Mapping in Python

Co-author

• Kim Ruhl University of Wisconsin

Prerequisites

• matplotlib Introduction

• Visualization Rules

Outcomes

• Use geopandas to create maps

# Uncomment following line to install on colab
#! pip install fiona geopandas xgboost gensim folium pyLDAvis descartes

import geopandas as gpd
import matplotlib.pyplot as plt
import pandas as pd

from shapely.geometry import Point

%matplotlib inline

Mapping in Python

In this lecture, we will use a new package, geopandas, to create maps.

Maps are really quite complicated… We are trying to project a spherical surface onto a flat figure, which is an inherently complicated endeavor.

Luckily, geopandas will do most of the heavy lifting for us.

Let’s start with a DataFrame that has the latitude and longitude coordinates of various South American cities.

Our goal is to turn them into something we can plot – in this case, a GeoDataFrame.

df = pd.DataFrame({
    'City': ['Buenos Aires', 'Brasilia', 'Santiago', 'Bogota', 'Caracas'],
    'Country': ['Argentina', 'Brazil', 'Chile', 'Colombia', 'Venezuela'],
    'Latitude': [-34.58, -15.78, -33.45, 4.60, 10.48],
    'Longitude': [-58.66, -47.91, -70.66, -74.08, -66.86]
})

In order to map the cities, we need tuples of coordinates.

We generate them by zipping the latitude and longitude together to store them in a new column named Coordinates.

df["Coordinates"] = list(zip(df.Longitude, df.Latitude))
df.head()

	City	Country	Latitude	Longitude	Coordinates
0	Buenos Aires	Argentina	-34.58	-58.66	(-58.66, -34.58)
1	Brasilia	Brazil	-15.78	-47.91	(-47.91, -15.78)
2	Santiago	Chile	-33.45	-70.66	(-70.66, -33.45)
3	Bogota	Colombia	4.60	-74.08	(-74.08, 4.6)
4	Caracas	Venezuela	10.48	-66.86	(-66.86, 10.48)

Our next step is to turn the tuple into a Shapely Point object.

We will do this by applying Shapely’s Point method to the Coordinates column.

df["Coordinates"] = df["Coordinates"].apply(Point)
df.head()

	City	Country	Latitude	Longitude	Coordinates
0	Buenos Aires	Argentina	-34.58	-58.66	POINT (-58.66 -34.58)
1	Brasilia	Brazil	-15.78	-47.91	POINT (-47.91 -15.78)
2	Santiago	Chile	-33.45	-70.66	POINT (-70.66 -33.45)
3	Bogota	Colombia	4.60	-74.08	POINT (-74.08 4.6)
4	Caracas	Venezuela	10.48	-66.86	POINT (-66.86 10.48)

Finally, we will convert our DataFrame into a GeoDataFrame by calling the geopandas.DataFrame method.

Conveniently, a GeoDataFrame is a data structure with the convenience of a normal DataFrame but also an understanding of how to plot maps.

In the code below, we must specify the column that contains the geometry data.

See this excerpt from the docs

The most important property of a GeoDataFrame is that it always has one GeoSeries column that holds a special status. This GeoSeries is referred to as the GeoDataFrame’s “geometry”. When a spatial method is applied to a GeoDataFrame (or a spatial attribute like area is called), this commands will always act on the “geometry” column.

gdf = gpd.GeoDataFrame(df, geometry="Coordinates")
gdf.head()

	City	Country	Latitude	Longitude	Coordinates
0	Buenos Aires	Argentina	-34.58	-58.66	POINT (-58.66000 -34.58000)
1	Brasilia	Brazil	-15.78	-47.91	POINT (-47.91000 -15.78000)
2	Santiago	Chile	-33.45	-70.66	POINT (-70.66000 -33.45000)
3	Bogota	Colombia	4.60	-74.08	POINT (-74.08000 4.60000)
4	Caracas	Venezuela	10.48	-66.86	POINT (-66.86000 10.48000)

# Doesn't look different than a vanilla DataFrame...let's make sure we have what we want
print('gdf is of type:', type(gdf))

# And how can we tell which column is the geometry column?
print('\nThe geometry column is:', gdf.geometry.name)

gdf is of type: <class 'geopandas.geodataframe.GeoDataFrame'>

The geometry column is: Coordinates

Plotting a Map

Great, now we have our points in the GeoDataFrame.

Let’s plot the locations on a map.

This will require 3 steps

1. Get the map

2. Plot the map

3. Plot the points (our cities) on the map

1. Get the map

An organization called Natural Earth compiled the map data that we use here.

The file provides the outlines of countries, over which we’ll plot the city locations from our GeoDataFrame.

Luckily, geopandas already comes bundled with this data, so we don’t have to hunt it down!

# Grab low resolution world file
world = gpd.read_file(gpd.datasets.get_path("naturalearth_lowres"))
world = world.set_index("iso_a3")

world.head()

	pop_est	continent	name	gdp_md_est	geometry
iso_a3
FJI	889953.0	Oceania	Fiji	5496	MULTIPOLYGON (((180.00000 -16.06713, 180.00000...
TZA	58005463.0	Africa	Tanzania	63177	POLYGON ((33.90371 -0.95000, 34.07262 -1.05982...
ESH	603253.0	Africa	W. Sahara	907	POLYGON ((-8.66559 27.65643, -8.66512 27.58948...
CAN	37589262.0	North America	Canada	1736425	MULTIPOLYGON (((-122.84000 49.00000, -122.9742...
USA	328239523.0	North America	United States of America	21433226	MULTIPOLYGON (((-122.84000 49.00000, -120.0000...

world is a GeoDataFrame with the following columns:

• pop_est: Contains a population estimate for the country

• continent: The country’s continent

• name: The country’s name

• iso_a3: The country’s 3 letter abbreviation (we made this the index)

• gdp_md_est: An estimate of country’s GDP

• geometry: A POLYGON for each country (we will learn more about these soon)

world.geometry.name

'geometry'

Notice that the geometry for this GeoDataFrame is stored in the geometry column.

A quick note about polygons

Instead of points (as our cities are), the geometry objects are now polygons.

A polygon is what you already likely think it is – a collection of ordered points connected by straight lines.

The smaller the distance between the points, the more readily the polygon can approximate non-linear shapes.

Let’s see an example of a polygon.

world.loc["ALB", 'geometry']

Notice that it displayed the country of Albania.

# Returns two arrays that hold the x and y coordinates of the points that define the polygon's exterior.
x, y = world.loc["ALB", "geometry"].exterior.coords.xy

# How many points?
print('Points in the exterior of Albania:', len(x))

Points in the exterior of Albania: 24

Let’s see another

world.loc["AFG", "geometry"]

# Returns two arrays that hold the x and y coordinates of the points that define the polygon's exterior.
x, y = world.loc["AFG", 'geometry'].exterior.coords.xy

# How many points?
print('Points in the exterior of Afghanistan:', len(x))

Points in the exterior of Afghanistan: 69

Notice that we’ve now displayed Afghanistan.

This is a more complex shape than Albania and thus required more points.

2. Plotting the map

fig, gax = plt.subplots(figsize=(10,10))

# By only plotting rows in which the continent is 'South America' we only plot SA.
world.query("continent == 'South America'").plot(ax=gax, edgecolor='black',color='white')

# By the way, if you haven't read the book 'longitude' by Dava Sobel, you should...
gax.set_xlabel('longitude')
gax.set_ylabel('latitude')

gax.spines['top'].set_visible(False)
gax.spines['right'].set_visible(False)

plt.show()

Creating this map may have been easier than you expected!

In reality, a lot of heavy lifting is going on behind the scenes.

Entire university classes (and even majors!) focus on the theory and thought that goes into creating maps, but, for now, we are happy to rely on the work done by the experts behind geopandas and its related libraries.

3. Plot the cities

In the code below, we run the same commands as before to plot the South American countries, but , now, we also plot the data in gdf, which contains the location of South American cities.

# Step 3: Plot the cities onto the map
# We mostly use the code from before --- we still want the country borders plotted --- and we
# add a command to plot the cities
fig, gax = plt.subplots(figsize=(10,10))

# By only plotting rows in which the continent is 'South America' we only plot, well,
# South America.
world.query("continent == 'South America'").plot(ax = gax, edgecolor='black', color='white')

# This plot the cities. It's the same syntax, but we are plotting from a different GeoDataFrame.
# I want the cities as pale red dots.
gdf.plot(ax=gax, color='red', alpha = 0.5)

gax.set_xlabel('longitude')
gax.set_ylabel('latitude')
gax.set_title('South America')

gax.spines['top'].set_visible(False)
gax.spines['right'].set_visible(False)

plt.show()

Adding labels to points.

Finally, we might want to consider annotating the cities so we know which cities are which.

# Step 3: Plot the cities onto the map
# We mostly use the code from before --- we still want the country borders plotted --- and we add a command to plot the cities
fig, gax = plt.subplots(figsize=(10,10))

# By only plotting rows in which the continent is 'South America' we only plot, well, South America.
world.query("continent == 'South America'").plot(ax = gax, edgecolor='black', color='white')

# This plot the cities. It's the same syntax, but we are plotting from a different GeoDataFrame. I want the
# cities as pale red dots.
gdf.plot(ax=gax, color='red', alpha = 0.5)

gax.set_xlabel('longitude')
gax.set_ylabel('latitude')
gax.set_title('South America')

# Kill the spines...
gax.spines['top'].set_visible(False)
gax.spines['right'].set_visible(False)

# ...or get rid of all the axis. Is it important to know the lat and long?
# plt.axis('off')


# Label the cities
for x, y, label in zip(gdf['Coordinates'].x, gdf['Coordinates'].y, gdf['City']):
    gax.annotate(label, xy=(x,y), xytext=(4,4), textcoords='offset points')

plt.show()

Case Study: Voting in Wisconsin

In the example that follows, we will demonstrate how each county in Wisconsin voted during the 2016 Presidential Election.

Along the way, we will learn a couple of valuable lessons:

1. Where to find shape files for US states and counties

2. How to match census style data to shape files

Find and Plot State Border

Our first step will be to find the border for the state of interest. This can be found on the US Census’s website here.

You can download the cb_2016_us_state_5m.zip by hand, or simply allow geopandas to extract the relevant information from the zip file online.

state_df = gpd.read_file("https://datascience.quantecon.org/assets/data/cb_2016_us_state_5m.zip")
state_df.head()

	STATEFP	STATENS	AFFGEOID	GEOID	STUSPS	NAME	LSAD	ALAND	AWATER	geometry
0	01	01779775	0400000US01	01	AL	Alabama	00	131173688951	4593686489	MULTIPOLYGON (((-88.04374 30.51742, -88.03661 ...
1	02	01785533	0400000US02	02	AK	Alaska	00	1477946266785	245390495931	MULTIPOLYGON (((-133.65582 55.62562, -133.6249...
2	04	01779777	0400000US04	04	AZ	Arizona	00	294198560125	1027346486	POLYGON ((-114.79968 32.59362, -114.80939 32.6...
3	08	01779779	0400000US08	08	CO	Colorado	00	268429343790	1175112870	POLYGON ((-109.06025 38.59933, -109.05954 38.7...
4	09	01779780	0400000US09	09	CT	Connecticut	00	12542638347	1815476291	POLYGON ((-73.72777 41.10070, -73.69595 41.115...

print(state_df.columns)

Index(['STATEFP', 'STATENS', 'AFFGEOID', 'GEOID', 'STUSPS', 'NAME', 'LSAD',
       'ALAND', 'AWATER', 'geometry'],
      dtype='object')

We have various columns, but, most importantly, we can find the right geometry by filtering by name.

fig, gax = plt.subplots(figsize=(10, 10))
state_df.query("NAME == 'Wisconsin'").plot(ax=gax, edgecolor="black", color="white")
plt.show()

Find and Plot County Borders

Next, we will add the county borders to our map.

The county shape files (for the entire US) can be found on the Census site.

Once again, we will use the 5m resolution.

county_df = gpd.read_file("https://datascience.quantecon.org/assets/data/cb_2016_us_county_5m.zip")
county_df.head()

	STATEFP	COUNTYFP	COUNTYNS	AFFGEOID	GEOID	NAME	LSAD	ALAND	AWATER	geometry
0	04	015	00025445	0500000US04015	04015	Mohave	06	34475567011	387344307	POLYGON ((-114.75562 36.08717, -114.75364 36.0...
1	12	035	00308547	0500000US12035	12035	Flagler	06	1257365642	221047161	POLYGON ((-81.52366 29.62243, -81.32406 29.625...
2	20	129	00485135	0500000US20129	20129	Morton	06	1889993251	507796	POLYGON ((-102.04195 37.02474, -102.04195 37.0...
3	28	093	00695770	0500000US28093	28093	Marshall	06	1828989833	9195190	POLYGON ((-89.72432 34.99521, -89.64428 34.995...
4	29	510	00767557	0500000US29510	29510	St. Louis	25	160458044	10670040	POLYGON ((-90.31821 38.60002, -90.30183 38.655...

print(county_df.columns)

Index(['STATEFP', 'COUNTYFP', 'COUNTYNS', 'AFFGEOID', 'GEOID', 'NAME', 'LSAD',
       'ALAND', 'AWATER', 'geometry'],
      dtype='object')

Wisconsin’s FIPS code is 55 so we will make sure that we only keep those counties.

county_df = county_df.query("STATEFP == '55'")

Now we can plot all counties in Wisconsin.

fig, gax = plt.subplots(figsize=(10, 10))

state_df.query("NAME == 'Wisconsin'").plot(ax=gax, edgecolor="black", color="white")
county_df.plot(ax=gax, edgecolor="black", color="white")

plt.show()

Get Vote Data

The final step is to get the vote data, which can be found online on this site.

Our friend Kim says,

Go ahead and open up the file. It’s a mess! I saved a cleaned up version of the file to results.csv which we can use to save the hassle with cleaning the data. For fun, you should load the raw data and try beating it into shape. That’s what you normally would have to do… and it’s fun.

We’d like to add that such an exercise is also “good for you” (similar to how vegetables are good for you).

But, for the example in class, we’ll simply start with his cleaned data.

results = pd.read_csv("https://datascience.quantecon.org/assets/data/ruhl_cleaned_results.csv", thousands=",")
results.head()

	county	total	trump	clinton
0	ADAMS	10130	5966	3745
1	ASHLAND	8032	3303	4226
2	BARRON	22671	13614	7889
3	BAYFIELD	9612	4124	4953
4	BROWN	129011	67210	53382

Notice that this is NOT a GeoDataFrame; it has no geographical information.

But it does have the names of each county.

We will be able to use this to match to the counties from county_df.

First, we need to finish up the data cleaning.

results["county"] = results["county"].str.title()
results["county"] = results["county"].str.strip()
county_df["NAME"] = county_df["NAME"].str.title()
county_df["NAME"] = county_df["NAME"].str.strip()

Then, we can merge election results with the county data.

res_w_states = county_df.merge(results, left_on="NAME", right_on="county", how="inner")

Next, we’ll create a new variable called trump_share, which will denote the percentage of votes that Donald Trump won during the election.

res_w_states["trump_share"] = res_w_states["trump"] / (res_w_states["total"])
res_w_states["rel_trump_share"] = res_w_states["trump"] / (res_w_states["trump"]+res_w_states["clinton"])
res_w_states.head()

	STATEFP	COUNTYFP	COUNTYNS	AFFGEOID	GEOID	NAME	LSAD	ALAND	AWATER	geometry	county	total	trump	clinton	trump_share	rel_trump_share
0	55	035	01581077	0500000US55035	55035	Eau Claire	06	1652211310	18848512	POLYGON ((-91.65045 44.85595, -90.92225 44.857...	Eau Claire	55025	23331	27340	0.424007	0.460441
1	55	113	01581116	0500000US55113	55113	Sawyer	06	3256410077	240690443	POLYGON ((-91.55128 46.15704, -91.23838 46.157...	Sawyer	9137	5185	3503	0.567473	0.596800
2	55	101	01581111	0500000US55101	55101	Racine	06	861267826	1190381762	POLYGON ((-88.30638 42.84210, -88.06992 42.843...	Racine	94302	46681	42641	0.495016	0.522615
3	55	097	01581109	0500000US55097	55097	Portage	06	2074100548	56938133	POLYGON ((-89.84493 44.68494, -89.34592 44.681...	Portage	38589	17305	18529	0.448444	0.482921
4	55	135	01581127	0500000US55135	55135	Waupaca	06	1936525696	45266211	POLYGON ((-89.22374 44.68136, -88.60516 44.678...	Waupaca	26095	16209	8451	0.621153	0.657299

Finally, we can create our map.

fig, gax = plt.subplots(figsize = (10,8))

# Plot the state
state_df[state_df['NAME'] == 'Wisconsin'].plot(ax = gax, edgecolor='black',color='white')

# Plot the counties and pass 'rel_trump_share' as the data to color
res_w_states.plot(
    ax=gax, edgecolor='black', column='rel_trump_share', legend=True, cmap='RdBu_r',
    vmin=0.2, vmax=0.8
)

# Add text to let people know what we are plotting
gax.annotate('Republican vote share',xy=(0.76, 0.06),  xycoords='figure fraction')

# I don't want the axis with long and lat
plt.axis('off')

plt.show()

What do you see from this map?

How many counties did Trump win? How many did Clinton win?

res_w_states.eval("trump > clinton").sum()

60

res_w_states.eval("clinton > trump").sum()

12

Who had more votes? Do you think a comparison in counties won or votes won is more reasonable? Why do you think they diverge?

res_w_states["trump"].sum()

1405284

res_w_states["clinton"].sum()

1382536

What story could you tell about this divergence?

Interactivity

Multiple Python libraries can help create interactive figures.

Here, we will see an example using bokeh.

In the another lecture, we will see an example with folium.

from bokeh.io import output_notebook
from bokeh.plotting import figure, ColumnDataSource
from bokeh.io import output_notebook, show, output_file
from bokeh.plotting import figure
from bokeh.models import GeoJSONDataSource, LinearColorMapper, ColorBar, HoverTool
from bokeh.palettes import brewer
output_notebook()
import json
res_w_states["clinton_share"] = res_w_states["clinton"] / res_w_states["total"]
#Convert data to geojson for bokeh
wi_geojson=GeoJSONDataSource(geojson=res_w_states.to_json())

Loading BokehJS ...

color_mapper = LinearColorMapper(palette = brewer['RdBu'][10], low = 0, high = 1)
color_bar = ColorBar(color_mapper=color_mapper, label_standoff=8,width = 500, height = 20,
                     border_line_color=None,location = (0,0), orientation = 'horizontal')
hover = HoverTool(tooltips = [ ('County','@county'),('Portion Trump', '@trump_share'),
                               ('Portion Clinton','@clinton_share'),
                               ('Total','@total')])
p = figure(title="Wisconsin Voting in 2016 Presidential Election", tools=[hover])
p.patches("xs","ys",source=wi_geojson,
          fill_color = {'field' :'rel_trump_share', 'transform' : color_mapper})
p.add_layout(color_bar, 'below')
show(p)