Plotting a map using geopandas and matplotlib

Question

I have small csv that has 6 coordinates from Birmingham England. I read the csv with pandas then transformed it into GeoPandas DataFrame changing my latitude and longitude colu

Answer 1

Try df.unary_union. The function will aggregate points into a single geometry. Jupyter Notebook can plot it

Answer 2

The GeoPandas documentation contains an example on how to add a background to a map (https://geopandas.readthedocs.io/en/latest/gallery/plotting_basemap_background.html), which is explained in more detail below.

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You will have to deal with tiles, that are (png) images served through a web server, with a URL like

http://.../Z/X/Y.png, where Z is the zoom level, and X and Y identify the tile

And geopandas's doc shows how to set tiles as backgrounds for your plots, fetching the correct ones and doing all the otherwise difficult job of spatial syncing, etc...

---

Installation

Assuming GeoPandas is already installed, you need the contextily package in addition. If you are under windows, you may want to pick a look at How to install Contextily?

Use case

Create a python script and define the contextily helper function

import contextily as ctx

def add_basemap(ax, zoom, url='http://tile.stamen.com/terrain/tileZ/tileX/tileY.png'):
    xmin, xmax, ymin, ymax = ax.axis()
    basemap, extent = ctx.bounds2img(xmin, ymin, xmax, ymax, zoom=zoom, url=url)
    ax.imshow(basemap, extent=extent, interpolation='bilinear')
    # restore original x/y limits
    ax.axis((xmin, xmax, ymin, ymax))

and play

import matplotlib.pyplot as plt
from shapely.geometry import Point
import geopandas as gpd
import pandas as pd

# Let's define our raw data, whose epsg is 4326
df = pd.DataFrame({
    'LAT'  :[-22.266415, -20.684157],
    'LONG' :[166.452764, 164.956089],
})
df['coords'] = list(zip(df.LONG, df.LAT))

# ... turn them into geodataframe, and convert our
# epsg into 3857, since web map tiles are typically
# provided as such.
geo_df = gpd.GeoDataFrame(
    df, crs  ={'init': 'epsg:4326'},
    geometry = df['coords'].apply(Point)
).to_crs(epsg=3857)

# ... and make the plot
ax = geo_df.plot(
    figsize= (5, 5),
    alpha  = 1
)
add_basemap(ax, zoom=10)
ax.set_axis_off()
plt.title('Kaledonia : From Hienghène to Nouméa')
plt.show()

---

Note: you can play with the zoom to find the good resolution for the map. E.g./I.e. :

... and such resolutions implicitly call for changing the x/y limits.

Answer 3

Just want to add the use case concerning zooming whereby the basemap is updated according to the new xlim and ylim coordinates. A solution I have come up with is:

• First set callbacks on the ax that can detect xlim_changed and ylim_changed
• Once both have been detected as changed get the new plot_area calling ax.get_xlim() and ax.get_ylim()
• Then clear the ax and re-plot the basemap and any other data

Example for a world map showing the capitals. You notice when you zoom in the resolution of the map is being updated.

import geopandas as gpd
import matplotlib.pyplot as plt
import contextily as ctx


figsize = (12, 10)
osm_url = 'http://tile.stamen.com/terrain/{z}/{x}/{y}.png'
EPSG_OSM = 3857
EPSG_WGS84 = 4326

class MapTools:
    def __init__(self):
        self.cities = gpd.read_file(
            gpd.datasets.get_path('naturalearth_cities'))
        self.cities.crs = EPSG_WGS84
        self.cities = self.convert_to_osm(self.cities)

        self.fig, self.ax = plt.subplots(nrows=1, ncols=1, figsize=figsize)
        self.callbacks_connect()

        # get extent of the map for all cities
        self.cities.plot(ax=self.ax)
        self.plot_area = self.ax.axis()

    def convert_to_osm(self, df):
        return df.to_crs(epsg=EPSG_OSM)

    def callbacks_connect(self):
        self.zoomcallx = self.ax.callbacks.connect(
            'xlim_changed', self.on_limx_change)
        self.zoomcally = self.ax.callbacks.connect(
            'ylim_changed', self.on_limy_change)

        self.x_called = False
        self.y_called = False

    def callbacks_disconnect(self):
        self.ax.callbacks.disconnect(self.zoomcallx)
        self.ax.callbacks.disconnect(self.zoomcally)

    def on_limx_change(self, _):
        self.x_called = True
        if self.y_called:
            self.on_lim_change()

    def on_limy_change(self, _):
        self.y_called = True
        if self.x_called:
            self.on_lim_change()

    def on_lim_change(self):
        xlim = self.ax.get_xlim()
        ylim = self.ax.get_ylim()
        self.plot_area = (*xlim, *ylim)
        self.blit_map()

    def add_base_map_osm(self):
        if abs(self.plot_area[1] - self.plot_area[0]) < 100:
            zoom = 13

        else:
            zoom = 'auto'

        try:
            basemap, extent = ctx.bounds2img(
                self.plot_area[0], self.plot_area[2],
                self.plot_area[1], self.plot_area[3],
                zoom=zoom,
                url=osm_url,)
            self.ax.imshow(basemap, extent=extent, interpolation='bilinear')

        except Exception as e:
            print(f'unable to load map: {e}')

    def blit_map(self):
        self.ax.cla()
        self.callbacks_disconnect()
        cities = self.cities.cx[
            self.plot_area[0]:self.plot_area[1],
            self.plot_area[2]:self.plot_area[3]]
        cities.plot(ax=self.ax, color='red', markersize=3)

        print('*'*80)
        print(self.plot_area)
        print(f'{len(cities)} cities in plot area')

        self.add_base_map_osm()
        self.callbacks_connect()

    @staticmethod
    def show():
        plt.show()


def main():
    map_tools = MapTools()
    map_tools.show()

if __name__ == '__main__':
    main()

Runs on Linux Python3.8 with following pip installs

affine==2.3.0
attrs==19.3.0
autopep8==1.4.4
Cartopy==0.17.0
certifi==2019.11.28
chardet==3.0.4
Click==7.0
click-plugins==1.1.1
cligj==0.5.0
contextily==1.0rc2
cycler==0.10.0
descartes==1.1.0
Fiona==1.8.11
geographiclib==1.50
geopandas==0.6.2
geopy==1.20.0
idna==2.8
joblib==0.14.0
kiwisolver==1.1.0
matplotlib==3.1.2
mercantile==1.1.2
more-itertools==8.0.0
munch==2.5.0
numpy==1.17.4
packaging==19.2
pandas==0.25.3
Pillow==6.2.1
pluggy==0.13.1
py==1.8.0
pycodestyle==2.5.0
pyparsing==2.4.5
pyproj==2.4.1
pyshp==2.1.0
pytest==5.3.1
python-dateutil==2.8.1
pytz==2019.3
rasterio==1.1.1
requests==2.22.0
Rtree==0.9.1
Shapely==1.6.4.post2
six==1.13.0
snuggs==1.4.7
urllib3==1.25.7
wcwidth==0.1.7

Note especially requirement for contextily==1.0rc2

On windows I use Conda (P3.7.3) and don't forget to set the User variables:

GDAL c:\Users\<username>\Anaconda3\envs\<your environment>\Library\share\gdal

PROJLIB c:\Users\<username>\Anaconda3\envs\<your environment>\Library\share