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Elevation Data Accuracy/Precision/Error
Elevation Data Accuracy/Precision/Error
Hello. I’m not a surveyor, but did complete a college course many decades ago and have used transit/rod and full station equipment to determine rim elevation of groundwater monitoring wells (based on an arbitrary datum) very occasionally over the years. Recently used an auto level to determine rim elevations. My question is, if a licensed survey was asked to determine rim elevations of monitoring wells (using typical survey instruments) so that we can use a tape (graduated in 100ths of a foot) to measure down from that point to determine groundwater elevation, how accurate (i.e. plus or minus how many inches, etc.) would your data be. What equipment would you use. Is it ridiculous to think that accuracy could be 0.01 feet based on a benchmark located 1000 feet away.
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It depends on what accuracy you request. Depending on that I would chose the method:
GNSS: 0.05-0.10 feet (may be degraded if wells are in the woods)
Total Station: 0.03 feet
leveling: 0.01 feet with fiberglass or wooden rod and digital level
leveling: 0.003 feet with invar rod and digital level
Each method has costs associated with it that are correlated to the accuracy
edit: you asked for inches, it was hard enough for me to convert from meters to feet 😉
Larry,
In our industry it is common, and indeed required by statutes in some states, to report measured distances to the nearest hundredth of a foot or thousandth of a meter. Similarly with measured directions, angles or vectors being reported to the nearest arc second. I wouldn’t necessarily assume a distance (or depth) that was merely reported with an accuracy in hundredths of feet to be necessarily precise. Especially, in the example you’ve stated, with merely measuring down the well with a 100′ tape.
Reliably determining an elevation within 0.01′ from a BM 1000′ distant can be done with proper procedure. Measuring down a hole from that determined point with a tape to ascertain a ground water elevation probably couldn’t be done with the same precision.
I don’t know if I’d go so far as to call it ridiculous, but it could be construed as misleading. I would personally probably report well levels to the nearest 0.1′. And that in itself might be subject to an unspecified level of error depending on conditions and measurement techniques.
When doing work of this type I use a digital level to determine the height of the top of the PVC well pipe. With such a device relative elevations of plus/minus 0.01’ are realistic.
I thought you were going to end your first sentence with…. but I did stay at a Holiday Inn Express last night.
Is it ridiculous to think that accuracy could be 0.01 feet based on a benchmark located 1000 feet away.
I wish there were more engineers that understood that simple concept.
In my experience with monitoring well surveys, local accuracy is more important than datum accuracy. When characterizing aquifer slope and direction, having the well elevations in a local area accurate with respect to each other matters more than how accurate the elevations are with respect to a published datum. For a small site like a gas station, rim elevations with a relative accuracy of 0.01′ are easily attainable with a total station or auto level. For the purpose of site characterization, it won’t matter much if those elevations are all shifted from NAVD88 by a few tenths of a foot.
As the size of the site under study gets larger, datum accuracy becomes more important, and care must be taken to align the project approach with the desired results.
The industry standard for monitor well locations is 0.10 horizontal and 0.01 vertical. We have done thousands using the method you describe for vertical and yes it is very achievable. The string that the technician often uses to both measure down and collect a water sample at the same time is ineherently less accurate and can be a significant source of error. Many companies have come to us because their ground water contours didn’t make sense based on the elevations reported by others. This was typically because trigs or GPS were the source of the elevations. But the measure down by the environmental tech is often not accurate.
I’ve had requests for monitoring well accuracy to be .01′ over half a township area, 18 sections.
Of course, that’s impossible without an insane amount of work and time (probably not even then).
You have to talk down the client at that point and get a realistic contract. We would normally report them to .1′ which is probably a bit iffy but then who’s going to show you’re wrong.
If the wells are clustered then that’s a different story, .01′ is usually possible for something like a gas station. Often the pipe itself isn’t cut to facilitate .01′ even if you use an arrow or all the pipes are to be shot on the north edge.
Then there’s the issue of measuring down, there are devices for that. But, how accurate are they? I don’t know. So .01′ in 1000′, that’s doable.
Total Station: 0.03 feet
leveling: 0.01 feet with fiberglass or wooden rod and digital level
Done correctly, a total station (5″ or better) can easily exceed the precision and accuracy of an automatic level with a fiberglass rod and a mark I eyeball. This is especially true if there is any significant change in elevation across the route of the level loop.
There is almost zero reason to not use a total station, except for our (mine included) bias.
And, knowing this, I still make the crew break out the auto level for critical checks.
-All thoughts my own, except my typos and when I am wrong.Is it ridiculous to think that accuracy could be 0.01 feet based on a benchmark located 1000 feet away.
That sort of accuracy isn’t ridiculous. It is more expensive.
When we move to the right of the decimal place, think of it as a logarithmic scale. We might say that 0.1′ costs 10x 1.0′, 0.01′ costs 10x 0.1′, and so on. And…that is probably at a “+/- 95% probability” based on decades of experience and checks and standard procedures. If you want that to be a verified 95% probability based on statistical analysis, the cost goes up. (And the time you need to wait to get the answer is directly proportional to the cost.)
So, everyone wants accuracy, but almost no one wants to pay and even fewer want to pay AND wait.
Realistically determining the accuracy that you NEED for you study is a critical analysis of the project.
-All thoughts my own, except my typos and when I am wrong.
So, everyone wants accuracy, but almost no one wants to pay and even fewer want to pay AND wait.
Realistically determining the accuracy that you NEED for you study is a critical analysis of the project.
Amen.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil PostmanWhat happens is the request for .01′ is sent out for the job and someone takes the job, hits the pipes with GPS, sends the data to the client showing .01′ XYZ.
Everyone is happy, since the data request is ridiculous anyway and there’s no way to prove any of it is wrong without spending a fortune and they don’t need it to the accuracy they “wanted” the world moves on and all is good.
same people that want dirt contours accurate to the tenth of a foot in a plowed field.
@mightymoe Except that in this case the accuracy is needed and the groundwater contours will not make sense without the accuracy and the money is wasted and the work has to be redone.
Thanks for all the feedback. Five years ago I surveyed the top of casing (TOC) elevations of 5 monitoring wells (MWs) we installed. Typically we would just use an arbitrary datum as a benchmark (or backshot), but for this project (for various reasons) it was critical to reference elevations from a known datum. I must have moved the tripod 6 to 8 times trying not to shoot long shots and having to turn corners. We recently installed 5 additional MWs, so I used established TOC elevations to calculate TOC elev for new wells, and I checked to see if some of the old data match. There was a difference of 0.15′ on a couple of the old MWs.
I find the auto level a bear to work with especially when the receiver is at a joint in the rod, or laser line is way high.
We use an electronic interface meter down the well to measure water levels and oil thickness (if present). It has 0.01 tick marks, so the regulators assume that everything is accurate to 0.01′.
Me thinks thats not an auto level youre using if you have a receiver on the rod.
I find the auto level a bear to work with especially when the receiver is at a joint in the rod, or laser line is way high.
That sounds like you are using a construction laser, not a survey grade level. A construction laser has a precision of maybe 1/16″ in 100 feet (50mm/1000m). A survey grade digital level will have an accuracy spec of around 1mm per 1000 meters.
Thanks for all the feedback. Five years ago I surveyed the top of casing (TOC) elevations of 5 monitoring wells (MWs) we installed. Typically we would just use an arbitrary datum as a benchmark (or backshot), but for this project (for various reasons) it was critical to reference elevations from a known datum. I must have moved the tripod 6 to 8 times…
These guys will tell you better, but it sounds like they’re wanting the known datum to do something like producing groundwater contours/flow paths. So it would be an important aspect as the work progresses. To do that you’ll want to use a survey grade leveling device – total station or auto level I suppose – and close the level by traversing back to your known point. This will “close” the survey and tell you if you have any blunders and more probably a closure error that can be adjusted using appropriate calcs. That adjusted leveling data will be a basis for all future work from which new known benchmarks can be set (and checked).
dd
If you really get to 0.01 ft accuracy, over a north-south extent of several miles, then you will need to do the geoid correction, which a lot of people aren’t familiar with because they can ignore in their ordinary work.
And for water work, you really should be using Dynamic Height, so the geoid correction is different than the Orthometric Correction more commonly presented.
.We had almost this same discussion in 2012 and 2013 where somebody was trying to hold 2 cm accuracy on aquifer monitoring wells over quite a few miles of South Dakota, and generally surveyors thought it was not practical.
The geoid model is so much better today and digital levels are good, but it would be a major campaign effort and expensive to even make a decent effort over significant distances.
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