poly_coord.py

import json
import matplotlib.pyplot as plt


def plot_polygon(coord):
	xs, ys = zip(*coord)  # create lists of x and y values
	plt.figure()
	plt.plot(xs, ys)
	plt.show()


def get_poly_coord(ways):
	def save_way():
		for point in way:
			coord.append([
				point['lon'], point['lat']
			])
		return i
	def check_swap(last_checked=-1):
		point_i = 0 if last_checked == -1 else -1
		for j, w_j in enumerate(ways_index_to_check):
			if ways[w_j][point_i] == way[point_i]:
				# we just need to swap the current one and save
				# (usually occurs when this is the first way)
				way.reverse()
				return save_way()
			elif ways[w_j][point_i] == ways[w_i][point_i]:
				# in this case it is the next one that needs to be swapped,
				# so we need to check this way again
				ways[w_i].reverse()
				return save_way()
	coord = []
	ways_index_to_check = [i for i in range(len(ways))]
	way_i = 0
	while len(ways_index_to_check) > 0:
		way = ways[ways_index_to_check[way_i]]
		del ways_index_to_check[way_i]  # does not need to be checked anymore
		for i, w_i in enumerate(ways_index_to_check):
			if ways[w_i][0] == way[-1]:
				# then we have found the next way!
				way_i = save_way()
				break
			elif ways[w_i][-1] == way[-1]:
				# check which way is the one to swap
				way_i = check_swap(last_checked=-1)
			else:
				pass
	save_way()
	# for i, way in enumerate(ways):
	# 	# this does not check for swaps/reordering
	# 	for point in way:
	# 		coord.append([
	# 			point['lon'], point['lat']
	# 		])
	return coord


if __name__ == '__main__':
	with open("overpass_result_rapallo.json", "r") as json_overpass:
		data = json.load(json_overpass)
		# this data has one node (the city center),
		# and many ways (the border of the municipality).
		# This is the query to retrieve this data:
		# place = "Rapallo"
		# overpass_query = f"""
		# [out:json];
		# area['admin_level'='2']['name'='Italia'];
		# rel['admin_level'='8']['name'=\"{place}\"](area);
		# out geom;
		# """
		members = data['elements'][0]['members']
		if members[0]['type'] == 'node':
			# the node is the city center (roughly), and it is
			# different from the boundaries of the municipality
			node_index = 0
		else:
			# last position
			node_index = -1

		poly_coord = get_poly_coord(
			[members[i]['geometry'] for i in range(
				1+node_index, len(members)+node_index
			)]
		)
		plot_polygon(poly_coord)