PyQGIS – Extending Only One End of Line

extendpyqgisqgis-3qgis-pluginsqgis-processing

From Shortening line with PyQGIS I was able to make a nice tool where the user can click one end of the line or the other and only that end will be shortened. the function is here:

def shorten_line(self, feat, len, point):
    '''Returns a linestring geometry shortened by the given length.
    Takes two arguments- a QgsFeature object and a double value'''
    g = feat.geometry()
    v = [v for v in g.vertices()]
    if QgsGeometry.fromPointXY(QgsPointXY(v[0])).equals(QgsGeometry.fromPointXY(point)):
        print("starting point chosen")
        reverse = QgsLineString(v[::-1])
        g = QgsGeometry(reverse)
        v = [v for v in g.vertices()]
    v.remove(v[-1])
    v.append(QgsPoint(g.interpolate(g.length()-len).asPoint()))
    n = QgsLineString(v)
    return QgsGeometry(n)

Where feat is the line feature, len is the length to be subtracted and point is the vertex that was clicked.
Now I want to make an extend line function that will do the same thing except it will add on to that end of the line instead of subtract. This attempt does not work:

def extend_line(self, feat, len, point):
    '''Returns a linestring geometry shortened by the given length.
    Takes two arguments- a QgsFeature object and a double value'''
    g = feat.geometry()
    v = [v for v in g.vertices()]
    if QgsGeometry.fromPointXY(QgsPointXY(v[0])).equals(QgsGeometry.fromPointXY(point)):
        print("starting point chosen")
        reverse = QgsLineString(v[::-1])
        g = QgsGeometry(reverse)
        v = [v for v in g.vertices()]
    v.remove(v[-1])
    v.append(QgsPoint(g.interpolate(g.length()+len).asPoint()))
    n = QgsLineString(v)
    return QgsGeometry(n)

How can I extend the end of the line that the user clicks?

Best Answer

You could use the method extendLine()

This particular implementation is untested (I have simply modified your function), but something like the following should work for you:

def extend_line(self, feat, length, point):
    g = feat.geometry()
    v = [v for v in g.vertices()]
    if QgsGeometry.fromPointXY(QgsPointXY(v[0])).equals(QgsGeometry.fromPointXY(point)):
        print("start point chosen")
        extended_line = g.extendLine(float(length), 0.0)
    elif QgsGeometry.fromPointXY(QgsPointXY(v[-1])).equals(QgsGeometry.fromPointXY(point)):
        print("end point chosen")
        extended_line = g.extendLine(0.0, float(length))
    # extendLine() returns a QgsGeometry object
    return extended_line

Related Solutions

[GIS] Extending line by specified distance in ArcGIS for Desktop

Well I think I've gotten it down for lines of any vertex count. I haven't attempted multipart lines since I've never messed with it in arcpy. The coding was made a bit more difficult since there isn't write access to the lastPoint property for Geometry objects. Instead of using the slope (which was my initial thought), I used the code from this SO question. It doesn't rely on trigonometry, so it should be slightly more efficient. The following code works by moving the endpoint of a line to a new coordinate that lies along the prolongation of a line from the last two vertices. I tested it on a shapefile.

from math import hypot
import collections
from operator import add
import arcpy

layer = arcpy.GetParameterAsText(0)
distance = float(arcpy.GetParameterAsText(1))

#Computes new coordinates x3,y3 at a specified distance
#along the prolongation of the line from x1,y1 to x2,y2
def newcoord(coords, dist):
    (x1,y1),(x2,y2) = coords
    dx = x2 - x1
    dy = y2 - y1
    linelen = hypot(dx, dy)

    x3 = x2 + dx/linelen * dist
    y3 = y2 + dy/linelen * dist    
    return x3, y3

#accumulate([1,2,3,4,5]) --> 1 3 6 10 15
#Equivalent to itertools.accumulate() which isn't present in Python 2.7
def accumulate(iterable):    
    it = iter(iterable)
    total = next(it)
    yield total
    for element in it:
        total = add(total, element)
        yield total

#OID is needed to determine how to break up flat list of data by feature.
coordinates = [[row[0], row[1]] for row in
               arcpy.da.SearchCursor(layer, ["OID@", "SHAPE@XY"], explode_to_points=True)]

oid,vert = zip(*coordinates)

#Construct list of numbers that mark the start of a new feature class.
#This is created by counting OIDS and then accumulating the values.
vertcounts = list(accumulate(collections.Counter(oid).values()))

#Grab the last two vertices of each feature
lastpoint = [point for x,point in enumerate(vert) if x+1 in vertcounts or x+2 in vertcounts]

#Convert flat list of tuples to list of lists of tuples.
#Obtain list of tuples of new end coordinates.
newvert = [newcoord(y, distance) for y in zip(*[iter(lastpoint)]*2)]    

j = 0
with arcpy.da.UpdateCursor(layer, "SHAPE@XY", explode_to_points=True) as rows:
    for i,row in enumerate(rows):
        if i+1 in vertcounts:            
            row[0] = newvert[j]
            j+=1
            rows.updateRow(row)

I set the symbology to arrow at end for categories based on OID so that it will be easier to see the separation between features. Labeling was set to count vertices. enter image description here

[GIS] Extending Polyline in ArcGIS without intersecting line

The most simple workaround to this problem is to create a buffer around your line, then you convert the buffer into a line ("Feature to line" using both the buffers and the original lines) and finally you run the "extend" tool (Info/advanced licence needed for feature to line, and Editor/standard licence needed for extend, but there are free solutions for the feature to line tool).

EDIT (Thanks Hornbydd for the remarks): you can directly create the line buffer in an edit session (editor toolbar > Editor > Buffer). This tool will work with selected feature only. The extend tool is also available as an editing tool (editor toolbar > Editor > more editing tools > advanced editing)

Related Question