Intersection w/ FLs and Supers to Change X-slope

For those of us still using Civil 3D 2009 or less:

There are lots of times when grading an intersection of two roads with steep slopes that you need to adjust a lane cross slope to make the intersection drain properly.  Lets review how to grade an intersection using Featurelines (FL) – the fastest way if you don’t need to run pavement quantities off the Corridor model, until you upgrade to Civil 3D 2010, 2011, etc… – and also lets look at a fast way to adjust road cross slopes using Superelevation points.

The first step is to extend the Corridor Model up to the curb radii.  Unless the roads are coming together at exactly 90-degree angles you will need to add a region to the corridor model and create an assembly that is only the right – or left – lane (see image below).

2009.11.23-Starting point

Additionally, in the image above you will also see I have elected to draw two construction lines connect the center of the curb radii to the point of tangency – or curb radii ends – with the main road.  These construction lines are critical for using the Apparent Intersection (APPINT) osnap to connect the curb radii end points across the chord of the curves.

Using Apparent Intersection Osnaps

To draw the chord featureline, use a 3D view to snap to the end of the Corridor model.

 2009.11.23-APPINT snap 1

Next, use a Plan view and the APPINT osnap to snap to the Corridor Featureline that is representing the edge of pavement (EOP) on the main road and the construction polyline (see images below). 2009.11.23-APPINT snap 22009.11.23-APPINT snap 3

After you have two Featurelines connecting the ends of the curb radii – drawn across the arc chord – , use the Create Featurelines from Alignment tool to create a Featurelines that represents the centerline of the road. You should end up with the below after adding these featurelines to your surface that already contains the corridor model as a paste.

2009.11.23-Connecting the ChordsLooking at the image above, you will see the left side of the road – looking up station – looks fine but the right side needs some work. At this stage it is probably a good idea to use a surface style with a contour interval of two-tenths of a foot (0.2’).  We ultimately want to be able to stake-out this curb line using the surface featurelines to create points or from spot elevations that read the surface; these spot elevations reading to nearest hundredth of a foot (X.XX’).  At a minimum, our contours should look good pulled at 2-tenths of a foot (0.2′).  At 2-tenths of a foot we can catch any anomalies like shown in the picture above.  If we were only looking at contours to the nearest 1-foot, it may not show, so we may not catch it, so our spot elevations or points could be wrong.  Consistency of accuracy, something typically forgotten from our 4th grade science class for most Civil Engineers or Techs.

Stepping down from the soapbox, the next operation is to simply:
1. draw an AutoCAD Arc
2. convert it into a FL
3. add elevations points at 5′ intervals – more or less – and
4. drape the arc FL onto the surface.

As you can see in the image below before I draped the FL I added some further definition to the surface to straighten the contours between the centerline\EOP and the chord FL.  Doing this will give me nice straight cross slope grades on my left lane.  To straighten, simply use AutoCAD plines – snapping to the contours to inherit elevations – to draw lines where you want the contours to go and then add those plines to the surface.

2009.11.23-Arc Featureline drap onto Surface

Now for the right side of the intersection:

Since this is not my first Rodeo, the issue is apparent.  The contours look all nasty because roads coming together at a -2% cross slope, and a -2% cross slope naturally want to create a low point on the curve.  However since the main road – road to the north – has such a large longitudinal slope (4%) it creates a flat area instead of a low point.  In this situation we want to have the water actually flow down to the main road cutter line and avoid placing an inlet altogether.

2009.11.23-Int Right side issueI’m not going to lie to you.  There was a little trial and error to get to the solution I’m about to present.  But that should not be a shock to anyone.  Another word for trial and error is "design".  So, looking at the picture above, of course you can see if we would drape an arc FL on the surface it would retain the same bad elevations as shown.  So, what we need to do is set a high, or low, point on the curve.  Again, in this situation we want to set a high point so water drains across the intersection to save some money on inlets and pipes.

Draw a construction line from the alignment end to the CL of the radii.  Then draw an arc FL on the curb radii.  You can use the chord FL for end snaps (see below image).

Finally, draw a FL at +2% along the construction line.  Set an elevation point on the arc FL as shown in the image below (green dot).  As you can see the contours on the left side of the arc look good.  But the right side is still a little flat.  Thus comes in the part where we use Superelevation Points to raise the right side of the road!

2009.11.23-Right radii High Point

Go into the “Alignment Properties” of the secondary road.  Then go into the "Superelevation" tab.  See the image below to set two points along the alignment to control slope.

Two things to note:
1. When you add a station location you need to change all the slopes to be negative (see image).
2. After you click "OK" on Alignment Properties you need to rebuild the Corridor manual – an automatic rebuild will not be tripped.

2009.11.23-Super SettingsAfter you rebuild the Corridor the surface will update automatically but there may be some cleanup.  See the image below for some FL adjustments you may need to perform.

2009.11.23-FL adjustmentsAfter re-snapping the FLs to the Corridor you should end up with the following.

2009.11.23-Smoothing FLs… and as you an see the intersection will drain quite nicely and we are DONE!

2009.11.23-Final IntersectionTHAT’ IT…!


Check out the below videos to see the entire process. It only takes ~9:00 minutes.  The 2010 video highlights that with the intersection tools in 2010 you get a far more complete product in the same amount of time and it is also far more dynamic to road layout changes.

Civil 3D 2009 video of the above process –>

Civil 3D 2010 video of the same process using the intersection tools –>


  1. Josh Petersen says:

    Thanks Mark!
    If there is one thing I could take away from this that I will use right away, it’s utilizing the 0.2′ interval contours. I always forget about some of the simple “customizing” tricks that you can use to your advantage.

  2. Every little bit helps…