user_area_centre = { longitude: centre_x, latitude: centre_y }
      tile_bounds   = { south: latitude_min, west: longitude_min, north: latitude_max, east: longitude_max }

      image = static_satellite_img
      draw_circle(image, tile_bounds, user_area_centre)

  def draw_circle(image, bounds, user_area_centre, circle_line_width = 3, circle_colour = 'red', circle_size = 0.1)
    # Approximate where the centre point is in the image.
    width   = (bounds[:east] - bounds[:west]).abs
    x_scale = image.columns / width

    height  = (bounds[:south] - bounds[:north]).abs
    y_scale = image.rows / height

    x = (centre[:longitude] - bounds[:west]) * x_scale
    y = (centre[:latitude] - bounds[:south]) * y_scale

    circle = Magick::Draw.new
    circle.stroke(circle_colour)
    circle.fill('transparent')
    circle.stroke_width(circle_line_width)
    circle.ellipse(x, image.rows - y, image.columns * circle_size, image.columns * circle_size, 0, 360)
    circle.draw(image)
  end