So here’s the scenario, you have submitted plans for a sewer line (or profile) and a comment comes back that your pipe slope labels are wrong. You go through and check them and find out that you are getting the same results as the plan checker. You think Civil 3D is stupid it can’t even calculate pipe slopes correctly. Well the real problem is located between the chair on the plans on the plan checker side and between your chair and the computer on your end.
It seems somewhere along the way some of us forgot about significant figures. The basic concept of significant figures is that the resulting number can’t be more accurate than the least accurate number used in the calculation.
The problem usually comes up when the agency requires more significant figures in the slope label then in the Grade Break labels. In this above picture if we calculate the slope based on the grade break information:
(PVIE – PVIE)/(PVIS-PVIS) = slope
The best accuracy we can get from the slope would be -0.33%. If you do the math you will find calculating the slope based on the grade breaks is -0.3284746%. This number is obviously different than the slope Civil 3D is showing.
The only way we should be confirming the accuracy of the Civil 3D label is using this formula:
((PVIS-PVIS) x slope) +PVIE = PVIE
This way we are using the available values to ensure our results meet the requirements of the significant figures provided on the plans.
Once completing this explanation of the issue with the plan checker and they don’t agree. The only accurate solution is to increase the precision of the grade break labels. Creating a label to get the slope label to be correct may cause problems later on. Especially if the contractor builds the project off the slopes provided. For long lengths the ending PVIE will be incorrect value.