I was asked to provide a cost proposal to establish rim and invert elevations on 600 or so manholes distributed across a project 7 miles long and 3.5 miles wide for purposes of hydraulic modeling. There's one NAVD88 bench mark about a mile from one end of the project, one in the middle, and one about two miles from the other end. I asked about an accuracy specification. The first response is "We'd like them accurate to 0.01 foot." I explained the difference between accuracy, precision, and reported significant figures, and pressed him about what they really need. I told him that achieving 0.01 foot at the 95% confidence level throughout the project was not practically attainable. He hemmed and hawed for awhile, then allowed that they could live with 0.1 foot.
I ran a few simulations through Star*Net to come up with an acceptable design, then prepared and submitted a proposal. I got a call back today explaining that my cost is twice what they're accustomed to paying for work of this nature. I asked if he could furnish a survey report from one of those projects to see what they've been getting, but that request wasn't met with much enthusiasm. He said that he'd talk with some of the other PMs to see if they could relax the specification, but that the project budget couldn't stand the cost I proposed.
This isn't the first time I've run into this kind of thing. It seems that there are surveyors out there who are willing to sign off on accuracy statements that they either don't understand or don't care to meet, and that one of them will likely end up doing this work instead of me.
It always happens. Thats when your negotiating skills come into play, which I am still learning. :confounded:
I have seen projects where they specified 0.01 feet horizontal AND vertical for manholes.
This is why ALL civil engineers should be required to take a survey course as part of their degree. Not to make surveyors out of them, but so that they understand methodologies, achievable accuracies, and costs associated with different levels of accuracy
The Pittsburgh area has a massive problem of combined sewers (sanitary and storm) that overload during heavy rains. EPA has mandated they be separated, and the cost is something like 3 to 5 billion dollars. About 10 years ago there was a project put out to bid to survey 100,000+ manholes. We put together a team of several firms, and bid on the project. There were 5 bidders, if I recall correctly. Four of us were in the same range. One company from another city was about 1/3 of the cost. Of course they got the project. Then they got here and realized that wow, there are lots of trees, and sewer lines are in valleys! How different and unique! So they basically defaulted, and the company (a large engineering firm headquartered here) that was hired to oversee the project then took it over and did it for about what we originally bid. The whole thing stinks (pardon the pun).
That's a helluva lot of manholes to survey. Plus it may be wise to add some densification to the project area fwiw.
As for the scope. Yep, folks want things that look nice and tight and right on paper without knowing accuracy requirements. Looks like a project where multiple crews with ample spray paint would push the job along.
John Hamilton, post: 393110, member: 640 wrote: This is why ALL civil engineers should be required to take a survey course as part of their degree. Not to make surveyors out of them, but so that they understand methodologies, achievable accuracies, and costs associated with different levels of accuracy
Risky suggestion! This is why so many of them tell me surveying is easy - they obtained perfect answers in the week they spent surveying the back yard at the civil engineering campus. They didn't understand any of it.
99 out of 100 surveyors would RTK the MH lids once, measure down with the 25 ft level rod, report the results to 2 decimal places, and everyone - including 95 of those surveyors - would proceed in blissful ignorance. Knowledge can be a burden.
So I think that I would propose in language like this:
In accordance with common survey practice we intend to perform this work in the following manner: 1) Establish survey control at approximately 1000 foot spacings along the route of the subject drainage system using RTK GPS to establish horizontal control, and differential levelling to establish vertical control; 2) tie subject manhole rims at the point on the rim closest to the center of the manhole barrel from this control using a robotic total station by a single sideshot; 3) measure down from the rim to the inverts using a levelling rod or tape measure. Results will be reported in feet to 2 decimal points.
Embellish/modify as you see fit. You might leave it at that or you might add the following:
A statistical analysis of the proposed survey methodology indicates that results of a survey performed in this manner will have a relative precision of not greater than o.xx feet.
Mark Mayer, post: 393125, member: 424 wrote: 99 out of 100 surveyors would RTK the MH lids once, measure down with the 25 ft level rod, report the results to 2 decimal places, and everyone - including 95 of those surveyors - would proceed in blissful ignorance. Knowledge can be a burden.
So I think that I would propose in language like this:
In accordance with common survey practice we intend to perform this work in the following manner: 1) Establish survey control at approximately 1000 foot spacings along the route of the subject drainage system using RTK GPS to establish horizontal control, and differential levelling to establish vertical control; 2) tie subject manhole rims at the point on the rim closest to the center of the manhole barrel from this control using a robotic total station by a single sideshot; 3) measure down from the rim to the inverts using a levelling rod or tape measure. Results will be reported in feet to 2 decimal points.
Embellish/modify as you see fit. You might leave it at that or you might add the following:
A statistical analysis of the proposed survey methodology indicates that results of a survey performed in this manner will have a relative precision of not greater than o.xx feet.
For elevations you really need D&R observations. A single sideshot may be OK for location, but not elevation.
Paul in PA
Chris: I disagree, I think SOME knowledge about surveying is better than NO knowledge about it for someone who is contracting or specifying surveys and using the data. I see it all the time, they don't know what they want, they don't know how to ask for what they want, they don't know what is achievable economically, and they don't understand why it costs so much to get high accuracy.
When my father went to school in the late 40's for electrical engineering, ALL engineering students had to take a survey course, and civils had to take more and also go to summer camp. I am certainly not advocating that, but I can tell you that my father never professed to be a surveyor, but he understood the basics. It was a good intro to measurements, units, accuracy, precision, etc.
As I tell them..."cheap, accurate, or quick. Pick any two"
Jim Frame, post: 393101, member: 10 wrote: I was asked to provide a cost proposal to establish rim and invert elevations on 600 or so manholes distributed across a project 7 miles long and 3.5 miles wide for purposes of hydraulic modeling. There's one NAVD88 bench mark about a mile from one end of the project, one in the middle, and one about two miles from the other end. I asked about an accuracy specification. The first response is "We'd like them accurate to 0.01 foot." I explained the difference between accuracy, precision, and reported significant figures, and pressed him about what they really need. I told him that achieving 0.01 foot at the 95% confidence level throughout the project was not practically attainable. He hemmed and hawed for awhile, then allowed that they could live with 0.1 foot.
I ran a few simulations through Star*Net to come up with an acceptable design, then prepared and submitted a proposal. I got a call back today explaining that my cost is twice what they're accustomed to paying for work of this nature. I asked if he could furnish a survey report from one of those projects to see what they've been getting, but that request wasn't met with much enthusiasm. He said that he'd talk with some of the other PMs to see if they could relax the specification, but that the project budget couldn't stand the cost I proposed.
This isn't the first time I've run into this kind of thing. It seems that there are surveyors out there who are willing to sign off on accuracy statements that they either don't understand or don't care to meet, and that one of them will likely end up doing this work instead of me.
Plan out locations for secondary control points to be used for RTK data collection calibration. I always made one of my secondary control points at the lowest outfall, usually at the treatment plant, or as close as I could get to verify the low point in the collection system since this is the critical juncture for engineering analysis. Run two separate RS network surveys with a base receiver on each of the three primary control points and a rover receiver moving through each of the secondary control points. One run in morning and another in afternoon same day or next day capturing a different set of satellites. Run your minimally constrained adjustment for reality check to the other primary points. Run a constrained adjustment based on your analysis of the minimal adjustment.
Plan your RTK collection so you can calibrate on at least 3 secondary control points and check another or at least 1-2 collection points (manholes) from a previous setup.
I have performed this process on at least three different sewer collection systems (thousands of manholes), other utility systems, and many sets of monitoring wells checking each one randomly with total stations and auto level procedures to know my tolerances and what to expect. You would be surprised at the results.
Trust your instinct to expect and report to 0.1', however don't be surprised by your actual results. Engineering hydraulic analysis typically does not need the tolerance originally asked for unless the collection system has problem flat spots (low percentage flows) in which case you can always supplement in those areas. Talk with the client asking exactly why they need the analysis I.e., trouble flow areas, pipe sizes, I & I, etc.) to help in accessing their accuracy needs.
Ric Moore, post: 393138, member: 731 wrote: Trust your instinct to expect and report to 0.1', however don't be surprised by your actual results. Engineering hydraulic analysis typically does not need the tolerance originally asked for.
There's a big difference, at least in my mind, between delivering a product that I believe will satisfy the end result on the one hand and certifying to an accuracy specification on the other.
I couldn't agree more. Consult with your client and explain that. Eventually they will come to appreciate and respect your advice/recommendations/explanations.
Hydraulic modelling to 0.01' for a given storm event for a stormwater system with 600 manholes. I'd hate to be the engineer signing off on that. Nearest 0.1' for rims and inverts is accurate enough, trust me!
They have no idea what they actually need. Just survey it and do a good job.
John Hamilton, post: 393110, member: 640 wrote: This is why ALL civil engineers should be required to take a survey course as part of their degree. Not to make surveyors out of them, but so that they understand methodologies, achievable accuracies, and costs associated with different levels of accuracy
Almost all CE degree programs used to require, and many (although far less than in times past) still require one surveying course in the curriculum. It is usually a field class in which they run a level loop and a small traverse, but learn very little about error theory, accuracy vs. precision, or other concepts surveyors consider to be basic knowledge.
One might think that this would give them a sense of the impossibility of making perfect measurements, or that they might learn at least the concept of the propagation of errors. But more often than not, that limited exposure to surveying tends to give the future CE the impression that there isn't much to know to surveying and that it is merely a subset of the technical functions of civil engineering.
IMO, it would be better to give them an overview course that covers all subjects/topics that would be covered in a 4-year surveying degree, giving them a totally inadequate amount of time to gain any real understanding of any particular topic and thereby impress upon the future CE that surveying is indeed, fully a separate profession from civil engineering that they really know very little about.
I started seeing the .01' requirement a while back for locating wells.
It was possible for a small site (gas station or something like that) using levels, difficult even for those when they want it on the rim of a plastic pipe at the "arrow". The pipe is jagged and the arrow has more than .01' across it.
Then that same requirement started showing up in our larger well surveys (township size type), clearly a standard that just couldn't be met.
It would be interesting to know who started a requirement like that, maybe just one guy in an office somewhere and it took off from there like an urban legend kinda thing.
MightyMoe, post: 393215, member: 700 wrote: I started seeing the .01' requirement a while back for locating wells.
It was possible for a small site (gas station or something like that) using levels, difficult even for those when they want it on the rim of a plastic pipe at the "arrow". The pipe is jagged and the arrow has more than .01' across it.
Then that same requirement started showing up in our larger well surveys (township size type), clearly a standard that just couldn't be met.
It would be interesting to know who started a requirement like that, maybe just one guy in an office somewhere and it took off from there like an urban legend kinda thing.
Do you mean for monitoring wells? We have that requirement here and have had it for a long time for all monitoring wells. There is a form that has to be filled out, certified, signed and sealed called a Form 'B'. The one place I worked did a ton of them (it was an environmental consulting firm that had its own civil/survey department). Even on the large sites as you described (landfills, etc) we would run long level loops. If we didn't close our loop within a couple hundredths or so we ran it again. I'd say most or nearly all of those were within 0.01'. It is possible, but it's a pain.
Dan Patterson, post: 393277, member: 1179 wrote: If we didn't close our loop within a couple hundredths or so we ran it again. I'd say most or nearly all of those were within 0.01'. It is possible, but it's a pain.
Note that closing a loop within 0.01' isn't the same as achieving 0.01' accuracy on all stations. That's not the way error propagation works.
Jim Frame, post: 393286, member: 10 wrote: Note that closing a loop within 0.01' isn't the same as achieving 0.01' accuracy on all stations. That's not the way error propagation works.
I understand that, but I'm just telling you how we did it. I suppose we could have run several loops in alternate orders, and then run the results through a matrix and done a least squares adjustment. That was not happening at that place....trust me.....
We did make GPS and/or total station observations of the wells in addition to the leveling, so there were some other checks, but I don't know that those would've identified a discrepancy of 0.01' magnitude.
My point wasn't to criticize you or your methods, it was to complain (further) about clients asking for something not economically attainable and then finding surveyors willing to certify to accuracies that they didn't attain.
Dan Patterson, post: 393277, member: 1179 wrote: ....If we didn't close our loop within a couple hundredths or so we ran it again. I'd say most or nearly all of those were within 0.01'. It is possible, but it's a pain.....
Your former employer should invest in a digital level. Closures under a hundreth are the norm with those things given even moderate care and attention to practice.
