Shapely: Cut a piece from a linestring at two cutting points

Question

the well known function

from shapely.geometry import *
from shapely.wkt import loads

def cut(line, distance):
# Cuts a line in two at a distance from its starting point
if distance <= 0.0 or distance >= line.length:
    return [LineString(line)]
coords = list(line.coords)
for i, p in enumerate(coords):
    pd = line.project(Point(p))
    if pd == distance:
        return [
            LineString(coords[:i+1]),
            LineString(coords[i:])]
    if pd > distance:
        cp = line.interpolate(distance)
        return [
            LineString(coords[:i] + [(cp.x, cp.y)]),
            LineString([(cp.x, cp.y)] + coords[i:])]

splits a shapely linestring into two lines at distance.

What I need to do is to cut a piece of a certain length from the line, at a certain position along the line

Example line:

line = loads("LINESTRING (12.0133696 47.8217147, 12.0132944 47.8216655, 12.0132056 47.8215749, 12.0131542 47.8215034, 12.0130522 47.8212931, 12.0129941 47.8211294, 12.0130381 47.8209553, 12.0131116 47.8208718, 12.013184 47.8208107, 12.0133547 47.8207312, 12.0135537 47.8206727, 12.013915 47.8206019, 12.0141624 47.8205671, 12.0144317 47.8204965)")

I tried an approach with getting the difference between some linestrings that i got by appliyng above cut functio, but the results are not good due to shapely limitations.

Any ideas?

Answer 1

You can also use intersection and normalize the interpolate to get a curve within specific range :

def cut(line, distance):
    if distance <= 0.0 :#line.length:
        return [None, LineString(line)]
    elif distance >= 1.0:
        return [LineString(line), None]
    coords = list(line.coords)
    for i, p in enumerate(coords):
        pd = line.project(Point(p), normalized=True)
        if pd == distance:
            return [
                LineString(coords[:i+1]),
                LineString(coords[i:])]
        if pd > distance:
            cp = line.interpolate(distance, normalized=True)
            return [
                LineString(coords[:i] + [(cp.x, cp.y)]),
                LineString([(cp.x, cp.y)] + coords[i:])]

def cut_piece(line,distance1, distance2):
    """ From a linestring, this cuts a piece of length lgth at distance.
    Needs cut(line,distance) func from above ;-) 
    """
    l1 = cut(line, distance1)[1]
    l2 = cut(line, distance2)[0]
    result = l1.intersection(l2)
    return result

Example:

cir = Point(0.0, 0.0).buffer(200).exterior
cir.coords = list(cir.coords)[::-1] # to flip the curve direction
print(cir.is_ccw)
d = cut_piece(cir,0.25, 0.75)

example

Answer 2

I'll answer myself, and am happy about improvements:

def cut_piece(line,distance, lgth):
    """ From a linestring, this cuts a piece of length lgth at distance.
    Needs cut(line,distance) func from above ;-) 
    """
    precut = cut(line,distance)[1]
    result = cut(precut,lgth)[0]
    return result