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Regridding fails for geostationary coordinate systems #5485

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stephenworsley opened this issue Sep 8, 2023 · 3 comments · May be fixed by #5745
Open

Regridding fails for geostationary coordinate systems #5485

stephenworsley opened this issue Sep 8, 2023 · 3 comments · May be fixed by #5745
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Type: Bug

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@stephenworsley
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🐛 Bug Report

When one of the cubes involved in regridding has coordinates with a geostationary coordinate system, their units ought to be in radians, however iris insists that they ought to be in meters and regridding will fail when it gets to this line:

iris/lib/iris/analysis/_regrid.py

Lines 876 to 881 in c526ce0

if coord.units != "m":
msg = (
"Unsupported units for coordinate system. "
"Expected 'metres' got {!r}.".format(coord.units)
)
raise ValueError(msg)

This logic should be changed to allow for gesostationary coordinates with radian units.

See Proj for a description of the geostationary projection: https://proj.org/en/9.3/operations/projections/geos.html
@stephenworsley stephenworsley self-assigned this Sep 8, 2023
@stephenworsley
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See the CF conventions for this projection for a more concrete statement of expected units: http://cfconventions.org/cf-conventions/cf-conventions.html#_geostationary_projection

The use of projection_x_coordinate and projection_y_coordinate was deprecated in version 1.9 of the CF Conventions. The initial definition of this projection used these standard names to identify the projection coordinates even though their canonical units (meters) do not match those required for this projection (radians).

Note that this also suggests we ought to be able to handle x and y coordinates with the names projection_x_angular_coordinate and projection_y_angular_coordinate.

@trexfeathers
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In case it helps:

iris/lib/iris/plot.py

Lines 984 to 996 in e9c77c0

def _check_geostationary_coords_and_convert(x, y, kwargs):
# Geostationary stores projected coordinates as scanning angles (
# radians), in line with CF definition (this behaviour is unique to
# Geostationary). Before plotting, must be converted by multiplying by
# satellite height.
x, y = (i.copy() for i in (x, y))
transform = kwargs.get("transform")
if isinstance(transform, cartopy.crs.Geostationary):
satellite_height = transform.proj4_params["h"]
for i in (x, y):
i *= satellite_height
return x, y

@trexfeathers
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See the CF conventions for this projection for a more concrete statement of expected units: http://cfconventions.org/cf-conventions/cf-conventions.html#_geostationary_projection

The use of projection_x_coordinate and projection_y_coordinate was deprecated in version 1.9 of the CF Conventions. The initial definition of this projection used these standard names to identify the projection coordinates even though their canonical units (meters) do not match those required for this projection (radians).

Note that this also suggests we ought to be able to handle x and y coordinates with the names projection_x_angular_coordinate and projection_y_angular_coordinate.

Need to remember to update guess_coord_axis() when we add these new coordinate names.

@stephenworsley stephenworsley linked a pull request Feb 14, 2024 that will close this issue
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@bjlittle bjlittle assigned bjlittle and unassigned trexfeathers Feb 16, 2024
@stephenworsley stephenworsley mentioned this issue Feb 16, 2024
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Type: Bug
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Add support for Geostationary coord_system stephenworsley/iris
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@bjlittle @trexfeathers @stephenworsley