Join Eric Chappell for an in-depth discussion in this video Understanding grading objects, part of AutoCAD Civil 3D: Site Design.
- [Instructor] In this chapter, we're going to study grading objects. But, before we get too deep, let's just talk a bit about what they are and how they behave. In this drawing I've got a grading object and you're actually looking at the same grading object in both views. In the left view we're looking at a plan view, top down, with a pretty simple visual style. Just your standard CAD style. In the right side, we're looking at it in a 3D perspective and I've got shading turned on so that you can see it a bit better. And the existing ground surface is shown as a grid so you can get a better idea of its shape.
If you can read contours well, then I think this probably makes sense to you. You can see that we're kind of in a ravine here, a depression, but the grid will help you over on the right side to see that as well. So, this green, these green lines, the shape that you're seeing is a grading object. And how it's created is, you start with a feature line. And the feature line is this orange shape here. And that feature line has its geometry, its shape, and also its elevations.
And when you create a grading object, what you do is you project slopes from that feature line to a target. In this case, the target is the surface. So it's projecting a slope outward from that feature line and it's trying to find an intersection along that slope with the surface. And this happens to be a 3-1 slope. Now, the slope is defined by grading criteria.
And right now the grading criteria in play says that, when I'm in Fill, I want to use a 3-1 fill slope and when I'm in Cut, I want to use a 3-1 cut slope. And you can actually assign them differently. Maybe you want your cut slope to be 2-1. There are all different kinds of combinations that we can do and we're going to cover that in an upcoming video, but just to name a few, you can project along a certain distance. You can project to a certain elevation. You can project to an elevation difference.
Quite a few choices there as far as what your target is and how you define your slopes. Now what makes grading objects so powerful is that this was all calculated automatically. Along these imaginary lines of slope Civil 3D has gone out and calculated every individual point along this perimeter to figure out exactly where this slope intersects with the very irregular geometry of the existing ground surface. This takes heavy duty calculations and would take forever to do by yourself by hand, so it's a really powerful, really powerful feature of Civil 3D.
And then on top of that, it's also dynamic. What I mean is this. If I were to go in and change this feature line. Let's say we use the Raise Lower command to lower it down 25 feet. I'm going to type in a minus 25 here. Civil 3D is going to recalculate all of those tie in slopes based on the new elevation. And you're actually going to see this thing flip from green to red, indicating that I'm now in cut instead of fill. And if you look at the 3D view, you can see how the shape is now beneath the surface and I'm projecting upward and outward at a slope of 3-1 to re-find that intersection with the existing ground surface.
Not only does it work if I change elevations, but if I edit the geometry in any way. In fact if I, for example, if I take this feature line and I'm going to move it over into this slope, into this steeper area and you'll see that we have a much deeper cut now on the left side of the feature line than we do on the right. It has automatically recalculated the slope based on the new position of the feature line. If I changed its shape, if I made it bigger, if I rotated it, moved it.
All of those operations would automatically trigger the grading object to recalculate its slopes. And that dynamic activity makes it ten times more powerful than the fact that it did the calculation initially in the first place. But what's unique about the grading object is that all of these feature lines are grouped together and they behave as one object, one thing. But there are still feature lines embedded in there. So, if you do things like draw feature lines across a grading object, you can get that interaction that we talked about earlier.
So, you just need to know that grading objects are a feature line derivative and they do understand sites. They work within sites. They even interact with each other. If one grading object runs into another they will try to kind of clean each other up and find intersections between each other's slopes. So they're very powerful, very sophisticated and very useful objects that you can use to perform grading design in Civil 3D. And in this chapter, we're going to learn all about them. So now you know a little bit about what grading objects are made of and what they do.
- What is site design?
- Creating feature lines
- Breaking, joining, trimming, and extending feature lines
- Adding and removing elevation points
- Raising and lowering feature lines
- Creating grading objects
- Building a grading scheme
- Creating curbs and parking lot surfaces
- Labeling a grading design
- Creating a storm design
- Adding inlets and pipes to a storm design