MH Measure downs
The more experienced I have become in surveying the more comfortable I have come with imperfections that I cannot control, with measure downs being one of them. Engineers, construction inspectors and pipe layers become very concerned when their is minimal flow but the truth of the matter is that most of the time fluids will create their own flow line as long one end is lower. The invent of pipe camera systems illustrates this perfectly if you have ever watched a camera in a system.....it travels along hitting puddles in the pipe and the majority of time nothing is done and the system works just fine. But if an inspector or pipe layer discovers a low point at a joint before backfilling and it is a major issue to work out. My 2 cents, Jp
I doubt this is any more accurate than the methods many on here are describing, but 4 months ago or so there was a discussion about this here where Wendell listed a commercial invert measuring stick that read the angle. I had to do some of this recently and, having "low overhead" (read, being cheap and poor), I'm not going to buy a fancy stick. So I used a regular stick (level rod) and the stock iOS app for leveling that comes with an iPhone and just reduced observed measurements. Good to the nearest degree or so. If I wanted better accuracy I thought I might add a pointed foot to the stick and/or use a nice digital calibrated and level strapped to it.
The thing Wendell linked to was an invert measurer:
I carry an inexpensive carpenter's clinometer I'm the truck for use in dipping manholes. It's only good for a couple of degrees, but when the invert is way back from the rim it's better than guesstimating.
I also carry a Pipe Mic, but it's such a hassle to use that I only bother if I'm doing a lot of manholes.
Many years ago I had a job locating sewer manholes (or "maintenance" holes as they are now called in this day of gender neutrality) for a small city. We duct-taped a four foot lath to the Philly rod with two feet of the lath extending below the rod so it wouldn't get all yucky. While probing I noticed a distinct drop below the inlet pipe that was about 6 inches deep at every MH. Two city workers accompanied us to open the lids. One explained that a former employee dug a "stilling well" at every inlet to catch metal items that got flushed down toilets, whether on purpose or not. He would then go into the very shallow manholes and scoop out coins, rings and other heavy items. He said the guy recovered many diamond rings that probably fell off while washing hands.
I was wondering about that. I thought 0.05 was a bit large to miss rims by, if they had good control.
To emphasize my point ........ 0.05' is a lot if you are shooting the same point on the structure. But considering the topography of the typical lid and its frame, it's not so much. Then you should be doing your measure downs to the point that you rim tied. Which, if you tie the center of the lid - which I think is very commonly done - isn't possible to do. Then there is the matter of correcting for slope and getting the rod on the actual invert. All these things are potential sources of error that do quickly add up to well in excess of a tenth, IMHO.