What is the term used for the inside bottom elevation of a sewer line or drain pipe?

The term used for the inside bottom elevation of a sewer line or drain pipe is "invert." This designation is crucial in civil engineering and construction, particularly when dealing with drainage and sewage systems, as it indicates the lowest point within a pipe where fluids can flow. Understanding the invert elevation helps engineers and contractors to design systems that ensure proper flow and minimize issues like blockages or backups.

On the other hand, the term "benchmark" refers to a fixed point of reference used for measuring elevation; it does not specifically relate to the inside of pipes. "Grade" generally describes the slope or steepness of land or surfaces rather than the internal measurements of a pipe. "Elevation" is a more general term for the height of a point above or below a reference level, such as sea level, but it does not specifically denote the bottom of a sewer line or drain pipe. Thus, "invert" is the most precise and relevant term for the context of sewer lines and drainage systems.