(pipe technology) The top point of the inside open section of a pipe or box conduit.
The soffit is the highest point of the internal surface of a pipe or culvert at any cross-section. The soffit is also referred to as the pipe obvert.
So it is not quite the Crown of the Pipe. Here is an image I found that hopefully explains it better.
via Blogger http://www.swmm5.net/2013/12/soffit-level.html