The invert level of an existing pipe is the result when you subtract the depth/height of the lowest elevation/level of the inside/inner portion of the existing pipe from the surface reference elevation most commonly known as benchmark (BM). Benchmark (BM) can usually be found on permanent structure in the surface/ground.
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Invert Level is the bottom of the pipe, reduced level is the middle of the pipe.
In civil engineering, the invert level is the base inside level of a pipe, trench or tunnel the carries liquid, considered the "floor" level. It is used to determine the hydraulic gradient.
The invert is the bottom of the inside of the pipe. It is the major level reference level when installing pipework of civil works, ie storm water etc.
A manhole invert is the pipeline running across the bottom of the manhole, from the point where the incoming pipe enters until the outgoing pipe discharges the outflow. It is frequently expressed as an elevation level where this pipeline sits.
Invert elevations can usually be found directly on as-built drawings of the sewer facility. The simplest way to calculate an invert elevation of a manhole is to measure the depth of the manhole from the surface to the pipe invert. The invert elevation equals the ground or rim elevation minus this depth. If you do not know the rim elevation or invert elevation, a surveyor must measure the invert and/or rim elevation with reference to a nearby elevation benchmark (see: http://commons.wikimedia.org/wiki/Image:Elevation_Benchmark.jpg). Also, upstream and downstream invert elevations in a manhole typically differ by about 0.1 feet.