Limitation of Synced Road Models in Civil 3D

In MiTS, we allow users to sync the road models to Civil 3D as corridors via our MC Integrator. Step-by-step sync guide can be referred for detailed instructions on road model sync from MiTS to Civil 3D.

MiTS and Civil 3D project file sample

What is the issue? #

While the sync is successful and all your road design in MiTS project file is correct, you may find that your cross section in Civil 3D somehow generated differently as compared to MiTS at some parts. And this difference may not be immediately obvious at first. Refer figure below and notice the protruding slope line highlighted in red circle.

From the cross section comparison above, we detected that the slope generated in Civil 3D showed incorrect behavior where the slope extends past the intersection with the ground level (target surface). This error refers to daylight overshooting where the slope line (daylight) is supposed to stop exactly at the target surface but instead, it goes beyond the surface and misses the correct intersection point.

This daylight overshooting error also causes the cut and fill volume computation in MiTS and Civil 3D to have significant discrepancy. This discrepancy is caused by the extra area of cut or fill generated due to the daylight overshooting error.

What causes the issue? #

If the synced road corridor from the MiTS project file is observed in Civil 3D, you may notice that parts of the road corridor have unusual orientation.

This is the bowtie error in Civil 3D where corridors overlap or self-intersect with each other instead of forming a clean mesh. The bowtie error in the corridor leads to distorted triangulation that eventually causes the daylight overshooting error.

Once the road model in MiTS is synced to Civil 3D, notice the error listed under the event viewer. If this “Mask ‘0’ NOT added to corridor surface ‘CorridorSurface-RoadDatum/CorridorSurface-RodTop’ as boundary due to crossing polygon” type of errors are listed, most likely that your corridors have this bowtie error.

So whose fault is this? #

This is a bug on the Civil 3D side, which is yet unresolved. MiTS behavior is more correct and can handle this kind of situation better than Civil 3D.

How to solve? #

But still, you can remodel in MiTS to ensure that you sidestep the Civil 3D issue. For example, you can change the spiral length setting to a smaller value (not necessarily have to be 0).

However, depending on your road design, it might not always be the same case all the time. If you find that changing the spiral length does not solve the issue, there might be another issue that is triggering the bowtie error in Civil 3D. It is worth noting that even though your road design has no error in MiTS, the error on bowtie might still occur due to limitation in Civil 3D.

Comparison between MiTS and Civil 3D #

With the above limitation observed in Civil 3D, it can be seen that MiTS approach in road design is correct. The road modelling in MiTS follows the alignment design based on established design standards, where parameters such as design speed, minimum curve radius, and minimum spiral length are defined according to the design guidelines.MiTS don’t produce “nonsense” corridor like Civil 3D.