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.
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. 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.
@dmyhill So that is why I get -5' at the inverts and only -3' at the center line trough.
@lurker entering a manhole for any reason is never a wise thing to do for than a few reasons, the first being that your insurance company would be extremely unhappy about it, another being, at least in my area of practice, most are a minimum of 4' deep and, by definition, a confined space entry. OSHA would have a field day if they caught us doing that. The reason I mention it is because shortly before I started with one of the companies I have worked for, a crew chief climbed down into a 10' storm manhole and collapsed at the bottom, the IM went down to help him and they both died as a result of methane gas being present. My crews are not allowed to enter manholes for any reason, they are also not allowed to walk foundation walls. we do most new building and home construction with basements. I refuse to locate a foundation that is not backfilled.
@jitterboogie Manholes can be entered in compliance with OSHA and confined space regulations. Why does everyone assume I'm talking about entering them in an unsafe manner?
I wasn't lambasting you. I agree completely and have been OSHA trained beyond where most people even want to know.
I was just agreeing with the dont send anyone down into a manhole unless absolutely necessary. And with land surveying I can't see why you ever would descend into one.
We had a sanisew manhole that required a 35' rod to measure.
I asked the GIS tech I supervised who trained him how to get the rim elevation with that big of a triangle.....
Tyler. Tyler left before i got there.....
Tyler was a computer programmer turned GIS. Tech.
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.
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.
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. 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.
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? It seems like you are adding your disagreement to the rim to the disagreement on the measure down.
Just curious.
I'm making my own measure downs and applying them to my rim ties. Then comparing the final results to theirs. Obviously our measure downs are not the same, just as my rim ties are not. And I'm not saying that they are wrong, but rather that the typical measure down has a +/- error much greater than the least count of the measuring device.
An issue with the rim ties is that I tie a point on the rim that is closest to the center of the MH barrel. They, like a lot of surveyors, tie the center of the MH cover. The cover is not a flat plate, and the rims are commonly sloped to match the profile and crossfall of the road.
Interesting. The center of the MH plate vs center of structure makes a difference, that is certain.
I was wondering about that. I thought 0.05 was a bit large to miss rims by, if they had good control.
I was taught to shoot the rim nearest the center of the structure. Makes it easier when another crew follows behind to match my work. But, I have since learned that there are many ways that people consider to be "the right way" to measure a manhole.
Based on your procedure, I am guessing that your method is far more accurate than the previous work.
@dmyhill I was taught, and therefore taught my crews, that we shot the rim elevation over the "out" pipe. It is about repeatability. Normally the lids are within a couple of hundredths of being level if steeply canted we would shoot the lid over both the out invert and at the high point also.
Andy
Interesting. The reason we shot nearest the center was for repeatability, but to also be able to use the shot for horizontal calculations.
If you shot over the out, did you also shoot the center of structure?
As for the slope on the lid, 2% cross slope equals 0.04' drop over the lid (and it is common for there to be closer to 3.5% cross slope - match curb top at crown, which would make it 0.07' across the lid). But, none of that is relevant to the accuracy of the dip, if you always measure from your shot.
The other advantage of shooting the center of the lid, is that in a typical 48" structure, (if you shoot and measure from the center) you know two sides of your triangle, making it easy to reduce the third leg for vertical.
@dmyhill Just another procedure. We shoot centerline lid, centerline barrel, and measure point for the dips. Depending on the style of manhole this is 1, 2, or 3 different shots. Size of barrel is also determined so slant can be converted to vertical for the distance below the measure point. I have remeasured my own and others dips and typically only find .03' or less or a significant blunder. Someone is making a mistake if you cannot match within .2' on a 10' deep manhole.
@lurker I'm only talking vertical for shooting over the out pipe. For horizontal we did shoot the center of the lid.
My MH code has an attribute field for center of lid or center of structure. Depending on the clients requirements, the MH will get shot twice.
I'd like to come up with a way to shot them directly. First, I hate sticking the fiberglass rod down it the sh___t. As hard as you try you just can't sanitize that thing. Secondly, it just in not that accurate.
I've been toying with getting one of the Leica hidden point poles with multiple prisms and some long sticks rod.
Yes, everyone wants the manhole location...and getting the field notes and drafters to get that on the plans, it is usually easier to simply add in a shot for the manhole lid.
@john-putnam The fun part is when the bend in the pipe is NOT made at the manhole. There is a large (42 inch) sanitary sewer line near me that has manholes and then a joint downstream there is a precast bend. Don't ask me why it was done that way, it just was.
Andy
We always had 2 rods.
The level loop rod and the 💩 stick.
Unless you were new. You only had a 💩 stick then. And never ate lunch after dipping manholes.
🤨 🤔 😑
Yeah, I have dedicated rod for dips. I always wear surgical gloves, wash the rod down with simple green or some other cleaner and then wipe the rod down with bleach wipes. I'm know the thing is still not even close to clean.
I was taught to always measure from the most northerly point on the rim and have been doing so for decades. When I was a neophyte I reported depths to the nearest 0.01'. After I got some experience i changed to 0.05'. Now that I am old I only report them to the nearest 0.1' regardless of what accuracy the crew reports.
My thoughts are that ±0.1' can't really matter in the real world...
