{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import hvplot.xarray  # noqa"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`vectorfield` accepts 2d arrays of magnitude and angle on a grid and produces an array of vectors. x and y can be 2d or 1d coordinates. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import numpy as np\n",
    "import xarray as xr\n",
    "import geoviews as gv\n",
    "import cartopy.crs as ccrs"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def sample_data(shape=(20, 30)):\n",
    "    \"\"\"\n",
    "    Return ``(x, y, u, v, crs)`` of some vector data\n",
    "    computed mathematically. The returned crs will be a rotated\n",
    "    pole CRS, meaning that the vectors will be unevenly spaced in\n",
    "    regular PlateCarree space.\n",
    "\n",
    "    \"\"\"\n",
    "    crs = ccrs.RotatedPole(pole_longitude=177.5, pole_latitude=37.5)\n",
    "\n",
    "    x = np.linspace(311.9, 391.1, shape[1])\n",
    "    y = np.linspace(-23.6, 24.8, shape[0])\n",
    "\n",
    "    x2d, y2d = np.meshgrid(x, y)\n",
    "    u = 10 * (2 * np.cos(2 * np.deg2rad(x2d) + 3 * np.deg2rad(y2d + 30)) ** 2)\n",
    "    v = 20 * np.cos(6 * np.deg2rad(x2d))\n",
    "\n",
    "    return x, y, u, v, crs\n",
    "\n",
    "xs, ys, U, V, crs = sample_data()\n",
    "\n",
    "mag = np.sqrt(U**2 + V**2)\n",
    "angle = (np.pi/2.) - np.arctan2(U/mag, V/mag)\n",
    "\n",
    "ds = xr.Dataset({'mag': xr.DataArray(mag, dims=('y', 'x'), coords={'y': ys, 'x': xs}),\n",
    "                 'angle': xr.DataArray(angle, dims=('y', 'x'), coords={'y': ys, 'x': xs})}, \n",
    "                attrs={'crs': crs})\n",
    "ds"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "ds.hvplot.vectorfield(x='x', y='y', angle='angle', mag='mag', hover=False).opts(magnitude='mag')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Geographic Data\n",
    "If a dataset has an attr called `crs` which is a cartopy object or a proj4 string, then just by setting the option `geo=True` will use the correct crs."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "ds.hvplot.vectorfield(x='x', y='y', angle='angle', mag='mag',\n",
    "                      hover=False, geo=True).opts(magnitude='mag') * gv.tile_sources.CartoLight"
   ]
  }
 ],
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  "language_info": {
   "name": "python",
   "pygments_lexer": "ipython3"
  }
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 "nbformat_minor": 2
}