Generate a PNG map image from a shapefile

map_gen.py

#!/usr/bin/env python3

import math
import shapefile
from PIL import Image, ImageDraw

# Longitude range to display in degrees
x_range = [-180,180]
# Latitude range to display in degrees
y_range = [-90,90]
# Minimum size of the larger dimension of the image
min_size_px = 1500

# wget https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/50m/cultural/ne_50m_admin_0_countries_lakes.zip
# unzip ne_50m_admin_0_countries_lakes.zip
# rm ne_50m_admin_0_countries_lakes.zip *.cpg *.dbf *.prj *.README.html *.VERSION.txt *.shx
sf = shapefile.Reader("ne_50m_admin_0_countries_lakes.shp")

shapes = sf.shapes()

x_range = [math.floor(x_range[0]), math.ceil(x_range[1])]
y_range = [math.floor(y_range[0]), math.ceil(y_range[1])]
x_diff = x_range[1] - x_range[0]
y_diff = y_range[1] - y_range[0]

if x_diff >= y_diff:
    pixels_per_degree = math.ceil(min_size_px / x_diff)
else:
    pixels_per_degree = math.ceil(min_size_px / y_diff)

# Oversize and then resize at the end to allow
# to apply anti-aliasing filter
oversample = 4

x_size = x_diff * pixels_per_degree * oversample
y_size = y_diff * pixels_per_degree * oversample

img = Image.new("RGB", [x_size, y_size], "#8ab4f8")
canvas = ImageDraw.Draw(img)

# Mercator projection (linear mapping from degrees to pixels)
scale = x_size / x_diff
offset_x = -(x_range[0] * pixels_per_degree * oversample)
offset_y = -(y_range[0] * pixels_per_degree * oversample)

for country in shapes:
    # Countries can be multipart polygons
    # So get list of the start and end index values for each part
    parts = list(country.parts)
    # Last index isn't in the parts list, so add it
    parts.append(len(country.points))
    # Step through each pair of index value and draw the polygon
    for lower, upper in zip(parts, parts[1:]):
        # Apply scaling. X increases left to right, Y increased top to bottom
        country_scaled = [((point[0]*scale)+offset_x, y_size-(offset_y+(scale*point[1]))) 
                                for point in country.points[lower:upper]]
        canvas.polygon(country_scaled, fill ="#a8dab5", outline ="black")

img = img.resize((x_size//oversample, y_size//oversample), Image.LANCZOS)
img.save("map.png")
img.show()