diff --git a/src/rasterix/utils.py b/src/rasterix/utils.py
index 8a080b3..0d87c98 100644
--- a/src/rasterix/utils.py
+++ b/src/rasterix/utils.py
@@ -148,8 +148,9 @@ def get_affine(
     dx = (x[1] - x[0]).item()
     dy = (y[1] - y[0]).item()
     return Affine.translation(
-        x[0].item() - dx / 2, (y[0] if dy < 0 else y[-1]).item() - dy / 2
-    ) * Affine.scale(dx, dy)
+    x[0].item() - dx/2,
+    y[0].item() - dy/2
+) * Affine.scale(dx, dy)
 
 
 def get_crs_from_proj_zarr_convention(obj: xr.Dataset | xr.DataArray) -> CRS | None:
diff --git a/tests/test_raster_index.py b/tests/test_raster_index.py
index 4973653..d89941b 100644
--- a/tests/test_raster_index.py
+++ b/tests/test_raster_index.py
@@ -1059,3 +1059,26 @@ def test_tolerance_in_concat():
 
     with set_options(transform_rtol=1e-9):
         _assert_transforms_are_compatible(a1, a2)  # passes with 1e-9
+
+
+def test_get_affine_preserves_ascending_y():
+    nx, ny = 4, 3
+    lon = np.array([-179.5, -89.5, 0.5, 89.5])      # ascending
+    lat_asc  = np.array([-60.0, 0.0, 60.0])          # ascending
+    lat_desc = lat_asc[::-1]                          # descending
+    
+    ds_asc  = xr.Dataset(
+        {"foo": (("lat", "lon"), np.zeros((ny, nx)))},
+        coords={"lat": lat_asc, "lon": lon},
+    )
+    ds_desc = xr.Dataset(
+        {"foo": (("lat", "lon"), np.zeros((ny, nx)))},
+        coords={"lat": lat_desc, "lon": lon},
+    )
+    
+    out_asc  = assign_index(ds_asc,  x_dim="lon", y_dim="lat")
+    out_desc = assign_index(ds_desc, x_dim="lon", y_dim="lat")
+    
+    # The materialized coords should round-trip the input direction
+    np.testing.assert_allclose(out_asc.lat.values,  lat_asc)
+    np.testing.assert_allclose(out_desc.lat.values, lat_desc)