WBY,

Okay, here's the scoop (Zen Dude-style): 

First, fundamentally, a Key Station forces the software to calculate a sectional solution at the station value entered, based on the first template that it finds back-station from the key station entered.

So:

a)    A template is dropped at some station and it starts driving forward along the path of the corridor

b)    At every Interval (defined at the time of that template drop) InRoads is calculating a section layout at that particular station. 

c)    As the template steps from station to station, jumping from interval location to interval location (because it doesn't care about anything in between the Intervals) it's also watching for any other influences, applying them to that section. These are Point Controls, Superelevation, Parametric Constraints, End Condition Exceptions, and so on. The application of these influences has a hierarchy of superiority, but that's another subject.

d)    When a Key Station is encountered in a corridor, the modeling reacts just as if it was one of the 'regular' modeling stations introduced by the Interval setting.

That means that a Key Station is just a user-defined station where there is a desire to model based on the template that is moving forward along the alignment toward that station location. It's analogous to an Event Point along an alignment. You can view stationing at 'regular' stations but also view the Event location. The 'regular' stations are based on the Interval, but special locations can be manually added either as Events, or in the case of a corridor, Key Stations.

But I know what the real question is - When do you need them and when don't you need them?

I'm sure it's true that some users add them without needing to and the corridor has a station modeling redundancy built into it. And there is one key dialog box that can completely affect this entire conversation - the Roadway Designer Options. If there were an important batch of settings for a corridor, this would be one of the top dogs.

In particular, the Include Critical Sections area of this dialog box. I suspect that you understand the first four toggles, but the important one is the External Control Points toggle. That toggle can change the results of a model like no other toggle here.

This is a snippet from the Help file: "External control points are points which are controlled by other alignments, features, or parametric constraint entries. When point controls are assigned and use these types of parents, the vertices of the controlling alignments/features are not processed stations unless this option is checked on."

Okay, to be honest, I didn't quite understand the full implications of that 'help' statement until I read it four times and did some testing.

Here's what I found:

At Template Drops - It doesn't matter how oddball a station is at a template drop. If a template drop exists at any station, it doesn't need a Key Station at that location, and that section will always process.

At Point Controls - "On-alignment" PIs, POB, POE, PCs and PTs only automatically process if External Control Points is toggled on. Otherwise, if a Point Control required modeling at those locations you would have to add Key Stations for InRoads to model them. 

PIs, PCs, PTs, and so on, of an alignment used as a Point Control are not treated the same as the PIs, PCs, PTs, and so on, of the alignment the corridor is running along. The modeling of those points are controlled by the Cardinal toggles. The toggle that says, "Model at Cardinal Points" should actually say, "Model at the Cardinal Points of the alignment that the corridor is running along". This does not include the Cardinal Points of any Point Controls.

At Transition Locations - Obviously, if you are transitioning between templates you are adding a template drop on either end of the transition so all of those stations would process.

However, there is a valuable use of Key Stations that can be used here in certain cases that I like to do.

Very quickly: (if anyone has questions on this just let me know)

The picture below shows a transition between two template drops. Both drops could be at some odd station and it wouldn't matter. You can see by the white transitional breaklines that it's gradually moving from one section (on the left) to the other section (on the right).

If you disabled the transition - what happens? Well, it still transitions. Imagine that. But, the transition takes place at the last even station based on the Interval assigned when that first template (on the left) was placed. This is clearly observable in the picture below.

Now, what if you wanted the "disabled" transition to occur within a 0.01' from the template drop on the right? Like at a bridge abutment.

Most people would just drop another instance of the template on the left 0.01' away from the drop on the right. Right? I know this is true because I've built my share of models this way.

Enter Key Stations.

Just pop down a key station just prior to that second template drop. And what do you get? A processed section based on the far left template, 0.01' from the template drop on the right.

A beautiful abrupt change of section.

So, if you model a road with a 25' interval, with Template #1 starting at station 10+00, and drop Template #2 at 20+00, and a Key Station at 19+99.99 - and disable the Template Transition, then it would run T1 until 19+99.99 and then transition to T2 at 20+00.

Even though the interval is 25', the additional drop at 19+99.99 will force that station to be processed in the model. And actually, enabled transitions will start and stop between actual Template Drop locations and are not influenced by a Key Station entry.

At Parametric Constraints - Under my testing if a Parametric Constraint Start / Stop Stations are entered, it will automatically model there.

At Superelevation Locations - You don't need to place Key Stations at Superelevation locations; those are automatically processed by the Roadway Designer.

At End Condition Exceptions - Under my testing these locations are automatically processed.

At PCs and PTs of a curve - These are automatically modeled only if Cardinals are toggled on under Tools > Options in Include Critical Sections.

At Vertical Crests & Sags (high & low spots) - These locations on vertical curves along the roadway need to be added as a Key Station otherwise they will be passed right on by during modeling based on the interval applied.

At Horizontal & Vertical Event Points - These are clearly based on the appropriate Tools > Options toggle.

To wrap this up, my suggestion is to toggle on the option to use External Control Points under the Roadway Designer Options. And only toggle it off when it makes sense.

Keep improving!!!

Civilly yours,

- zen