Civil site plans are full of elevation labels for a whole variety of crucial points. Included among these are the spot shots for the face of curb, a point two elevations; the top face-of-curb, and the bottom face-of curb. By definition of the underlying surface objects, points can have but a single elevation within AutoCAD Civil 3D. So the mystery becomes just how does one label two elevations for a single point?
The simple answer to this question is through the use of Expressions. When building your companies template, you may have noticed the option to define Expressions for many Civil 3D objects. Expressions allow us to accomplish all sorts of wild and crazy things, but in the simplest of terms, they let us perform a calculation and save the result as a property I can use elsewhere in my drawing.
In the case of curbs, we know they have a fixed height – typically six-inches. Knowing the height of my curb, we can establish a relationship between our surface elevation and the top or bottom elevation of our curb. If my surface elevation represents the flowline, or bottom-of-curb, adding six-inches would give me the proper top elevation. By contrast, subtracting six-inches from the top-of-curb elevation results in the correct bottom-of-curb elevation. How you model your surface will determine whether you should add or subtract from the base surface elevation.
In either case, Expressions are the tool to use within Civil 3D. In the video above, I share the process of creating an expression that subtracts six-inches from my base surface elevation, and how to add the result of that calculation to a spot elevation label.
Do you or your team use Expressions to construct your Civil 3D drawings? Let us know what creative uses you’ve imagined in the comments section below.