I suppose you could use the Snipper transformer to cut off the part of the line (starting at 0 ending at the wanted length. Then use the CoordinateExtractor to retrieve the coordinates of the new endpoint (index -1).

So the work flow would be

Determine which vector the start lat long was on

Nearest nieghbor

Calcuate the length to point on the vector (measure 1)

Add measure 1 to the lenght value in the attrbute

Expression calculator to get result value

Then use snipper based on the different result values? Will see if snipper can use different lengths.

Cheers

If the "Length" means the length measured along the road (polyline), the Snipper (Snipping Mode: Distance) would be useful, as @erik_jan suggested.
However, if the "end lat long" means a point whose location is determined by the vector direction and the "Length" (straight distance from the start point), you can calculate its coordinates with simple math expressions.

Assuming

• (xs, ys) = coordinates of the start point
• (ux, uy) = the unit vector
• L = the "Length" (straight distance from the start point)

then, coordinates of the "end lat long" (xe, ye) are:

• xe = xs + ux * L
• ye = ys + uy * L

or use gonio..

dX )))

)))

Angle iof vector of course.