[GIS] Get the vertices on a LineString either side of a Point

geometrypythonshapely

I have a shapely LineString and defined a shapely Point which lies along the LineString.

How can I find the vertices of the LineString which lie either side of the point? (split the line in two).

This is very similar to this blog post by the author:
http://sgillies.net/blog/1040/shapely-recipes/

Except, instead of cutting by distance along the line, I want to cut given a point that lies along the line.

Best Answer

I don't understand your question:

line = LineString([(0, 0), (2, 2)])
# create a point which lies along the line
point = line.interpolate(1)
line.contains(point)
True

enter image description here

You want the two lines which lie either side of the point ?

line1 = LineString([line.coords[0],(point.x, point.y)])
line2 = LineString([(point.x, point.y), line.coords[1]])

enter image description here

Upgrade 1: with a line with multiple vertices

enter image description here

You need to iterate through the segments of the LineString to find the one that contains the point

The LineString must be iterate as pair to divide the line in segments.

def pairs(lst):
    for i in range(1, len(lst)):
        yield lst[i-1], lst[i]

 line = LineString([(0,0),(1,2), (2, 2), (2,3), (4,2),(5,5)])

 for pair in pairs(list(line.coords)):
      if LineString([pair[0],pair[1]]).contains(point):
          print LineString([pair[0],pair[1]])

LINESTRING (2.00 2.00, 2.00 3.00)

And you can use the previous answer: a rapid solution, for example ( this can be done better):

line1 = []
line2 = []
cp = False
for pair in pairs(list(line.coords)):
    if cp == False:
       line1.append(pair[0])
    if cp == True:
       line2.append(pair[1])
    if LineString([pair[0],pair[1]]).contains(point):
       line1.append((point.x,point.y))
       line2.append((point.x,point.y))
       line2.append(pair[1])
       cp = True
line1 = LineString(line1)
line2 = LineString(line2)

Result:

enter image description here

Related Solutions

[GIS] Break a shapely Linestring at multiple points

Shapely has now the split function (see Split lines at points using Shapely)

points = MultiPoint([(4,5),(9,18),(6,5)]) 
line = LineString([(1,2),(8,7),(4,5),(2,4),(4,7),(8,5),(9,18),(1,2),(12,7),(4,5),(6,5),(4,9)])
from shapely.ops import split
splitted = split(line, points)
for lin in splitted:
      print lin
LINESTRING (1 2, 8 7, 4 5)
LINESTRING (4 5, 2 4, 4 7, 8 5, 9 18)
LINESTRING (9 18, 1 2, 12 7, 4 5, 6 5)
LINESTRING (6 5, 4 9)

[GIS] Slope at a point on a Line (LineString) using Shapely and python

You don't need a second list of points to find the slopes

import itertools
# iterate by pairs of points
slopes = [(second.y-first.y)/(second.x-first.x)for first, second in itertools.izip(points, points[1:])]

If you want the mean slope of the slopes

import numpy as np
def mean_slope(xs,ys):
    m = (((np.mean(xs)*np.mean(ys)) - np.mean(xs*ys)) /((np.mean(xs)**2) - np.mean(xs*xs)))
    return m
print mean_slope(slopes)
-2.2934712193638029

You can also use NumPy or SciPy

x, y = zip(*[(pt.x, pt.y) for pt in points])
slope, intercept = np.polyfit(x,y,1)
print slope
-2.2934712193638029
from scipy import stats
slope, intercept, r_value, p_value, std_err = stats.linregress(x,y)
print slope
-2.2934712193638029

If the LineString is straight all the values of slopes = slope

If you want the slope between two point

first = points[5]
second = points[6]
print (second.y-first.y)/(second.x-first.x) 
-2.2934712193638029 #straight line in my case
print np.polyfit([second.x,first.x],[second.y,first.y],1)[0]
-2.2934712193638029 
print stats.linregress([second.x,first.x],[second.y,first.y])[0]
-2.2934712193638029

Best Answer

Upgrade 1: with a line with multiple vertices

Related Solutions

[GIS] Break a shapely Linestring at multiple points

[GIS] Slope at a point on a Line (LineString) using Shapely and python

Related Question