[quote user="Barry Lothian"]I do not know how I can detect if a coordinate is south of the chainage line[/quote]
One way to find if a point is 'left' or 'right' of a line is to use vector algebra. Construct one vector V1 from the chainage element's direction at the point of intersection. Construct a second vector V2 which is the perpendicular from the point onto the chainage line. Calculate the cross product of the two vectors V3 = V1 x V2.
With the result of that calculation, you can discover all kinds of stuff. However, if the thought of vector algebra and computational geometry seems too daunting, then it's time to hire a consultant.