The Vector Direction of Style in Civil 3D

Tags alignment, alignment direction, vector, horizontal control, vertical control, tangents, free curve

Software hopefully grows more intuitive. AutoCAD Civil 3D’s interface is better at it than the not so model-based software of the past. Features must behave in a more consistent and predictable way by definition. However “intuitive” means different things to different people. A change of context can be befuddling to the best of us.

We Don't Know When to Look for Directions

I ran into the following scenario in a training class. The users were new to Civil 3D therefore their experience with both Civil 3D Features and the interface was limited. However, I watched both the instructor and class coach stumble over this one for a bit too.
Everyone’s more directionless CAD past and those built-in expectations about what we’re doing and how to do it can trip anyone up.

A Cross Over Corridor

Suppose you have a complex Alignment to build connecting two other roadways. Say an off ramp from a highway or a cross over between two parallel streets for example. The geometry of the new Alignment is directly related to the established start and end Feature geometry. The off ramp scenario might be simplified to a tangent to perpendicular solution and the cross over scenario to a tangent to tangent solution. Lots of geometric combinations are possible.

In the Civil 3D world we’re thinking Corridor and often Intersection solutions right away. That means Baseline (an Alignment and Profile pair) and specifically where in the maze of potential geometry is the most manageable Baseline pair located.
Given we’re in AutoCAD Civil 3D 2012+ with Axis of Rotation subassemblies, better Alignment control geometry tools, better Superelvation, Offset Alignments, etc - This “manageable” Baseline isn’t always the centerline.
How to look for the critical Baseline(s) is not the subject of this post, but it’s an important topic. In the simple example we’ll use a centerline. Don’t be fooled - sometimes the classic centerline is really just more work. Nuff said?

Corridor Alignment Geometry

Here’s the final Baseline Alignment geometry of a typical cross over. The key thing to note is the three fixed tangents that establish the “frame” of the alignment. The specifics of curve geometry in between is irrelevant at this point.
The start this tangent is driven by a north end intersection. This is established by a point on a tangent and a perpendicular to another existing alignment.
The same is true or the south (or end) tangent.
No matter which direction you construct the overall alignment you will have to have one of the start or end segments established in the opposite direction of the overall alignment.
Civil 3D is perfectly happy with this. Alignment Feature segments (entities) each have a direction independent of the overall alignment Feature itself.

A Ramp Alignment

In a ramp configuration we might have the segment direction establishment issue too. This can all be resolved in any number of configurations in the Alignment Geometry editor albeit with perhaps some tweaks to the Alignment Constraint properties, the Design Criteria, and the specifics of the horizontal control we’re working on.

Edit the Ramp Alignment

The Old Rules Don’t Matter

The end results are one thing.
Getting there is where we may find cause to stumble. Suppose we have our starting "frame" tangents established. Things might look like this.

pThe Alignment Tangents Alone

Civil 3D is happy to let us create alignments with Fixed segments of different types that are disconnected. They don’t need to be created in any particular direction either.
The order in which we create the segments may sometimes be important, but Civil 3D becomes more forgiving of even that in more recent releases.

Visible Directions Do Matter

However, when you start attempting to connect these disconnected parts together the interface assumes you’ve paid attention to the direction of the segment entities themselves. Hence the InstantOn Basic “Layout” Feature Styles include those sometimes critical, sometimes annoying, direction arrow components.

Sometimes it can be handy to temporarily even exaggerate the size of the direction arrows in these styles – The direction arrow is a Marker component. We want to use them for quick visual QA to help us identify the direction of the segments all the time.

Alignment Style Marker Edits

We ALWAYS want the direction arrow Markers to appear so the Fit Options property makes a difference. We don’t want the direction arrows to be omitted if we have tiny Alignment segments that are geometrically distant from one another.
That would make the direction invisible which is exactly what we are trying to avoid at design time.

Create a Curve Between the Tangents

Given the direction arrangement between the first two simple tangent segments when we attempt to create a simple Free Curve between them using a Through point Free Curve no such point exists where we expect one to work.

A Curve that works

However, this through point solution DOES work. It also makes the relative direction issues between the two tangents more obvious.
We can’t count of this to be visible to the user in the heat of battle.

A Not So Intuitive Response

If we attempt almost any other Free Curve or Spiral construction we’ll get a NO SOLUTION found response from Civil 3D at the command line.
Nothing’s hopefully wrong with our math and our answers to the prompts.
Civil 3D isn’t failing to perform the obvious. It’s not a bug.
We just did not pay attention to the segment directions that we are applying the Alignment Layout tool to.
What looks like the next start point simply isn’t.

The Reverse Subentity Segment Tool

The fix is already built right into the Alignment Layout toolbar. The tool’s called Reverse Sub-entity Direction. Click, Pick, and we are are done.

The Subentity Direction is Fixed

We have two things to consider.

We must remember that vector direction always matters in any Civil 3D linear Feature.
Our users need Styles available to make this reality as visible as possible.
Otherwise they waste time and justifiably will become more frustrated with the software.

A Better Sense of Style Makes More Possible