Monday, May 31, 2010

Interesting Intersections I've crossed

This week I ran into two interesting examples of alignments that just didn't play well with the automated intersection design tools.

Case 1- The Loop:
It isn't unusual for an alignment to cross itself, however the intersection tool won't recognise the crossing location.  Of course, the easy answer is to split it into 2 alignments. However, I wanted to build the intersection the pre-2010 way, manually creating curb return profiles and corridor regions the "old fashioned" way to see if I ran into any hiccups. 


I was worried that C3D might get confused with the whole "Target nearest to offset" thing in the corridor parameters, however this ended up building without  problem.  The most difficult part was getting the right OSNAP for throwing on the station label at the intersection (as seen in the screen cap).  I used trial and error until I got what I wanted.

One of the big things you need to remember in creating manual intersections is to make sure your crowns match. 

To manually lock a PVI just jump into the profile geometry editor. Use the select PVI to make sure you're locking the correct one and set the Lock option in geometry to True.


Case 2 - Two Ts:
This scenario came to me this week from a client.  Two J-shaped subdivision roads T into each other.  The first intersection builds without a hitch. The second intersection will not go so well.  In my client's case it was creating wacky profiles (deleting vertical curves and locking the profile at the first intersection).  When I tried it myself, I got the message, "The primary road for this T-junction will result in a circular dependency with the existing intersections."  In other words, Civil 3D gets just as confused as the friend who you tell, "Pick me up at the intersection of Jefferson and 3rd Ave..."  Which one?!

Your best bet for fixing this case is two split one of the alignments into two.  Of course you could do this one the pre-2010 way as well.

Hope this info comes in handy someday.

2 comments:

Anonymous said...

great post. I'm just now wrapping up a subdivision that I corridored with, intersections, culdesacs and roundabouts.

tony

aloraw said...

I have found that a circular reference can occur with though other intersections as well.

Example....

I have three roads (A B C).

Road A intersects with B with a T intersection with A being the primary.

Road B intersects with C with a T intersection with B being the primary.

Road C intersects with A with a T intersection with C being the primary.

I have no problem with creating the first two intersections (AB and BC), but then receive the circular reference error when creating the 3rd (CA).

If I extend Road A through C... I can specify A as the primary and have no problems. If I specify C as the primary, error.