GeoPandas – How to Split Line by Nearest Points

geopandasshapelysnappingsplitting

I have two GeoPandas Dataframes.
One is a combination of linestrings, when plotted is a single line. The other GeoDataFrame contains rows where each row is a unique point close to to the first line.

Now I would like to split the line using the locations of the points returning a GeoDataFrame.

Where my input looks like this:

Best Answer

First make sure you union your GeoDataFrames into a MultiLineString and MultiPoint

line = gdf_line.geometry.unary_union
coords = gdf_point.geometry.unary_union

Using shapely.ops.snap and shapely.ops.split it is possible to snap the points to the line (with a given tolerance) and use this to split the line. Result is a GeometryCollection

split(line, snap(coords, line, tolerance=1)

To combine this and return a GeoDataFrame use the following function:

import geopandas as gpd
from shapely.ops import split, snap


def split_line_by_nearest_points(gdf_line, gdf_points, tolerance):
    """
    Split the union of lines with the union of points resulting 
    Parameters
    ----------
    gdf_line : geoDataFrame
        geodataframe with multiple rows of connecting line segments
    gdf_points : geoDataFrame
        geodataframe with multiple rows of single points

    Returns
    -------
    gdf_segments : geoDataFrame
        geodataframe of segments
    """

    # union all geometries
    line = gdf_line.geometry.unary_union
    coords = gdf_points.geometry.unary_union

    # snap and split coords on line
    # returns GeometryCollection
    split_line = split(line, snap(coords, line, tolerance))

    # transform Geometry Collection to GeoDataFrame
    segments = [feature for feature in split_line]

    gdf_segments = gpd.GeoDataFrame(
        list(range(len(segments))), geometry=segments)
    gdf_segments.columns = ['index', 'geometry']

    return gdf_segments

Which can be plotted as follows (where I find the tolerance variable still trial and error):

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
gdf_line.plot(ax=ax, lw=6, color='gray')
gdf_segments.plot(ax=ax, column='index', lw=3, cmap='Paired')
gdf_points.plot(ax=ax, zorder=3)

--EDIT

the snap function is not similar as a nearest_point query. I end up using the function https://github.com/ojdo/python-tools/blob/master/shapelytools.py#L144 from https://github.com/ojdo/python-tools that provides many interesting functions for shapely

Related Solutions

[GIS] Using ST_split to split a line feature using nearby points

As has been suggested earlier and in the ST_Split documentation, you must first snap your line to the points and then call ST_Split.

If you are like me, an example is worth more than words:

DROP TABLE IF EXISTS split;
CREATE TABLE split AS(
SELECT
    (ST_Dump(ST_Split(ST_Snap(a.the_geom, b.the_geom, 0.00001),b.the_geom))).geom
FROM 
    line_table a
JOIN 
    point_table b 
ON 
    ST_DWithin(b.the_geom, a.wkb_geometry, "Your Tolerance to Search for Lines (e.g., a number)")
);

[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
....

Best Answer

Related Solutions

[GIS] Using ST_split to split a line feature using nearby points

[GIS] Split lines at points using Shapely

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