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I have enjoyed the discussion of this thread. As I gave the task more thought and anaysis it became more clear that the task was not simple. As it appears that there is still no satisfacory solution I thought I would offer the following. The first goal for me was to create a function that would create a midline between two non parallel lines. The mid-lines extents should be a function of the given line segments. This function could then be used in a program that would step through the line segments of one of the polylines and search the other polyline for relevant segments. The function "midline" accepts four points. The first two points, A1 and A2, are the ends of one line sement while the thrid and fourth points, B1 and B2, are the ends of an opposing ilne segments. The diagram below details the variables in the function. The program uses vectors as I prefer them over angles which present, for me, a variety of problems. uA = unit vector in the diection from A1 to A2 uB = unit vector in the direction from B1 to B2 uBisector = unit vector in the direction of the angle bisector of uA and UB The ends of the two lines are projected onto the bisecting line defining 4 points, A1M, A2M, B1M, B2M. I debated which of the points to output for the line to be drawn. I first used the closest and furthest points from the intersecttion point ABIntr but I found it more helpful to use the two intermediate points (A1M and A2M in the example above). Here's an example of the results after manually steppng alone the polyline. Looking at the area circled in red we find: To fill the gap we need a curve that starts with a radius of 0.1514 and ends with a radius of 0.1693. This can be done with a spline or you may find it acceptable to extend the two lines to the point of intersection. The best way to create the spline is to use the Control Vertex Method and use the two endpoints and the imaginary point of intersecton for the middle CV. This ensures tangency to the two lines. As can be seen below the distance to a random point along the spline (red) agree! Run the program "test" and specify the end points of a line segment on one of the polylines, then the endpoints on a line segment on the opposing polyline. I have found the results very accurate and although it may not be used for creating the complete "hybrid " polyline it is helpful in finding the correct line for a specific segment.

@JerryFiedler look at fanzy zone that is also part of powertoys. its the whole reason i got it when it first came out.