is this posible to extract coastline or border line from multipolygon geojson but keep them separate

Question

i have multipolygon cities , how to get coastline or border line that not intersect with other cities, and keep separate each cities,

is that possible? there any software or programing method (like python or js) to extract such data?

for example below i want get colourful line only but keep them separate each cities.

here the geojson file https://gist.github.com/bagusindrayana/d42d0806d00fff1e805e733256eb07bf

EDIT : i prefer using geopandas in python

Answer 1

With some overlays, you can take advantage of the fact that the outer boundaries aren't intersecting with any other city boundaries...

The following script shows how. I included some test data in the script and commented your sample dataset so the script stays working if your sample dataset would become unavailable in the future:

import math
import geopandas as gpd
from matplotlib import pyplot as plt
import matplotlib.colors as mcolors
import shapely

# Prepare (test) data.
poly1 = shapely.box(2, 0, 4, 3)
poly2 = shapely.box(0, 1, 2, 2)
cities = gpd.GeoDataFrame(data={"alt_name": ["Brussels", "Ukkel"]}, geometry=[poly1, poly2])
# cities = gpd.read_file("https://gist.githubusercontent.com/bagusindrayana/d42d0806d00fff1e805e733256eb07bf/raw/51e6ac2864c996a83be844fd562056f482d91caf/kota_indonesia.geojson")

# We want to work with the boundaries of the polygons rather than the polygons.
boundaries = cities.copy()
boundaries["geometry"] = cities.geometry.boundary

# We only want to keep the outer boundaries... so the pieces of boundary that don't
# intersect/touch the boundaries of a neighbouring city.
# First determine the pieces of boundary that do intersect with a neighbour.
intersections = boundaries.overlay(boundaries)
# Retain only the boundary intersections with another city.
intersections = intersections.loc[intersections["alt_name_1"] != intersections["alt_name_2"]]
# Now we can use these intersections to remove them from the initial boundaries.
boundaries_outer = boundaries.overlay(intersections, how="difference")

# Plot input + result.
fig, ax = plt.subplots(nrows=2, figsize=(15, 15))
colors = list(mcolors.TABLEAU_COLORS) * math.ceil(len(cities) / len(mcolors.TABLEAU_COLORS))
cities.plot(ax=ax[0], color=colors)
colors = list(mcolors.TABLEAU_COLORS) * math.ceil(len(boundaries_outer) / len(mcolors.TABLEAU_COLORS))
boundaries_outer.plot(ax=ax[1], color=colors)

plt.show()

Result: