Scaled Length vs Unified Length, 3D length-Drainage

Question

For ORD Drainage design, what is the difference between Scaled length vs 3D and Unified length? Which length value is being utilized to compute the system results?

Jonathan Smith · Answer

There's some history in these settings. Once upon a time, your drainage layout might not have used accurate coordinates for the positions of nodes - it might have been more of a schematic representation. In that case, you needed to be able to set the length of a conduit - hence the "Has User Defined Length" property. You could set this to true, and type in a length. Therefore you have two properties that define the length, because it can be either User Defined, or the distance between the two nodes, which is known as the Scaled Length. 
 You wouldn't want to have to keep track of which one was being used for annotation, so the Length (Unified) property always tells you the length that is being used in the calculations. This is the "centre to centre" length between the two nodes. 
 Length (Construction) is the "clipped" length of the conduit, between the connection points at the nodes (often the inner faces of the walls of the nodes) so this is the length of conduit that will be constructed. 
 The User Defined Length may also come in useful in the case where you are doing an unsteady-state analysis, with very short conduits. These can be unstable, but that's a different topic.