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Fix incorrect affine for ascending-Y coordinate inputs #91

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Fix incorrect affine for ascending-Y coordinate inputs #91

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18125b4
fix affine determination for ascending y coordinate
mdsumner May 13, 2026
6bcd759
unit test for y ascending
mdsumner May 13, 2026
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5 changes: 3 additions & 2 deletions src/rasterix/utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -127,8 +127,9 @@ 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)


_ZarrConventionRegistration = TypedDict("_ZarrConventionRegistration", {"proj:": str})
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23 changes: 23 additions & 0 deletions tests/test_raster_index.py
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Original file line number Diff line number Diff line change
Expand Up @@ -944,3 +944,26 @@ 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)