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FIX: Change PlateCarree vector handling #1920

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FIX: Change PlateCarree vector handling #1920

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37 changes: 35 additions & 2 deletions lib/cartopy/crs.py
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Original file line number Diff line number Diff line change
Expand Up @@ -469,7 +469,23 @@ def transform_vectors(self, src_proj, x, y, u, v):
# Rotate the input vectors to the projection.
#
# 1: Find the magnitude and direction of the input vectors.
vector_magnitudes = (u**2 + v**2)**0.5
vector_magnitudes = np.sqrt(u**2 + v**2)
if isinstance(src_proj, PlateCarree):
# This means we were given lon/lat coordinates and
# east/north vector components
# When projecting the components, we need to account for the
# fact that we are using PlateCarree to represent our vectors
# and scale the u component for the transformation to maintain
# the proper angles
scale_factor = np.cos(y / 180 * np.pi)
# Force these values due to floating point precision near
# the poles with cos()
pole_points = np.isclose(np.abs(y), 90)
if np.any(pole_points):
warnings.warn('Vector transforms near the pole '
'may not have been transformed correctly')
scale_factor[pole_points] = np.inf
u = u / scale_factor
vector_angles = np.arctan2(v, u)
# 2: Find a point in the direction of the original vector that is
# a small distance away from the base point of the vector (near
Expand Down Expand Up @@ -530,8 +546,25 @@ def transform_vectors(self, src_proj, x, y, u, v):
source_y + y_perturbations)
target_x_perturbed = proj_xyz[..., 0]
target_y_perturbed = proj_xyz[..., 1]

x_delta = target_x_perturbed - target_x
if isinstance(self, PlateCarree):
# Account for scaling to the North/South components in the
# opposite manner as the source handling
# (i.e. inverse scale factor), with target_y being
# the latitudes we are going to
scale_factor = np.cos(target_y / 180 * np.pi)
# Force these values due to floating point precision near
# the poles with cos()
pole_points = np.isclose(np.abs(target_y), 90)
if np.any(pole_points):
warnings.warn('Vector transforms near the pole '
'may not have been transformed correctly')
scale_factor[pole_points] = np.inf
scale_factor[pole_points] = np.inf
x_delta = x_delta * scale_factor
projected_angles = np.arctan2(target_y_perturbed - target_y,
target_x_perturbed - target_x)
x_delta)
if reversed_vectors.any():
projected_angles[reversed_vectors] += np.pi
# 5: Form the projected vector components, preserving the magnitude
Expand Down
8 changes: 4 additions & 4 deletions lib/cartopy/tests/test_vector_transform.py
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Original file line number Diff line number Diff line change
Expand Up @@ -144,11 +144,11 @@ def test_with_transform(self):
[7.5, 7.5, 7.5, 7.5, 7.5],
[10., 10., 10., 10., 10]])
expected_u_grid = np.array([[np.nan, np.nan, np.nan, np.nan, np.nan],
[np.nan, 2.3838, 3.5025, 2.6152, np.nan],
[2, 3.0043, 4, 2.9022, 2]])
[np.nan, 2.3893, 3.5097, 2.6194, np.nan],
[2, 3.0005, 4, 2.8977, 2]])
expected_v_grid = np.array([[np.nan, np.nan, np.nan, np.nan, np.nan],
[np.nan, 2.6527, 2.1904, 2.4192, np.nan],
[5.5, 4.6483, 4, 4.47, 5.5]])
[np.nan, 2.6486, 2.1878, 2.4138, np.nan],
[5.5, 4.6497, 4, 4.4702, 5.5]])

x_grid, y_grid, u_grid, v_grid = vec_trans.vector_scalar_to_grid(
src_crs, target_crs, (5, 3), x_nps, y_nps, u_nps, v_nps)
Expand Down