Converting polygon to list of coordinates with shapely

Question

I received a code, that expects a list of coordinates such as this as input

area_polygon = [{'lat': 46.037286, 'lng': 14.471329},
                {'lat': 46.036733, 'lng': 14.467378},
                {'lat': 46.034822, 'lng': 14.468441}]

However, I have input data in form of shapely.Polygon and it looks like this:

POLYGON ((14.471329 46.037286, 14.467378 46.036733, 14.468441 46.034822))

I work with shapely and GeoPandas libraries and I know how to switch lats and longs. But I don't know how to do everything else to transform a Polygon to the form presented.

Answer 1

Here is another solution using the mapping():

returns a new, independent geometry with coordinates copied from the context.

import json
from shapely.geometry import Polygon, mapping

poly = Polygon([[14.471329,46.037286],[14.467378,46.036733],[14.468441,46.034822]])

poly_mapped = mapping(poly)

poly_coordinates = poly_mapped['coordinates'][0]

poly_ = [{'lat': coords[1],'lon': coords[0]} for coords in poly_coordinates]

print(json.dumps(poly_))

This results in:

[{"lat": 46.037286, "lon": 14.471329}, {"lat": 46.036733, "lon": 14.467378}, {"lat": 46.034822, "lon": 14.468441}, {"lat": 46.037286, "lon": 14.471329}]

Mind that the above output will contain one point two times, the first and last points, to avoid that use the following piece of code:

poly_ = poly_[:-1]

print(json.dumps(poly_))

This results in:

[{"lat": 46.037286, "lon": 14.471329}, {"lat": 46.036733, "lon": 14.467378}, {"lat": 46.034822, "lon": 14.468441}]

---

References:

• Extract points/coordinates from a polygon in Shapely

Answer 2

Easier with simple list comprehension:

poly = Polygon([[14.471329,46.037286],[14.467378,46.036733],[14.468441,46.034822]])
poly.wkt
POLYGON ((14.471329 46.037286, 14.467378 46.036733, 14.468441 46.034822, 14.471329 46.037286)
area_polygon = [{'lat':i[1],'lon':i[0]} for i in list(poly.exterior.coords)]
print(area_polygon)
[{'lat': 46.037286, 'lon': 14.471329}, {'lat': 46.036733, 'lon': 14.467378}, {'lat': 46.034822, 'lon': 14.468441}, {'lat': 46.037286, 'lon': 14.471329}]

Answer 3

Input data you have is probably a string(?) and it is almost in WKT (Well-known Text) format. Therefore, shapely.wkt.loads method helps you. But first, you have to have closed coordinates. That's, the first (lat, long) must also be at the end of the string like:

           vvvvvvv                        vvvvvvv
'POLYGON ((1.0 2.0, 10.0 20.0, 20.0 20.0, 1.0 2.0))'

The code:

import re
import numpy as np
from shapely import wkt

def add_closing_coordinates(d):
    """ Adds the first 'lat long' to the end"""
    i = re.search(r"\d", d).start()
    j = re.search(r'(\d)[^\d]*$', d).start() + 1
    c = d.index(',')    
    return d[:j] + ", " + d[i:c] + d[j:]

data = 'POLYGON ((1.0 2.0, 10.0 20.0, 20.0 20.0))'
data_wkt = add_closing_coordinates(data)
print(f"data_wkt: {data_wkt}")
# OUT:                                               vvvvvvv
# data_wkt: POLYGON ((1.0 2.0, 10.0 20.0, 20.0 20.0, 1.0 2.0))

polygon = wkt.loads(data_wkt)
coords = np.dstack(polygon.boundary.xy).tolist()[0][:-1]
print(f"coords: {coords}")
# OUT:
# coords: [[1.0, 2.0], [10.0, 20.0], [20.0, 20.0]]


expected_list_of_coordinates_for_received_code = [{"lat": x, "lng": y} for x, y in coords]
print(expected_list_of_coordinates_for_received_code )
# OUT:
# [{'lat': 1.0,  'lng': 2.0},
#  {'lat': 10.0, 'lng': 20.0},
#  {'lat': 20.0, 'lng': 20.0}]