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Efficient algorithms to split and join lines?.md

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From Efficient algorithms to split and join lines?

You can use effectively Shapely, and Fiona to read a shapefile for example:

    import fiona
    # open a line shapefile
    file = fiona.open('lines.shp')
    # first element of the shapefile
    first = file.next
    print first
    {'geometry': {'type': 'LineString', 'coordinates': [(203317.23, 90448.75), (203679.62, 90105.68), (203882.57, 89902.74), (204143.49, 89641.81), (204394.75, 89385.72), (204563.87, 89235.93)]}, 'id': '0', 'properties': {u'id': "1"}}

Now import Shapely

    from shapely.geometry import Point, LineString, shape
    geom = shape(first['geometry'])
    # now it is a Shapely geometry
    print geom
    LINESTRING (203317.23 90448.75, 203679.62 90105.68, 203882.57 89902.74, 204143.49 89641.81, 204394.75 89385.72, 204563.87 89235.93)

Splitting

We can split the line in segments, each pair of coordinates (Point) define a line segment:

    def pair(list):
        '''Iterate over pairs in a list -> pair of points '''
        for i in range(1, len(list)):
            yield list[i-1], list[i]

    for seg_start, seg_end in pair(geom.coords):
        line_start = Point(seg_start)
        line_end = Point(seg_end)
        segment = LineString([line_start.coords[0],line_end.coords[0]])
        print segment
    LINESTRING (203317.23 90448.75, 203679.62 90105.68)
    LINESTRING (203679.62 90105.68, 203882.57 89902.74)
    LINESTRING (203882.57 89902.74, 204143.49 89641.81)
    LINESTRING (204143.49 89641.81, 204394.75 89385.72)
    LINESTRING (204394.75 89385.72, 204563.87 89235.93)

You could do it directly with Fiona:

    for seg_start, seg_end in paires(first['geometry']['coordinates']):
        ....

Union

  1. If the line is straight, simply use the first and the last point to make the LineString
    LINESTRING (203317.23 90448.75, 204563.87 89235.93)
  1. In other cases, with shapely, you can use union, cascaded_union(geoms) or unary_union(geoms) and the result is a MULTILINESTRING;

With union:

    line = LineString()
    for element in line['geometry']['coordinates']:
       geom =shape(line['geometry'])
       profile = profile.union(geom)
    print line
    MULTILINESTRING ((203317.23 90448.75, 203679.62 90105.68), (203679.62 90105.68, 203882.57 89902.74), (203882.57 89902.74, 204143.49 89641.81), (204143.49 89641.81, 204394.756 89385.72), (204394.75 89385.72, 204563.87 89235.93))

or directly with shapely:

    line = LineString()
    for seg_start, seg_end in paires(geom.coords):
         line = line.union(LineString([Point(seg_start).coords[0],Point(seg_end).coords[0]]))

Intersection

The shapely function is geom1.intersection(geom2)

  1. for two straight lines, it is simply
   line1.intersection(line2) # the result is a Point
  1. for two lines with segments, it is
    (union of segments of line1).intersection(union of segments of line2) # the result is a Point or a MultiPoint (more than a single intersection)

And saving the results is easy with Fiona