How can I show a km ruler on a cartopy / matplotlib plot?

Question

How can I show a km ruler for a zoomed in section of a map, either inset in the image or as rulers on the side of the plot?

E.g. something like the 50 km bar on the

Answer 1

With the addition of the geodesic module in CartoPy 0.15, we can now fairly easily compute exact lengths on a map. It was a bit tricky to figure out how to find two points on a straight-on-a-map line which are the right distance-on-a-sphere apart. Once the direction on the map is specified, I perform an exponential search to find a point far enough away. I then perform a binary search to find a point close enough to the desired distance.

The scale_bar function is simple enough, but it has a lot of options. The basic signature is scale_bar(ax, location, length). ax is any CartoPy axes, location is the position of the left-side of the bar in axes coordinates (so each coordinate is from 0 to 1), and length is the length of the bar in kilometres. Other lengths are supported like with the metres_per_unit and unit_name keyword arguments.

Extra keyword arguments (like color) are simply passed to text and plot. However, keyword arguments specific to text or plot (like family or path_effects) must be passed in as dictionaries through text_kwargs and plot_kwargs.

I've included examples of what I think are the common use cases.

Please share any questions, comments, or criticisms.

scalebar.py

import numpy as np
import cartopy.crs as ccrs
import cartopy.geodesic as cgeo


def _axes_to_lonlat(ax, coords):
    """(lon, lat) from axes coordinates."""
    display = ax.transAxes.transform(coords)
    data = ax.transData.inverted().transform(display)
    lonlat = ccrs.PlateCarree().transform_point(*data, ax.projection)

    return lonlat


def _upper_bound(start, direction, distance, dist_func):
    """A point farther than distance from start, in the given direction.

    It doesn't matter which coordinate system start is given in, as long
    as dist_func takes points in that coordinate system.

    Args:
        start:     Starting point for the line.
        direction  Nonzero (2, 1)-shaped array, a direction vector.
        distance:  Positive distance to go past.
        dist_func: A two-argument function which returns distance.

    Returns:
        Coordinates of a point (a (2, 1)-shaped NumPy array).
    """
    if distance <= 0:
        raise ValueError(f"Minimum distance is not positive: {distance}")

    if np.linalg.norm(direction) == 0:
        raise ValueError("Direction vector must not be zero.")

    # Exponential search until the distance between start and end is
    # greater than the given limit.
    length = 0.1
    end = start + length * direction

    while dist_func(start, end) < distance:
        length *= 2
        end = start + length * direction

    return end


def _distance_along_line(start, end, distance, dist_func, tol):
    """Point at a distance from start on the segment  from start to end.

    It doesn't matter which coordinate system start is given in, as long
    as dist_func takes points in that coordinate system.

    Args:
        start:     Starting point for the line.
        end:       Outer bound on point's location.
        distance:  Positive distance to travel.
        dist_func: Two-argument function which returns distance.
        tol:       Relative error in distance to allow.

    Returns:
        Coordinates of a point (a (2, 1)-shaped NumPy array).
    """
    initial_distance = dist_func(start, end)
    if initial_distance < distance:
        raise ValueError(f"End is closer to start ({initial_distance}) than "
                         f"given distance ({distance}).")

    if tol <= 0:
        raise ValueError(f"Tolerance is not positive: {tol}")

    # Binary search for a point at the given distance.
    left = start
    right = end

    while not np.isclose(dist_func(start, right), distance, rtol=tol):
        midpoint = (left + right) / 2

        # If midpoint is too close, search in second half.
        if dist_func(start, midpoint) < distance:
            left = midpoint
        # Otherwise the midpoint is too far, so search in first half.
        else:
            right = midpoint

    return right


def _point_along_line(ax, start, distance, angle=0, tol=0.01):
    """Point at a given distance from start at a given angle.

    Args:
        ax:       CartoPy axes.
        start:    Starting point for the line in axes coordinates.
        distance: Positive physical distance to travel.
        angle:    Anti-clockwise angle for the bar, in radians. Default: 0
        tol:      Relative error in distance to allow. Default: 0.01

    Returns:
        Coordinates of a point (a (2, 1)-shaped NumPy array).
    """
    # Direction vector of the line in axes coordinates.
    direction = np.array([np.cos(angle), np.sin(angle)])

    geodesic = cgeo.Geodesic()

    # Physical distance between points.
    def dist_func(a_axes, b_axes):
        a_phys = _axes_to_lonlat(ax, a_axes)
        b_phys = _axes_to_lonlat(ax, b_axes)

        # Geodesic().inverse returns a NumPy MemoryView like [[distance,
        # start azimuth, end azimuth]].
        return geodesic.inverse(a_phys, b_phys).base[0, 0]

    end = _upper_bound(start, direction, distance, dist_func)

    return _distance_along_line(start, end, distance, dist_func, tol)


def scale_bar(ax, location, length, metres_per_unit=1000, unit_name='km',
              tol=0.01, angle=0, color='black', linewidth=3, text_offset=0.005,
              ha='center', va='bottom', plot_kwargs=None, text_kwargs=None,
              **kwargs):
    """Add a scale bar to CartoPy axes.

    For angles between 0 and 90 the text and line may be plotted at
    slightly different angles for unknown reasons. To work around this,
    override the 'rotation' keyword argument with text_kwargs.

    Args:
        ax:              CartoPy axes.
        location:        Position of left-side of bar in axes coordinates.
        length:          Geodesic length of the scale bar.
        metres_per_unit: Number of metres in the given unit. Default: 1000
        unit_name:       Name of the given unit. Default: 'km'
        tol:             Allowed relative error in length of bar. Default: 0.01
        angle:           Anti-clockwise rotation of the bar.
        color:           Color of the bar and text. Default: 'black'
        linewidth:       Same argument as for plot.
        text_offset:     Perpendicular offset for text in axes coordinates.
                         Default: 0.005
        ha:              Horizontal alignment. Default: 'center'
        va:              Vertical alignment. Default: 'bottom'
        **plot_kwargs:   Keyword arguments for plot, overridden by **kwargs.
        **text_kwargs:   Keyword arguments for text, overridden by **kwargs.
        **kwargs:        Keyword arguments for both plot and text.
    """
    # Setup kwargs, update plot_kwargs and text_kwargs.
    if plot_kwargs is None:
        plot_kwargs = {}
    if text_kwargs is None:
        text_kwargs = {}

    plot_kwargs = {'linewidth': linewidth, 'color': color, **plot_kwargs,
                   **kwargs}
    text_kwargs = {'ha': ha, 'va': va, 'rotation': angle, 'color': color,
                   **text_kwargs, **kwargs}

    # Convert all units and types.
    location = np.asarray(location)  # For vector addition.
    length_metres = length * metres_per_unit
    angle_rad = angle * np.pi / 180

    # End-point of bar.
    end = _point_along_line(ax, location, length_metres, angle=angle_rad,
                            tol=tol)

    # Coordinates are currently in axes coordinates, so use transAxes to
    # put into data coordinates. *zip(a, b) produces a list of x-coords,
    # then a list of y-coords.
    ax.plot(*zip(location, end), transform=ax.transAxes, **plot_kwargs)

    # Push text away from bar in the perpendicular direction.
    midpoint = (location + end) / 2
    offset = text_offset * np.array([-np.sin(angle_rad), np.cos(angle_rad)])
    text_location = midpoint + offset

    # 'rotation' keyword argument is in text_kwargs.
    ax.text(*text_location, f"{length} {unit_name}", rotation_mode='anchor',
            transform=ax.transAxes, **text_kwargs)

demo.py

import cartopy.crs as ccrs
import cartopy.feature as cfeature
import matplotlib.pyplot as plt
from scalebar import scale_bar

fig = plt.figure(1, figsize=(10, 10))
ax = fig.add_subplot(111, projection=ccrs.Mercator())
ax.set_extent([-180, 180, -85, 85])
ax.coastlines(facecolor='black')
ax.add_feature(cfeature.LAND)

# Standard 6,000 km scale bar.
scale_bar(ax, (0.65, 0.4), 6_000)

# Length of the bar reflects its position on the map.
scale_bar(ax, (0.55, 0.7), 6_000, color='green')

# Bar can be placed at any angle. Any units can be used.
scale_bar(ax, (0.4, 0.4), 3_000, metres_per_unit=1609, angle=-90,
          unit_name='mi', color='red')
# Text and line can be styled separately. Keywords are simply passed to
# text or plot.
text_kwargs = dict(family='serif', size='xx-large', color='red')
plot_kwargs = dict(linestyle='dashed', color='blue')
scale_bar(ax, (0.05, 0.3), 6_000, text_kwargs=text_kwargs,
          plot_kwargs=plot_kwargs)

# Angles between 0 and 90 may result in the text and line plotted at
# slightly different angles for an unknown reason.
scale_bar(ax, (0.45, 0.15), 5_000, color='purple', angle=45, text_offset=0)

# To get around this override the text's angle and fiddle manually.
scale_bar(ax, (0.55, 0.15), 5_000, color='orange', angle=45, text_offset=0,
          text_kwargs={'rotation': 41})

plt.show()