Using Feature Lines for grading in Civil 3D has been an area of the program that has continued to evolve with each release. Feature Lines in Civil 3D were originally based on the concepts and tools that many of us used in Land Desktop by grading with 3D polylines and then adding functionality that we could only wish for in LDT.
One of the scenarios that users always tried to avoid when grading with 3D polylines was creating 3D polylines that crossed which in turn created crossing breaklines. If you were careful and made sure to snap those 3D polylines together at the point they crossed so that the lines were crossing at exactly the same elevation then you were OK. However, this was a scenario that was a common problem for many users.
With Feature Lines in Civil 3D you can now safely create crossing Feature Lines and you may even want to do it intentionally once you learn how to control them. First you need to know that Feature Lines are included in Sites in Civil 3D. This means that Feature Lines will interact with other Feature Lines that are included in the same Site. (If you don’t want them to interact with each other put them in different Sites.) So, if you create two Feature Lines that cross each other a Split Point will be created at the intersecting location. This Split Point insures that the Feature Lines cross at exactly the same elevation.
Now that you know a Split Point is created you are probably wondering what elevation it is created at and which Feature Line it is added to. To answer both of those questions Autodesk uses what they call the “Last One Wins” rule. Basically, which ever Feature Line was created last is the one with priority. The Feature Line that was created first and is crossed by the new Feature Line has the Split Point added to it at the elevation that it crosses the new Feature Line. This modifies the slopes of the segments on the original Feature Line that connect to the new Split Point.
To control the priority of crossing Feature Lines, and override the “Last One Wins” rule, start by assigning different Styles to the crossing Feature Lines. Then go to the Prospector, expand the Site, Right-Click on the Feature Line collection and select Properties.
Select the Options tab in the Feature Line Site Properties dialog box. Here you can adjust the order of priority the Feature Lines are given for Split Point Resolution according to their Style.
Finally, the best way to understand crossing Feature Lines is to give it a try. You can use a very simple example. Start a new drawing in Civil 3D using your Template. Then draw two Feature Lines using the Grading >> Draw Feature Line command. Draw the first line vertically with an elevation of 0 and the second line horizontally with an elevation of 10 so they cross. Now examine the Feature Line by using the Grading >> Edit Feature Line Elevations >> Elevation Editor command.
You will see the Split Point shown on the vertical line as displayed above, but not on the horizontal line. This is because the vertical line was drawn first. You can also try assigning different styles to the two Feature Lines and changing the order of the styles in the Split Point Resolution settings to see the changes after changing the priority of the Feature Lines.