Welding individual line segments into one LineString using Shapely

Question

I'm using Shapely in python and I'm given a MultiLineString with a bunch of Linestring objects. I can guarantee that all the LineString objects are simple lines with only 2 vertices and that they are all part of one single line (no branches).

I want to "connect-the-dots" and create a single LineString.

Do I need to write a recursive welding method for this or is there a faster way?

Answer 1

You can use shapely's ops.linemerge to accomplish this:

from shapely import geometry, ops

# create three lines
line_a = geometry.LineString([[0,0], [1,1]])
line_b = geometry.LineString([[1,1], [1,0]])
line_c = geometry.LineString([[1,0], [2,0]])

# combine them into a multi-linestring
multi_line = geometry.MultiLineString([line_a, line_b, line_c])
print(multi_line)  # prints MULTILINESTRING ((0 0, 1 1), (1 1, 2 2), (2 2, 3 3))

# you can now merge the lines
merged_line = ops.linemerge(multi_line)
print(merged_line)  # prints LINESTRING (0 0, 1 1, 2 2, 3 3)

# if your lines aren't contiguous
line_a = geometry.LineString([[0,0], [1,1]])
line_b = geometry.LineString([[1,1], [1,0]])
line_c = geometry.LineString([[2,0], [3,0]])

# combine them into a multi-linestring
multi_line = geometry.MultiLineString([line_a, line_b, line_c])
print(multi_line)  # prints MULTILINESTRING ((0 0, 1 1), (1 1, 1 0), (2 0, 3 0))

# note that it will now merge only the contiguous portions into a component of a new multi-linestring
merged_line = ops.linemerge(multi_line)
print(merged_line)  # prints MULTILINESTRING ((0 0, 1 1, 1 0), (2 0, 3 0))

Answer 2

shapely.ops.linemerge works if the lines are contiguous ("tips" coincide with the "tails" of the constituent lines), but if they are non-contiguous (if there's a gap between the tips and tails) it returns another MultiLineString. If your constituent lines are well-ordered (with one line ending near the start of the next line) but have a tip-to-tail gap, you can extract the coordinates and use them make a new simple line. This approach also works for multi-lines made of more complex sublines (i.e. sublines with more than two points).

import shapely

# Make a MultiLineString to use for the example
inlines = shapely.geometry.MultiLineString(
    [shapely.geometry.LineString([(0,0),(0,0.9)]), 
     shapely.geometry.LineString([(0,1),(1,1)])]
)

# Put the sub-line coordinates into a list of sublists
outcoords = [list(i.coords) for i in inlines]

# Flatten the list of sublists and use it to make a new line
outline = shapely.geometry.LineString([i for sublist in outcoords for i in sublist])

Answer 3

I think you could do it with Shapely using shapely.ops.linemerge() method.

It looks like it could take a list of lines as input and merge them. I used the 'polygonize' method before and it does take a list of lines.

Take a look at the doc here: http://toblerity.org/shapely/manual.html#shapely.ops.linemerge

Answer 4

shapely.ops.linemerge() failed for some of my lines so I had to do it manually. It seems to fail for the lines that "returned" to itself, i.e., going through the same point more than once. For my case, I know that the lines are in the correct order so it was easy to write a small function to merge them.

from shapely.geometry import LineString
from typing import List
def merge_lines(lines: List[LineString]) -> LineString:
    last = None
    points = []
    for line in lines:
        current = line.coords[0]
    if last is None:
        points.extend(line.coords)
    else:
        if last == current:
            points.extend(line.coords[1:])
        else:
            print('Skipping to merge {} {}'.format(last, current))
            return None
    last = line.coords[-1]
return LineString(points)

Hope it helps someone