MH Measure downs
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.
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.
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.