In your experience, what magnitude of +/- error would you assign to the average pipe measure down? Assuming that it is made with appropriate care and attention. I'm thinking of a long existing pipe, not new construction. And 10+ feet deep, if that makes a difference.?ÿ
If you are measuring to the invert of the pipes (invert in/out) I would expect to be within 0.05 feet without some type of obstruction or an offset cone.?ÿ If you are measuring to the invert in the middle of the manhole I would expect no error unless the cone is offset.
Andy
10'+ deep .05' is definitely reasonable. Shallow manholes 3' to 5' deep can easily get a .10' of error unless you actually go in. It can be difficult because of the increased slant measure to the inverts.
At one time, I actually tried to define how accurate I was without entering. I ended up in the 0.05' range plus or a minus. But, that was about repeatability as well. The WAY you do it matters. There are so many things to consider: Are you actually on the invert? Is there anything under the rod? Etc.?ÿ
I would be far more comfortable rounding IE's to the nearest tenth, but that is not accepted.
And I agree a shallow manhole is typically more difficult.
I always reduce the hypotenuse (raw distance reading). It helps a lot.
Frankly guys, I think 0.05' is a bit optimistic. I read the rod to the nearest 0.05', but that's not definitive of error.
I'm currently engaged in surveying a run of Sanitary manholes. I happen to have a topo map in hand done by local company. I used to work for them and know that they do good work. Average at worst, usually better.?ÿ In the area they covered I'm using their vertical control, which was established using a digital levels (per a survey they filed).
I'm matching their rim elevations in the +/-0.05' range. Inverts in the +/- 0.1'- 0.2' range. Furthermore, I've got as-builts from 1958 on a few that are over a tenth off both of us.
This is not the first time I've had a similar experience. I've had projects were crews inadvertently overlapped each others work. The measure downs routinely disagreed by more than a tenth, sometimes a good deal more.?ÿ ?ÿ ?ÿ ?ÿ?ÿ
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I say it all boils down to methodology. You are at the mercy of the type of structure you have to measure. If you have run of manholes, whatever your magnitude of error happens to be, the percentage of flow should make sense. The aberrations in long-existing pipes will throw an additional kink in your measuring. I don't know if you can quantify such a thing.
I would be far more comfortable rounding IE's to the nearest tenth, but that is not accepted.
I usually round them to the nearest 0.05', even though I know they're not always that accurate.
It depends on the project.?ÿ If I know that the drop is critical I'll bear down and do the best I can within the realm of available equipment and methods.?ÿ But for a run-of-the-mill topo of a site where I have 8 tenths of drop over five hundred feet of sanitary sewer (an example from a recent topo), I'm happy just stabbing the rod in the middle of the manhole and reading the intercept of the hook laid across the opening.
If it is so flat that .05' makes a difference then it probably needs a new system anyway.
Safety concerns aside I know they got a lot less accurate when we stopped going down them.?ÿ
It surprises me that no companies stepped up to the challenge and got the gear and training to safely obtain interior measurements.
I am still waiting for some sort of simplistic scanner to be deployed as the solution.
Company to company there can be a huge difference in how much effort is afforded getting good measurements. When I started we would go to field with all publicly available as-builts and really go the extra mile to solve the system, at my next company I was promptly informed that anything more than two minutes a manhole was a waste of time and to deploy the blocked or could not open notation when needed to pass the buck.
A dual prism rod is the way to go for shooting them, in my opinion, but most surveyors can't afford specialty tools unless they have electronics so very few people use them.
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It depends on the project.?ÿ
That right there is key.?ÿ If you're doing an actual sewer or drain project, then it's more critical.?ÿ If you're doing a gas or water project, and they just want to know where the existing utilities are so they can plan the new line, then it doesn't necessarily have to be gnat's ass.
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I've seen guys try to be super careful and accurate, reading to the hundredth, and then I look at their readings and it's looking like the flow is going the wrong way.?ÿ And I can see that the inlet is above the standing water, and the water is right at the outlet invert, and they got it wrong.?ÿ So, there's that
In this survey I have 2 places where runs between manholes are under 20 feet. There is flow apparent, so I know there is at least minimal slope, which would mean perhaps 0.05' feet of fall between them. But the raw measure down numbers don't show it, due to the precision of the measurements. I'm going to have to apply some judgement in order to keep some engineer's head on his shoulders.?ÿ?ÿ
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.?ÿ 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.?ÿ 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.?ÿ 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.?ÿ ?ÿMy 2 cents, Jp
@jp7191?ÿ Well said. As an example of those imperfections, look closely at the concrete trough invert in the bottom of a manhole. It isn't a nice smooth surface, more like a wobbly one. I take a few dips, get a feel for the average, and call it good.
@dmyhill?ÿ So that is why I get -5' at the inverts and only -3' at the center line trough.
Curious about your statement...if you match the rims to 0.05, and the inverts to 0.2, how do you match the "measure down", if you were to reduce that from their data??ÿ It seems like you are adding your disagreement to the rim to the disagreement on the measure down.
Just curious.
It was eye opening to the engineering intern that worked with me for a summer...(we staked and then asbuilt a plat full of minimum slope sewer)
Even more is to watch how the MH channel gets created in the first place. A laborer goes down there with a mortar board and a trowel. He muds it in and makes it looks good. But at no point is a straight edge involved.?ÿ
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.?ÿ 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.?ÿ 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.?ÿ Good to the nearest degree or so.?ÿ 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:
