Grading groups are a quick and easy way of placing sloped surfaces in your terrain model. Unfortunately, there are people out there who believe that they are the work of the devil. The purpose of this post is to provide you with a few best practices when working with these tools. The proper use of these tools will enable you to create high quality terrain models for your projects.
Should I or Shouldn't I?
Grading groups should be used when consistent slopes need to be projected from a graded feature line. Grading groups enable us to keep a site feature with slopes modeled according to criteria when the graded feature line is modified. Grading groups can be used to define graded site features such as basins, berms, swales and ditches, curb and gutter in parking lots, daylighting, and building pads.
Sometimes When We Touch
Consider a drawing containing the three grading groups below. These three groups are in the same site. The groups are generating individual surfaces. The intent was to keep them self-contained for volume analysis; however when we applied the "grade to surface" criteria around the perimeter, the grading groups touch.
Even though the grading criteria's instruction was to grade to a specific surface, when the gradings began to intersect, they stopped at that intersection. The resulting feature lines created at the daylight line clean up at the intersection and, like feature lines are supposed to do, share the same elevations at the intersection:
If these features were located this close together on a site and the proposed slope grading overlapped, having it clean up like this is helpful. However, if we were analyzing two different building location scenarios, having an overlap in the same site could adversely affect our volume calculations; in this case, we should place them in separate sites.
We should avoid overlapping grading groups in the same site unless we know the grading will be constructible. The old adage "Garbage in, garbage out" applies here. When grading groups are not cooperating, they will break.
Keep Your TIN Lines to Yourself
When grading group surfaces are created, the outermost feature forms a type of boundary. When multiple grading groups are pasted into a surface, TIN lines will not be generated from one grading group to another.
However, as we place data into the surface between the grading groups, TIN lines will generate from the edge of the grading groups to this data. So, feature lines and/or points could be used to define transitions between grading groups.
Partners in Crime
Grading groups can be influenced by feature lines. If there is a feature line that crosses a grading group and is in the same site, the grading group will project to meet the elevation of the feature line and adversely affect your grading group surface. Sites are a great way to break up the partnership.
We use the Delete Grading command because that's what it's there for. Selecting any part of the grading will activate the contextual ribbon and expose the Delete Grading button. Using that button ensures that all of the grading components in that grading object will be eliminated. Using AutoCAD erase sometimes leaves pieces behind.
All the King's Horses & All the King's Men
One great thing about Civil 3D is that UNDO works… on most occasions. UNDO is an AutoCAD command and AutoCAD doesn't necessarily understand how to put Civil 3D objects back together. So if we accidently delete a grading object and use UNDO to bring it back, we may not get all of it back.
Who Makes the Rules?
Moving or rotating a grading group is best done by moving the matriarch, the original feature line. Grading groups and surfaces will follow.
We should save criteria often used in our templates. Recreating criteria over and over sucks. Editing generic criteria like Grade to Distance will generate that change to any grading groups in the drawing with that criterion applied.
In the Interim
I love creating interim surfaces with grading groups when I need to derive elevations for parking islands. The purpose of an interim surface isn't necessary to become part of a bigger surface but to provide elevations for feature lines. So if I wanted to apply a 2% slope across this parking lot, I could create a grading group surface graded out to a distance of 200' at a 2% slope. Then I could apply the elevations from this interim surface to the feature line parking islands.
If you are working on that 120 acre site, with those twelve massive building footprints, each with their own parking lot configuration, and a network of roads rolling over the rolling terrain, be very memory conscious. Unless you have an UBER-workstation, model your roads in a roadway source file and model your building sites in individual source files. When there are lots of things talking to each other in one file, that's when things start breaking. This is not because the program is buggy, it's because the workstation is buggy. It becomes buggy when you give it too much to do. The 32-bit workstation running the 3GB switch will need to have data spooned-fed to it in small doses. The 64-bit workstation with 16GB of RAM will want to swallow it whole.