In a surface model TBC has a tolerance for vertical accuracy of 0.1mm. This means that two breaklines or two points that are in nearly the same location have to be at least 0.1mm apart to avoid being flagged in the resultant surface model. To be safe use 1mm and in US Feet use .01’.
In a corridor model however if you set the tolerance on the corridor model to Best and BL across (recommended) rather than default the tolerance for two secrion drops in the model to be separated by is 0.02’ in the station direction. If you use the default it is larger than that value.
This means that if you enter a vertical alignment for a wall footing that contains vertical steps, you should enter the top of the step at say station 100.00 and the bottom of the step as 100.02 in order to generate a near vertical step. If you go less than the tolerance then TBC will throw out one of the section drops when they are too close to each other.
The same applies when you use a table of elevation or delta elevation values - use stations separated by at least 0.02’ to get the desired result.
When entering offsets a vertical step laterally can be just 0.01’ or 1mm as per the surface model tolerances.