To elaborate some.
Because superimposing works by drawing a line perpendicular from the parent
alignment to the superimposed alignment, then pulling an elevation from the
superimposed profile at the superimposed alignments' station of intersection
with that line, the only time profile will line up is when they are parallel
_over their entire length_. If they are parallel at the start, they will
superimpose as desired, but as soon as you reach a PT, they will shift on
you.
Check out the attached png. These are two parallel alignments, with similar
curve properties. Notice how the stationing has shifted? If you were to
superimpose the profile from the north alignment, you'd get it's elevation
for 33+00 shown at approx 35+00 on the southern alignments profile.
Does that make sense?
--
James Wedding, P.E.
Technology Manager &
Associate
Jones & Boyd, Inc.
Dallas, TX
XP/2 on P4-3.4/1G
LDT 2006 & C3D2006/SP1