilt_system.pointing_direction.alt.to_value(u.rad),
     )
 
     x_projected = x_initial - trans[2][0] * z_initial / trans[2][2]
diff --git a/ctapipe/coordinates/tests/test_coordinates.py b/ctapipe/coordinates/tests/test_coordinates.py
index 97cb00d14c9..1c93e70e274 100644
--- a/ctapipe/coordinates/tests/test_coordinates.py
+++ b/ctapipe/coordinates/tests/test_coordinates.py
@@ -249,7 +249,7 @@ def test_ground_to_tilt_many_to_many():
         TiltedGroundFrame(pointing_direction=pointing_direction)
     )
 
-    assert tilted.shape == (2,3)
+    assert tilted.shape == (2, 3)
 
 
 def test_camera_missing_focal_length():
@@ -300,7 +300,7 @@ def test_ground_to_tilt_many_to_many_roundtrip():
     tilted = ground[:, np.newaxis].transform_to(
         TiltedGroundFrame(pointing_direction=pointing_direction)
     )
-    back = tilted[:,0].transform_to(GroundFrame())
+    back = tilted[:, 0].transform_to(GroundFrame())
 
     assert u.isclose(ground.x, back.x, atol=1e-12 * u.m).all()
     assert u.isclose(ground.y, back.y, atol=1e-12 * u.m).all()