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Bugfix shape masking #6129

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Bugfix shape masking #6129

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ebee977
Updates to allow varying shape crs
hsteptoe Aug 22, 2024
e32f911
Ruff fixes
hsteptoe Aug 22, 2024
76eae38
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Aug 22, 2024
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86 changes: 56 additions & 30 deletions lib/iris/_shapefiles.py
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Original file line number Diff line number Diff line change
Expand Up @@ -17,14 +17,16 @@
import shapely.geometry as sgeom
import shapely.ops

import iris.analysis.cartography
from iris.warnings import IrisDefaultingWarning, IrisUserWarning


def create_shapefile_mask(
geometry,
cube,
minimum_weight=0.0,
):
geometry: shapely.Geometry,
cube: iris.cube.Cube,
minimum_weight: float = 0.0,
geometry_crs: cartopy.crs = None,
) -> np.array:
"""Make a mask for a cube from a shape.

Get the mask of the intersection between the
Expand All @@ -43,13 +45,29 @@ def create_shapefile_mask(
to be unmasked.
Requires geometry to be a Polygon or MultiPolygon
Defaults to 0.0 (eg only test intersection).
geometry_crs : :class:`cartopy.crs`, optional
A :class:`~iris.coord_systems` object describing
the coord_system of the shapefile. Defaults to None,
in which case the geometry_crs is assumed to be the
same as the `cube`.

Returns
-------
:class:`np.array`
An array of the shape of the x & y coordinates of the cube, with points
to mask equal to True.

Notes
-----
For best masking results, both the cube _and_ masking geometry should have a
coordinate reference system (CRS) defined. Masking results will be most reliable
when the cube and masking geometry have the same CRS.

If the cube has no coord_system, the default GeogCS is used where
the coordinate units are degrees. For any other coordinate units,
the cube **must** have a coord_system defined.

If a CRS is not provided for the the masking geometry, the CRS of the cube is assumed.
"""
from iris.cube import Cube, CubeList

Expand Down Expand Up @@ -116,7 +134,9 @@ def create_shapefile_mask(
return mask_template


def _transform_coord_system(geometry, cube, geometry_system=None):
def _transform_coord_system(
geometry: shapely.Geometry, cube: iris.cube.Cube, geometry_crs: cartopy.crs = None
) -> shapely.Geometry:
"""Project the shape onto another coordinate system.

Parameters
Expand All @@ -125,36 +145,52 @@ def _transform_coord_system(geometry, cube, geometry_system=None):
cube : :class:`iris.cube.Cube`
:class:`~iris.cube.Cube` with the coord_system to be projected to and
a x coordinate.
geometry_system : :class:`iris.coord_systems`, optional
A :class:`~iris.coord_systems` object describing
geometry_crs : :class:`cartopy.crs`, optional
A :class:`cartopy.crs` object describing
the coord_system of the shapefile. Defaults to None,
which is treated as GeogCS.
in which case the geometry_crs is assumed to be the
same as the `cube`.

Returns
-------
:class:`shapely.Geometry`
A transformed copy of the provided :class:`shapely.Geometry`.

"""
y_name, x_name = _cube_primary_xy_coord_names(cube)
import iris.analysis.cartography
_, x_name = _cube_primary_xy_coord_names(cube)

DEFAULT_CS = iris.coord_systems.GeogCS(
iris.analysis.cartography.DEFAULT_SPHERICAL_EARTH_RADIUS
)
target_system = cube.coord_system()
target_system = cube.coord_system().as_cartopy_projection()
if not target_system:
# If no cube coord_system do our best to guess...
if (
cube.coord(axis="x").units == "degrees"
and cube.coord(axis="y").units == "degrees"
):
# If units of degrees assume GeogCS
target_system = iris.coord_systems.GeogCS(
iris.analysis.cartography.DEFAULT_SPHERICAL_EARTH_RADIUS
)
warnings.warn(
"Cube has no coord_system; using default GeogCS lat/lon.",
category=IrisDefaultingWarning,
)
else:
# For any other units, don't guess and raise an error
target_system = iris.coord_systems.GeogCS(
iris.analysis.cartography.DEFAULT_SPHERICAL_EARTH_RADIUS
)
raise ValueError("Cube has no coord_system; cannot guess coord_system!")

if not geometry_crs:
# If no geometry_crs assume it has the cube coord_system
geometry_crs = target_system
warnings.warn(
"Cube has no coord_system; using default GeogCS lat/lon",
"No geometry coordinate reference system supplied; using cube coord_system instead.",
category=IrisDefaultingWarning,
)
target_system = DEFAULT_CS
if geometry_system is None:
geometry_system = DEFAULT_CS
target_proj = target_system.as_cartopy_projection()
source_proj = geometry_system.as_cartopy_projection()

trans_geometry = target_proj.project_geometry(geometry, source_proj)

# A GeogCS in iris can be either -180 to 180 or 0 to 360. If cube is 0-360, shift geom to match
if (
isinstance(target_system, iris.coord_systems.GeogCS)
Expand All @@ -165,16 +201,6 @@ def _transform_coord_system(geometry, cube, geometry_system=None):
trans_geometry = trans_geometry.difference(prime_meridian_line.buffer(0.00001))
trans_geometry = shapely.transform(trans_geometry, _trans_func)

if (not isinstance(target_system, iris.coord_systems.GeogCS)) and cube.coord(
x_name
).points[-1] > 180:
# this may lead to incorrect masking or not depending on projection type so warn user
warnings.warn(
"""Cube has x-coordinates over 180E and a non-standard projection type.\n
This may lead to incorrect masking. \n
If the result is not as expected, you might want to transform the x coordinate points of your cube to -180-180 """,
category=IrisUserWarning,
)
return trans_geometry


Expand Down
30 changes: 25 additions & 5 deletions lib/iris/util.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2122,9 +2122,17 @@ def _strip_metadata_from_dims(cube, dims):
return reduced_cube


def mask_cube_from_shapefile(cube, shape, minimum_weight=0.0, in_place=False):
def mask_cube_from_shapefile(
cube: iris.cube.Cube,
shape: shapely.Geometry,
shape_crs: cartopy.crs = None,
minimum_weight: float = 0.0,
in_place: bool = False,
):
"""Take a shape object and masks all points not touching it in a cube.

This function allows masking a cube with any shape object,
(e.g. Natural Earth Shapefiles via cartopy).
Finds the overlap between the `shape` and the `cube` in 2D xy space and
masks out any cells with less % overlap with shape than set.
Default behaviour is to count any overlap between shape and cell as valid
Expand All @@ -2133,10 +2141,12 @@ def mask_cube_from_shapefile(cube, shape, minimum_weight=0.0, in_place=False):
----------
cube : :class:`~iris.cube.Cube` object
The `Cube` object to masked. Must be singular, rather than a `CubeList`.
shape : Shapely.Geometry object
shape : shapely.Geometry object
A single `shape` of the area to remain unmasked on the `cube`.
If it a line object of some kind then minimum_weight will be ignored,
because you cannot compare the area of a 1D line and 2D Cell.
shape_crs : cartopy.crs.CRS, default=None
The coordinate reference system of the shape object.
minimum_weight : float , default=0.0
A number between 0-1 describing what % of a cube cell area must
the shape overlap to include it.
Expand All @@ -2156,8 +2166,6 @@ def mask_cube_from_shapefile(cube, shape, minimum_weight=0.0, in_place=False):

Notes
-----
This function allows masking a cube with any cartopy projection by a shape object,
most commonly from Natural Earth Shapefiles via cartopy.
To mask a cube from a shapefile, both must first be on the same coordinate system.
Shapefiles are mostly on a lat/lon grid with a projection very similar to GeogCS
The shapefile is projected to the coord system of the cube using cartopy, then each cell
Expand All @@ -2174,8 +2182,20 @@ def mask_cube_from_shapefile(cube, shape, minimum_weight=0.0, in_place=False):
>>> shape = shapely.geometry.box(-100,30, -80,40) # box between 30N-40N 100W-80W
>>> masked_cube = mask_cube_from_shapefile(cube, shape)

Warning
-------
For best masking results, both the cube _and_ masking geometry should have a
coordinate reference system (CRS) defined. Masking results will be most reliable
when the cube and masking geometry have the same CRS.

If the cube has no coord_system, the default GeogCS is used where
the coordinate units are degrees. For any other coordinate units,
the cube **must** have a coord_system defined.

If a CRS is not provided for the the masking geometry, the CRS of the cube is assumed.

"""
shapefile_mask = create_shapefile_mask(shape, cube, minimum_weight)
shapefile_mask = create_shapefile_mask(shape, cube, minimum_weight, shape_crs)
masked_cube = mask_cube(cube, shapefile_mask, in_place=in_place)
if not in_place:
return masked_cube
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