How to accurately find the vertical intersection of two colinear linestrings

This one comes up a fair bit, and I finally got around to recording a video to show you a neat TBC trick that I am sure many people are not fully aware of.

In the example - the user here has two arcs that overlap at a curb return that are colinear in the overlap area, but have separate vertical profiles. They want to join the two lines together at the point where the verticals intersect to create a continuous curb return, but without any vertical steps in the profile.

TBC is really a 2D CAD with 3D capability. When you run a snap like the intersection snap, it is computing the intersection in 2D and then where you create a break point, it computes the vertical from the existing line and creates VPIs at the break point on the two lines that result. It does not even attempt to compute a vertical intersection point today - that is not to say that a command cannot be written to do this, but how do you solve the problem using TBC tools that exist today.

In the example we show you that when you open a Profile View of Arc 1, that you can then open the profile view of Arc 2 - and if you do so while holding the shift key down, it will recompute Arc 2 into the stationing along Arc 1 and display both Arc profiles in the profile view for Arc 1.

Now that you have that, when you want to create the break location, you can exaggerate the vertical in the profile view and then zoom in close to the point where the two profiles cross and snap the station of the crossing location to break Arc 1 at that location. You cannot break Arc 2 using this method, because the Stationing extracted from the Profile View is for Arc 1 only.

To break Arc 2 - once you have broken Arc 1 - you can use End Point snap to break Arc 2.

If you were trying to break Arc 2 first, then you can use the Right Click, Station at Point Snap and then for the point location, use the Right Click Snap - Offset Line, Pick Arc 1 and then pick the Station by free click in the Profile Viewer and then set Offset 0 - this will compute the correct break location for Arc 2.

In my tests I have found that this works well and is accurate to better than 0.01’ (typically in the 0.001’ range) which should be good enough for most purposes …)

Once you have added a second profile to the first profile, it will clear itself if you subsequently do a profile view on a different line. It will however remember the association if you do not open a profile for a different line, so if you close the profile viewer and then reopen it on the same line - you will still have the two profiles in one view. So to clear that select a different line and open the profile viewer on that one, and then it will have reset itself.

Video shows you how