Find traveling distance between line and point

Question

Is there a clever way to traverse a line string up until a point and return the distance to that point? As an example:

a = LineString([(1, 1), (9, 1), (9, 6), (4, 6), (4, 3)])
p = Point((4,4,))

The distance from the start of a, to point p would be:

  8  # 9-1
  5  # 6-1
  5  # 9-4
+ 2  # 2, p intesects line segment after 2 units
= 20 

I suppose I can just traverse the line and summing up the distance until p intersects the line segment, and calculate the last bit, but I get a feeling that there is something more elegant in the API that I haven't yet discovered.

Never heard of shapely until today, and I'm quite impressed by what I have seen so far.

Answer 1

You can use project() method.

project returns the distance along this geometric object to a point nearest the other object.

a = LineString([(1, 1), (9, 1), (9, 6), (4, 6), (4, 3)])
p = Point((4,4,))
a.project(p)
#20.0

Note that if you define a as LineString([(4, 3), (4, 6), (9, 6), (9, 1), (1, 1)]), you get 1.0.