Have the Terrain>Add boundary> Extract Graphics command generate a shape and not a line
The command Terrain>Add boundary with Setting "Extract Graphics" generates a line string. It should be a closed entity (shape). usually these entity will will used as a clip/mask later one, and will need to be turned into a shape so why not gen...
Able to convert 2D contour lines to 3D contours for terrain creation; Or directly convert 2D contour lines to 3D terrain. Old product Bentley Architecture has function: Elevated Contour fixed this issue.