If you import your plot plan, but your Terrain Perimeter does not convert in your plot plan, but when you try to convert the polyline to Terrain Perimeter, you did not get the option to convert to Terrain Perimeter, even though it shows that it is a closed.
Additionally, how can you make your terrain perimeter recognized so that you can convert it to Terrain Perimeter? Please refer to the attached file for further details.
Richard