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posted 01/17/07 by Anthony Governanti Site Topology Part 2; Grading

Ok, it’s been a while since I posted on Part 1, but I hope you can forgive the major interruption in service here. I’d like to take a moment to talk about Grading, and how it fits into our site topology discussion.

When we discuss grading in general, we need to separate it into two categories of features within Civil 3D; Grading and Feature lines. Grading is the use of the Grading layout tools to create the intelligent grading projects and groups that Civil 3D can use to design site grading. A Grading (capitol G) uses a feature line to start with, and then applies a grading criteria to project to different targets. These targets can be a surface, a distance by slope or grade, or an elevation by slope or grade, or to a relative elevation by slope or grade. Multiple Gradings can be grouped together into a… you guessed it Grading group, so that any change to one Grading will ripple or update the other Gradings.


View image
Here we see a simple Grading; note the green dashed line is a feature line, the projection has a diamond glyph, and the target line is another feature line.

Then we have Feature Lines, which are essentially 3D polylines, but with some extra data and behaviour attached. With the introduction of Civil 3D 2007, we were given a whole bunch of powerful feature line editing and creation tools. It is these tools that make feature lines very powerful, in that you can get most of the "grading" needed for a typical site done with just the lines, and then finish it off with the Grading tools.

So this all sounds well and good, but if any of you have tried to use the grading tools, you will have run across some problems at one time or another.Most of the issue that 99% of Civil 3D users run into when attempting to grade is breaking the site topology without realizing they did, and thus either creating a bad grading, or worse, a fatal error. Now I’m of the opinion that a lot of this is our fault (meaning Autodesk) because we should be trapping those errors and notifying the user of the problem, or better yet, should make it so it doesn’t error in the first place. Now all wishful thinking aside, what do we do to try and get our grading to work without a topology problem?

Suggestion #1 - Separate feature lines that may cross.
The first rule in our topology is that geometry is aware of other geometry and in the case of Grading or feature lines, if they cross, we could have issues. The issue is that a vertical point of intersection will be automatically added to any feature lines that intersect, at the point of intersection. And since this happens automatically without the user really knowing about it, this will cause blown shots in the grading, as the PI is set at zero elevation. To avoid this situation, create or move the feature lines that are crossing into separate sites.

Suggestion #2 - Separate grading groups that intersect.
The second issue is that Grading target lines that cross other target lines or feature lines may stop the projection from happening, giving an incomplete grading, or making it look like a solution wasn’t found. To make sure they still project, separate the grading groups into different sites as well.

Essentially if the gradings or feature lines don't need to interact, then make sure they don't by using different sites. The Site in Civil 3D is basically a bucket that you put geometry in that needs to interact or be aware of each other. Conversely, if that geometry needs to be seperated, then you put them into different buckets, or Sites. By adhering to these two suggestions, I think you'll find that grading will work a lot more for you, and you'll avoid "weirdness" with the feature lines getting zero elevations.

Site topology is something that we don’t really know is there, but having a better understanding of it can help us make better decisions as to how to setup a design in Civil 3D, and in the end avoid issues or problems that we may not be aware are there. I hope this information is useful to you; happy grading!

Till next time,
Cheers,
AG

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