Leveraging Corridors and Feature Lines

Over the past few releases of AutoCAD® Civil 3D®, there has been a lot of emphasis on corridor modeling, corridor modeling techniques, and advanced modeling workflows.  Much of this is because the Department of Transportation is getting deeper into Civil 3D workflows and larger companies are pushing for things to behave a certain way.  But even for those doing “simple” site plans and subdivision roads, Civil 3D 2019 has a great new way to increase your productivity and help streamline your workflows.

This article walks through a couple scenarios for when you could possibly use feature lines in your corridor modeling.  If you would like the data used for this article, please email me and I will send the files.

New in Civil 3D 2017

Let’s start with a couple features that came out in Civil 3D 2017.

In addition to alignments and profiles, feature lines can now be used as corridor baselines. Feature lines can also be selected when creating the corridor and when adding baselines (Figure 1).

Figure 1

A parking lot is a good example of where to apply this new workflow. When I design a parking lot, I typically design the edge of pavement in order to see true slopes of asphalt. I then use the stepped offset command to give me flow line and/or back of curb elevations, and sometimes I may use grading objects to keep some level of dynamics.  But now you can add the feature line to a corridor along with a curb & gutter assembly and have a much more dynamic and accurate surface.

An intersection is also a great example of using feature lines within your corridor model. Many people feature line intersections, but up until now it was more of a manual process and not very dynamic.  Extracting and targeting these feature lines is very simple and easy to use in corridor modeling (Figure 2).

The functionality of extracting feature lines, and keeping them dynamic, has been in Civil 3D for as long as I can remember.  However, this functionality has now been enhanced with many new options.  The options for selecting which feature lines to extract include:

  • Extract all feature lines at once
  • Select them one by one
  • Define Station Range
  • Select a subset based on a corridor region or a simple polyline

Figure 2

By selecting a polyline, you will then be given the option of which feature lines to include/exclude for the extraction procedure.  Select the Settings option to choose whether or not to dynamically link them to the corridor.  You can choose to add them to a site, or now use the “Siteless” feature line options.

The Assembly

We will need to create an assembly for our curb & gutter.  We will use this assembly for both examples. This will typically be a simple assembly, and can even be a copy/modification of your corridor’s full assembly.  Figure 3 is a screenshot of what I will be using.

Figure 3

Intersection Workflow

Yes, we could use the intersection tool to create this, but sometimes that doesn’t accomplish what we want to do. Sometimes you need just a bit more control, and feature lines will allow you to do so.

Let’s take a 4-way intersection, for example.  I first start with extracting the centerline feature lines as a dynamic link to either my profile or corridor.  In this case I chose to use my FG profile as the feature from which to extract a feature line. (We will keep both crowns maintained.)

1.  From the Home tab > Create Design panel > Feature Line drop-down > Create Feature Lines from Alignment (Figure 4)

Figure 4

2.  Select your alignment, then select the profile you wish to dynamically link to. You will then have a dynamic feature line—therefore, when your alignment and/or profile changes, your feature line does as well.
3.  Create your feature lines along the Lip of Gutter (Edge of Asphalt) using either a polyline and create from objects command, or manually draw in your feature line.
4.  Set the grades as you would like along that feature line. For this example, I will assume you know how to do so. 

The example I am using would look something like Figure 5.  The corridor has already been started, with gaps for the intersection area to be modeled.

Figure 5

Once we have our assembly created and our feature lines in place, we are ready to begin modeling the intersection.

5.  Select the Corridor, go into your Corridor Properties and select the Parameters tab.
6.  From the Parameters tab, select Add Baseline.  The Create Corridor Baseline dialog box will appear, select Feature Line from the Baseline type (Figure 6).

Figure 6

7.  Use the icon next to the Feature Line drop-down to select your feature line from the screen.  Once selected, you may be prompted to name the feature line, I chose to name mine NW Quad INT 1.  Select OK.
8.  The Baseline is now added to your corridor and we need to add the region and select the assembly.
9.  Right-click on the new baseline and select Add Region.
10. Choose your assembly, and select OK. Rebuild corridor if needed.

This will not put in the assembly along that feature line. And no need to do a stepped offset!  Pretty good so far, right?  Let’s finish up the intersection quadrant by targeting our centerline feature lines for width and elevation.  I use the contextual ribbon for some very useful shortcuts. Use these to really speed up your corridor modeling workflows!

11. Select your corridor from the screen.
12. From the contextual ribbon for said corridor, select Edit Targets from the Modify Region panel (Figure 7).

Figure 7

13. Select within the region for the intersection.  The Target Mapping dialog box will appear.
14. We first will select the targets to set our Width.  Select None next to Width Target for the asphalt portion.
15. Set the object type to Feature Lines, Survey Figures, and polylines.  Then select both centerline feature lines from the screen.
16. Select OK.
17. We now need to select the same feature lines for the Outside Elevation Target. Select None next to Outside Elevation Target and follow the same steps as outlined above for setting width targets.
18. Select OK to exit the Target Mapping dialog box and Select OK to complete the Corridor edits.

Repeat all the above for each intersection. Remember to use your contextual ribbon to assist.  There is a Match Parameters command that is very helpful in this case. This will allow you to set similar targets without going through all the same steps.

Your resulting intersection should look something like Figure 8.  Keep in mind, you may need to reverse feature lines from time to time as well.

Figure 8

Site/Parking Lot Workflow

This is a question I probably get at least once a month:
“Should I use corridors for parking lots?”  Before now, in order to corridor a parking lot, you would end up with numerous alignment, profiles, assemblies, etc.  It just wasn’t worth the hassle of managing all that data, and a lot of times resulted in a lot of re-work.

Now for parking lots of any shape and size, you can simply add each feature line to the corridor as a baseline (not separate corridors) and apply the correct assembly.  You also get the extended data a corridor offers with different materials and surface—something that can’t easily be done with just feature lines and stepped offsets.  The workflows are practically the same as I outlined within the intersection example, so I won’t repeat everything, but a quick outline of this new workflow is below.

1.  Add features lines to corridor as a baseline and apply the assembly.
2.  Repeat as needed for all curbing.
3.  After the first couple are in place, create a surface from your corridor model for the finished grade.

Figure 9

4.  As you add additional baselines, you can see your parking lot take shape!  You can allow the surface to tin from island to island, or even target using an asphalt subassembly.
5.  If the surface/corridor seems to be upside down, you simply need to select the feature line and use the reverse tool from the contextual ribbon (Figure 10).

Figure 10

6.  One common issue in corridor modeling is inside and outside corners. Avoiding “bowties” and “non-mitered” issues has always been painful.  Inspect some of the island and parking areas to see how Civil 3D has now fixed the majority of those issues.

Try this on your next site design to really see the benefit of using corridors in this situation. I would love to see your final product!


There are many ways to model a corridor and corridor intersection. Some like to create alignments and profiles, some like to use the intersection tool and modify from there, and some just ignore the intersection (I have from time to time).  This was my attempt to display a workflow that you may try and see if it fits your needs. So hopefully this gives you a little direct to at least give the feature line as baseline a shot within your corridor modeling workflows.

As always, I am interested to hear what you think and see how we can improve upon this topic.

Good Luck and Happy Modeling!!

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