Python: how to create a choropleth map out of a shapefile of Canada?

Question

My goal here is to create a choropleth map of Canada in Python. Suppose I have a dictionary with values referring to each Canadian province/territory:

myvalues={'Alberta': 1.0,
 'British Columbia': 2.0,
 'Manitoba': 3.0,
 'New Brunswick': 4.0,
 'Newfoundland and Labrador': 5.0,
 'Northwest Territories': 6.0,
 'Nova Scotia': 7.0,
 'Nunavut': 8.0,
 'Ontario': 9.0,
 'Prince Edward Island': 10.0,
 'Quebec': 11.0,
 'Saskatchewan': 12.0,
 'Yukon': 13.0}

Now I want to color each province based on the corresponding value in myvalues, using a continuous colormap (e.g., shades of red). How to do that?

So far I have only been able to plot the Canadian provinces/territory within matplotlib, but their shapes appear in a unique color, and I don't know how to change that according to the numbers in myvalues (maybe I need to play with patches but I don't know how).

This is where you can find the shapefile: http://www.filedropper.com/canadm1_1

And this is my code to date:

import shapefile
import matplotlib.pyplot as plt
import matplotlib.patches as patches
from matplotlib.patches import Polygon
from matplotlib.collections import PatchCollection
#   -- input --
sf = shapefile.Reader("myfolder\CAN_adm1.shp")
recs    = sf.records()
shapes  = sf.shapes()
Nshp    = len(shapes)
cns     = []
for nshp in xrange(Nshp):
    cns.append(recs[nshp][1])
cns = array(cns)
cm    = get_cmap('Dark2')
cccol = cm(1.*arange(Nshp)/Nshp)
#   -- plot --
fig     = plt.figure()
ax      = fig.add_subplot(111)
for nshp in xrange(Nshp):
    ptchs   = []
    pts     = array(shapes[nshp].points)
    prt     = shapes[nshp].parts
    par     = list(prt) + [pts.shape[0]]
    for pij in xrange(len(prt)):
     ptchs.append(Polygon(pts[par[pij]:par[pij+1]]))
    ax.add_collection(PatchCollection(ptchs,facecolor=None,edgecolor='k', linewidths=.5))
ax.set_xlim(-160,-40)
ax.set_ylim(40,90)

This is the image I am getting so far:

EDIT

I get the solution must be in the following lines:

cm    = get_cmap('OrRd')
cccol = cm(1.*arange(Nshp)/Nshp)

The above script creates a cccol array which in reality has this shape:

array([[ 1.        ,  0.96862745,  0.9254902 ,  1.        ],
       [ 0.99766244,  0.93356402,  0.84133796,  1.        ],
       [ 0.99520185,  0.89227221,  0.74749713,  1.        ],
       [ 0.99274125,  0.84306037,  0.64415227,  1.        ],
       [ 0.99215686,  0.78754327,  0.5740254 ,  1.        ],
       [ 0.99186467,  0.71989237,  0.50508269,  1.        ],
       [ 0.98940408,  0.60670514,  0.39927722,  1.        ],
       [ 0.97304114,  0.50618995,  0.32915034,  1.        ],
       [ 0.94105344,  0.40776625,  0.28732027,  1.        ],
       [ 0.88521339,  0.28115341,  0.19344868,  1.        ],
       [ 0.8220992 ,  0.16018455,  0.10345252,  1.        ],
       [ 0.73351789,  0.04207613,  0.02717416,  1.        ],
       [ 0.61959248,  0.        ,  0.        ,  1.        ]])

I don't know why it has 4 columns, but I figure that if I can somehow link the values of this array to those specified in the values dict, I can solve the problem. Any ideas?

EDIT 2

I have figured out the "trick" is in cccol = cm(). In order to relate this to the provinces, I tried to assign cccol = cm(myvalues.values(i) for i in myvalues.keys())

so that (in my mind at least) each color is assigned based on the relevant key and there are no misplacements. The problem is that I get an error:

TypeError: Cannot cast array data from dtype('O') to dtype('int32') according to the rule 'safe'.

How to work around this?

Answer 1

This doesn't directly answer your question but hopefully solves your problem just the same. Have you looked at GeoPandas? It provides a simple API for working with and plotting shapefiles. You can replicate your code, including plotting a choropleth, in just a few lines:

import geopandas as gpd
canada = gpd.read_file('CAN_adm1.shp')
canada.plot('myvalues', cmap='OrRd')

This example assumes your shapefile has an attribute on each province that contains the values you want to plot, and the attribute is called "myvalues". If the values aren't stored in the shapefile, you can use canada.merge to merge your values map onto the GeoDataframe.

One caveat: At this time GeoPandas does not have an easy way to plot the legend for choropleth colors. (issue reported here)