Update tile/image based on extent using Cartopy and Matplotlib FuncAnimation

Question

I want to create an animation of a trajectory using Cartopy and FuncAnimation. I have the trajectory stored in a GeoDataFrame with latitude, longitude and date time columns, which I want to visualise.

The trajectory moves across the globe, but I want to zoom in and therefore aim to have the basemap to be updated depending on the extent. I want to use satellite imagery as basemap, and I am using GoogleTiles from cartopy.io.img_tiles.

However, the image seems to only load at the initial extent, and is not updated in the new frames. The same happens when using 'stock_img()'. Interestingly, features such as coastlines and borders do load. To visualise my problem, view the images attached.

My function:

# Function to animate with moving basemaps
def plot_animation_moving(gdf, zoom_level = 8, extent_margin = 4, tail_length = 50, frames_between_points = 10, interval=100):

   tiler = GoogleTiles(style="satellite")
   mercator = tiler.crs

   # Set up the figure and axis
   fig, ax = plt.subplots(figsize=(12, 12), subplot_kw={'projection': mercator})
   
   # Initial extent
   initial_extent = [gdf.iloc[0]['lon']-extent_margin, gdf.iloc[0]['lon']+extent_margin, gdf.iloc[0]['lat']-extent_margin, gdf.iloc[0]['lat']+extent_margin]
   ax.set_extent(initial_extent)

   # Add background and features
   ax.add_image(tiler, zoom_level)
   # ax.stock_img()
   ax.add_feature(cfeature.BORDERS, linestyle=':')
   ax.coastlines(resolution='10m')
   ax.add_feature(cfeature.LAND)
   ax.add_feature(cfeature.OCEAN)
   ax.add_feature(cfeature.LAKES, alpha=0.5)
   ax.add_feature(cfeature.RIVERS)

   # Loop through each pair of points and create points in between
   lats_list = []
   lons_list = []
   dates_list = []
   for i in range(len(gdf)):
       
       # create lat, lon and date sequences
       ...
       lats_list.append(lats)
       lons_list.append(lons)
       dates_list.append(dates)

   # Concatenate points
   lats = np.concatenate(lats_list)
   lons = np.concatenate(lons_list)
   dates = np.concatenate(dates_list)

   # Create a point with tail object on the Basemap
   point, = ...
   tail, = ...
   date_text = ...

   # Update the point position
   def update(frame):

       # Update point, tail and date
       point.set_data([lons[frame]], [lats[frame]])
       current_date = pd.to_datetime(dates[frame])
       date_text.set_text(f'{current_date.strftime("%Y-%m-%d %H:00")}')

       # Update the map extent to follow the line
       ax.set_extent([lons[frame] - extent_margin, lons[frame] + extent_margin, lats[frame] - extent_margin, lats[frame] + extent_margin])
       
       return point, tail, date_text
   
   # Create the animation
   ani = FuncAnimation(fig, update, frames=tqdm.tqdm(range(len(lats)), file=sys.stdout), blit=False, interval=interval),
   ax.add_image(tiler, zoom_level) # --> doesn't help

   ani.save("example.mp4")

In the update function, I have tried adding the image again using ax.add_image(tiler, zoom_level) but it doesn't seem to help.

My question therefore is: how do I correctly update the basemap image so that it loads when the point is moving across the globe?

Answer 1

You could be sneaky and go one level deeper by passing the tiler to imshow at each frame :

def add_image(factory, *factory_args, **factory_kwargs):
    img, extent, origin = factory.image_for_domain(
        ax._get_extent_geom(factory.crs),
        factory_args[0],
    )
    ax.imshow(
        img,
        extent=extent,
        origin=origin,
        transform=factory.crs,
        *factory_args[1:],
        **factory_kwargs
    )

NB: You need to place add_image(tiler, zoom_level) right before update's return.