Civil 3D has long been able to drape (or project) feature lines onto a surface. This is a valuable grading tool and a great way to apply elevations to complex feature lines by creating a surface with the correct slopes using more simplified geometry. For example, a parking may need a 1% slope, but the curb around it has many curves and complex geometry. Calculating the elevation of each point on that feature line would be tedious and time consuming. Instead, draw that curb line in 2D with simple, familiar drafting tools, then create a simple surface with the correct slopes using basic geometry as breaklines for that surface. Finally, drape that simple 2D geometry over the surface and let Civil 3D calculate the elevations for you. This is a great tool. The only down side is that if there is a change needed to the original surface, the feature line does not update, you have to drape the feature line again. In the previous example if the required slope changed from 1% to 1.5 percent, you would need to update that feature line.
Recently Civil 3D added the ability to link the feature lines to a surface. That makes this process totally dynamic. When the surface changes the feature lines linked to it will automatically update. There are a couple different places you can set this up.
First, if you create a feature line from objects, you have the option to assign elevations. Then you can use the option to get the elevations from a surface and enable the option to make it relative to the surface. You can even set a value to keep the feature line a given distance above or below the surface.
But if you set the Relative Surface it then shows the actual elevation, but grayed out since it is linked to the selected surface. You can change the relative elevation here to raise or lower the feature line relative to the surface.
This makes the process of grading with feature lines dynamic and can be a big time saver. If you haven’t used this feature yet give it a try on your next project, it might just take your grading to a whole new level.