Shapely – How to Split Polygon by MultiLineString

polygonpythonshapelysplitting

I'm working on splitting a polygon given a set of linestrings using the shapely python library. The problem I'm facing is that a polygon cannot be split by a MultiLineString usingshapely.ops.split(geom, splitter).

The desired result in the image above is to split polygon (blue) with the linestrings (yellow, brown, red) and recieve 4 seperate polygons (1,2,3,4). The geometries in the image are provided below. It is preferred to do this within shapely.

    polygon = shapely.geometry.polygon.Polygon(
[(172288.6111791851, 442085.8603492165),
 (172300.89001713035, 442131.53561756504),
 (171916.83106749243, 442140.9016841368),
 (171910.779533863, 442014.47234046715),
 (172165.44614826166, 442016.17982713436),
 (172188.0201264064, 442022.13092037174),
 (172189.32585150487, 442047.592559792),
 (172159.87170649506, 442049.9277989104),
 (172169.13733267464, 442085.8603492165),
 (172288.6111791851, 442085.8603492165)])

coords = [((172159.87170649506, 442016.14245166205),
 (172159.87170649506, 442049.9277989104)),((172159.87170649506, 442049.9277989104),
 (172159.87170649506, 442134.97463880514)),((172169.13733267464, 442085.8603492165),
 (172169.13733267464, 442134.74867747363))]

lines = shapely.geometry.MultiLineString(coords)

Best Answer

adopted the answer from this post: Splitting Polygon by Linestring in GeoDjango?.

line_split_collection.append(polygon.boundary) # collection of individual linestrings for splitting in a list and add the polygon lines to it.
merged_lines = shapely.ops.linemerge(line_split_collection)
border_lines = shapely.ops.unary_union(merged_lines)
decomposition = shapely.ops.polygonize(border_lines)

The lines for splitting are slightly extended in both directions (0.01) in order to ensure that they cross the input polygon border, therefore making the unary_union work.

Related Solutions

[GIS] Split lines at points using Shapely

You have many solutions and I use here a simple example

1) the easiest way

from shapely.geometry import Point, LineString
line = LineString([(1,2),(2,4),(4,5)])
point = Point(2,4)

First, you must determine if the point is within the line (Determine if shapely point is within a linestring/multilinestring)

line.distance(point) < 1e-8
True
print LineString([line.coords[0],point.coords[:][0]])
LINESTRING (1 2, 2 4)
print LineString([point.coords[:][0], line.coords[-1]])
LINESTRING (2 4, 4 5)

2) with the shapely function split

from shapely.ops import split
result = split(line, point)
result.wkt
'GEOMETRYCOLLECTION (LINESTRING (1 2, 2 4), LINESTRING (2 4, 4 5))'

3) from Get the vertices on a LineString either side of a Point

from shapely.geometry import Point,LineString    
def split(line_string, point):
    coords = line_string.coords
    j = None    
    for i in range(len(coords) - 1):
        if LineString(coords[i:i + 2]).intersects(point):
           j = i
           break    
    assert j is not None    
    # Make sure to always include the point in the first group
    if Point(coords[j + 1:j + 2]).equals(point):
        return coords[:j + 2], coords[j + 1:]
    else:
        return coords[:j + 1], coords[j:]

line1,line2 = split(line,point)
line1 = LineString(line1)
line2 = LineString(line2)
print line1, line2
LINESTRING (1 2, 2 4) LINESTRING (2 4, 4 5)

4) from Shapely Split LineStrings at Intersections with other LineStrings

# First coords of line (start + end)
coords = [line.coords[0], line.coords[-1]] 
# Add the coords from the points
coords += point.coords
# Calculate the distance along the line for each point
dists = [line.project(Point(p)) for p in coords]
# sort the coordinates
coords = [p for (d, p) in sorted(zip(dists, coords))]
lines = [LineString([coords[i], coords[i+1]]) for i in range(len(coords)-1)]
for lin in lines:
   print lin
LINESTRING (1 2, 2 4)
LINESTRING (2 4, 4 5)

5) you can also examine and adapt

cut.py by Sean Gillies
Efficient algorithms to split and join lines?
Shapely: Cut a piece from a linestring at two cutting points
....

Related Question