When you model a curb and gutter for a parking island that has right angle corners (Internal or External corners), you will find that you should model the right angle corners as follows to get the elevation deltas of the TBC from the Flow Line to be exactly e.g. 0.5’
Change the Vertical Design Property Cross Slope Mode to Quick Mode and set a Quick Mode Density of e.g. 5.0’
In the diagonal of the corner, put a Free Dz instruction using a 0.5’ elevation delta. This will lock in the corner at exactly 0.5’ above the Flow Line.
Anywhere along the inbound and outbound lines from the corner you can place your Cross DZ instruction using a 0.5’ vertical offset. This will lock in the elevation delta between the TBC and the FL strings.
You will need to do this on each of the angular corners of the island. On curved corners you should not have to do this at all.
The reason for this is as shown in the diagram below