James Mussleman | Applications Expert
How do you create an existing grade surface when your surveyor isn’t utilizing Civil 3D? Recently I had two separate clients whose survey departments were not utilizing Civil 3D while the engineering department was. This results in an issue when the engineering department needs an existing grade surface in order to utilize Civil 3D to its full potential.
The best option would be to transition the survey department into using Civil 3D. Unfortunately, this was not an option in either case. In both the cases the clients were resorting to using the polylines that were generated from the surveyor in order to create this surface. On the surface (no pun intended) this appears to create an exact copy of the existing contours. Unfortunately looks can be deceiving. All they’ve done is create a surface that displays contours in the same place. This does not mean that all of the areas between the contours are exactly the same. If we turn the triangles of this surface on we can see how it has been constructed:
The TIN is constructed by connecting all of the vertices on the polyline to the one before and after. The issue is that this assumes it is a constant slope from one contour to the next. It does not handle depressions or mounds that lie between the contour interval. The more appropriate way to construct this survey would be from the surveyor’s triangles which are an exact representation of the TIN. I say the triangles and not the points because the triangles are the surveyor’s representation of how the points are connected. The benefit of using the triangles is that they have already been adjusted to accurately represent the existing grade. Also with this method we do not need to utilize the surveyor’s breaklines because they would already be built into the TIN. Here is an example of that same surface generated from TIN lines:
As stated previously, this will be an exact representation of the surveyor’s surface and not just the contours. The main benefit of this is that it reduces the potential error that can be introduced when using contours.
A quick note; if your surveyor can only give you the TIN in the form of lines Civil 3D can handle that as well. When creating the surface choose the option for Drawing Objects and set the object type to Lines. The last important step is to check on Maintain edges from objects.
The importance of that setting is that if we don’t check it, Civil 3D will reinterpret the triangles. By checking that option on it will keep the triangle edges right where they were.
I hope that this post will help you reconsider how you’re translating non Civil 3D survey data.
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