Added extra example demonstrating use of both crs keys.

src/geovista/cache/registry.txt

@@ -74,6 +74,7 @@ tests/images/examples.test__unstructured.tri_hammer.png 55a063f42722665ae3575d70
 tests/images/examples.test__unstructured.tri.png 17706b23041a027bd52be9950a86703a59b8f1282a82790ee7686832d0bc4196
 tests/images/examples.test__vector_data.flow_directions.png 26cea271234f57b721da235b0d1b9b6d35927003621ac3f4750cd8b52e99134e
 tests/images/examples.test__vector_data.vectors.png 72f1a30dd57b8c6cc635e866aa1ffd79bdfc10dd75cbf5ea8588f74781f0c490
+tests/images/examples.test__vector_data.true_winds.png b160ff21770e81514089fd1a58ed3bee137734aa78d830f1d7e3c522267f9d93
 tests/images/examples.test__vector_data.wind_arrows.png e73f77f2cb804e7d629be36ef8b0fcef1d64d653db005d29d2a2188575734809
 tests/images/examples.test__vector_data.winds_3d.png eb08f0d00a89d3429ab4e541d1b6c791d569d6bc6e359bf98d43f2937dff6753
 tests/images/examples.test__warp.from_unstructured__fvcom.png b31dba987a706601c60df6ab9c39cf03e83ed6d07acf84d2d10a798702762f08

src/geovista/examples/vector_data/true_winds.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+# Copyright (c) 2021, GeoVista Contributors.
+#
+# This file is part of GeoVista and is distributed under the 3-Clause BSD license.
+# See the LICENSE file in the package root directory for licensing details.
+
+"""
+"True" Winds on a Foreign Grid
+------------------------------
+
+This example demonstrates how to display "true" winds, when attached to data points
+given in a non-standard coordinate system.
+
+📋 Summary
+^^^^^^^^^^
+
+The example constructs some sample points on a polar stereographic grid, and assigns
+synthetic vector values to them, representing wind data.  These data are presented as
+"true" winds, i.e. northward and eastward components, so *not* related to the sample
+grid.  This is a fairly typical real-world case.
+
+In this case, the synthetic wind is a steady 4.0 ms-1 Eastward everywhere, so the
+meaning is clearly seen.
+
+We use the :meth:`geovista.bridge.Transform.from_points` method, passing the
+winds to the ``vectors`` keyword, and specifying the different coordinate reference
+systems of both the sample points (``crs``) and the vectors (``vectors_crs``).
+
+Please refer to other code examples, e.g. `wind_arrows <./wind_arrows.html>`_ for more
+details of the basic methods used.
+
+"""  # noqa: D205,D212,D400
+
+from __future__ import annotations
+
+# Use cartopy as a source for specifying Coordinate Reference Systems
+#  (which we can translate into PROJ via the .to_wkt() method)
+import cartopy.crs as ccrs
+import numpy as np
+
+import geovista as gv
+
+
+def main() -> None:
+    """Demonstrate plotting vectors on different CRS from points."""
+    # Create a mesh of individual points, adding vectors at each point.
+    # NOTE: this creates a mesh with 'mesh vectors' : a specific concept in PyVista.
+
+    # Create a CRS for "ordinary latitude-longitude"
+    crs_latlon = ccrs.Geodetic().to_wkt()
+    # Create a CRS for "polar stereographic"
+    crs_polar = ccrs.NorthPolarStereo().to_wkt()
+    # Create polar grid points
+    xx = np.linspace(-5.0e6, 5.0e6, 20)
+    yy = np.linspace(-4.0e6, 2.0e6, 20)
+    xx, yy = np.meshgrid(xx, yy)
+    # Make vector component arrays matching the XX and YY arrays
+    xx, yy = xx.flatten(), yy.flatten()
+    vx, vy = 4.0 * np.ones_like(xx), np.zeros_like(yy)
+
+    # Create a mesh of location points with attached vector information
+    mesh = gv.Transform.from_points(
+        xx, yy, vectors=(vx, vy), crs=crs_polar, vectors_crs=crs_latlon
+    )
+
+    # Create a new mesh containing arrow glyphs, from the mesh vectors.
+    arrows = mesh.glyph(factor=0.02)
+
+    # Add the arrows to a Plotter with other aspects, and display
+    p = gv.GeoPlotter()
+    # Scale the base layer slightly to ensure it remains 'below' other elements.
+    p.add_base_layer(radius=0.99)
+    p.add_mesh(arrows, color="red")
+    p.add_coastlines()
+    p.add_graticule()
+    p.add_axes()
+
+    # Set up a nice default view
+    selected_view = [
+        (0.6037050543418041, -0.011033743353827528, 3.069575190259155),
+        (0.0, 0.0, 0.0028896447726441954),
+        (-0.9809865744261353, -0.019981215106321615, 0.19304427429621296),
+    ]
+    p.camera_position = selected_view
+    p.show()
+
+
+if __name__ == "__main__":
+    main()