# Problem

Are pipe lengths and slopes based on the center-to-center distance between the structures, or based on edge-to-edge? (does it account for the width of the structure?)

# Solution

Currently, the width and length of the structure (manhole or catchbasin) is not considered with pipe length and slope calculations and is only used in the headloss calculations. Pipe lengths and slopes are calculated based on center-to-center distance. Meaning, the length and slope is based on the distance between the center-point X/Y coordinates of the nodes and the difference between the upstream invert and the downstream invert of the pipe.

In most cases, the impact that this has on the slope and hydraulic calculations is very minimal and engineers accept this assumption (especially considering the margin of error between other designed parameters versus how they are actually built). Meaning, the actual slopes in the real system from edge-to-edge would be slightly different than what the OpenFlows product shows you. Let's examine a more extreme example of two six-foot wide manholes. If the pipe between them is 100 feet long with a three foot drop, the difference in slope between center-to-center and edge-to-edge is 0.030 ft/ft vs. 0.032 ft/ft. Even if the pipe was only 50 feet long, the difference is only 0.060 ft/ft vs 0.068 ft/ft. Taking another example of a short pipe that may occur in a real system, if the length is 20 feet with a one foot drop and the structure diameter is three feet (a more typical width), the difference would be 0.050 ft/ft vs. 0.059 ft/ft

Note that profiles will still show the pipes connected to the edge of the structure but the stationing (horizontal distance) of the profile will be based on the pipe lengths.