[GRASSLIST:5058] Distances along roads

[Dan Jacobson]

Dear GRASS aficionados, how does one compute distances along roads?
Segments 1 2 3 joined by points a b c d. I would like to know the
accumulated distance at each of a b c and d, however I suppose I will
probably have to settle with just the distance between two nodes.
Well, at least GRASS can twist and turn with the vertices, hopefully.

Here weeks into looking for an answer, we see what I came up/down with today:

What is v.distance's "to_along" all about (here in GRASS 5.7).
Here I have a road starting at point a and consisting of lines 1,2 and 3:
a--1--b-\      3--d
         2--c-/
Why yes, I would like to compute the distance from point a to b, a to
c, and a to d, all via my road and not as the bird flies, however, I
have the funny feeling that what I am guessing they mean in the
extremely scant to_along documentation, and non existent examples,
must be something different. My trials oddly gave a c to d answer.

Indeed, I bet I am not going to get further than one segment, and must
make a new from= file with v.extract to measure each segment.

I had some success with v.net.iso, but that races along roads, not
stopping at points one wants to know the accumulated distance along
the road to... Nice for mile markers along roads, but today I want to
compute distances to my points a b c and d.

Yes I could use v.build.polylines but still I bet that is not going to
get me any further along in my quest to find how far along the road
points b c and d are.