Amen!!
I love when I see 3 decimals places to the centerline of a stonewall...
I've always thought all this ALTA positional tolerance emphasis is total BS. Doesn't signing and sealing a survey indicate that you are a professional, have studied this stuff and understand how to measure and have confidence in your results? I know this isn't necessarily true in all cases. ALTAs are are about jumping through ridiculous hoops and taking on way more liability than we should.
> How many attorney's actully understand statistics?...I'd say few.
>
> Attorneys are smarter than to challange a surveyor on something they know a surveyor has more knowledge than he/she does...
Attorneys should be challenging surveyors not on the math itself, but their use of math over boundary law principles. We should understand these concepts better than an attorney but as a whole I would say we don't.
There is a great book on statistical analysis of survey measurements:
"Errors in Practical Measurementin Surveying, Engineering, and Technology" by B. Austin Barry, published by John Wiley & Sons in 1978 and re-published by Landmark Enterprises in 1991.
Very readable and fairly easy to understand. See page 15 for excellent description of precision vs accuracy.
Does it work on Cats too?
The problem is that you have to heard them cats into one area before you even start to work on measuring them......good luck with that.;-)
Mr. Diekman...
very good post..
Lehigh University, Circa 1970's
Statistics was rolled into many courses but was not a separate requirement. That made it harder later on in advanced surveying courses at New Jersey Institute of Technology, circa 2000. 136 credits were required for a BS Engineering, I graduated with 142. At the time Lehigh was one of the few colleges to still require surveying camp for all civil engineers on top of surveying 1. I finally took a 3 credit correspondance course in statistics, as I could not find any advanced 4 credit courses.
Paul in PA
Kevin, 8 Hours Is Not Enough
Larry P's course is more of a review and very good for PDHs.
College level statistics courses are 3 credits even at the community college level. That means 45 hours of class time.
For the technical work of engineers and surveyors that 3 credits is not adequate and many institutions require a 4 credit course covering more advance concepts than most college level statistical work.
Paul in PA
Kevin, 8 Hours Is Not Enough
> College level statistics courses are 3 credits even at the community college level. That means 45 hours of class time.
Really? For a basic statistics course? Wow!
But I know I got really twisted in expectations by my college. Things went by pretty darn fast there. And Caltech's a pretty funky school, and pretty far from what most would consider "normal"... There were statistics courses there, but they would pretty much assume everyone knew what "mean" and "standard deviation" were before the course even started. Next thing you know, you're in partial differential equations... 😛
But like I said, that's a pretty funky school, and they'd get into the esoterica pretty fast. Understanding a simple "standard deviation" is generally enough for most purposes. And that's basically nothing but a measure of how much variability is in your data.
The simple rule of thumb in most cases (where we have what's called a "normal distribution" in the standard cryptic lingo, or a "bell curve" in the lingo most of us learned in middle school) is that about 2/3 of all samples will be within one "standard deviation" of the "mean". I think we all pretty much know what the "mean" is, e.g. what we usually think of as the "average". Then from the mean, 1/3 of all samples will be within one standard deviation lower, and 1/3 will be within one standard deviation higher.
A simple example would be the average height of US males. Using values taken from a Wikipedia article, the mean height of US males is 70 inches, and the standard deviation is 3 inches. This means that if we took the average height of all males in the US, we'd have 70 inches, and approximately 2/3 of all males are between 67 inches and 73 inches. That's really all there is to "standard deviation".
It gets more complicated if you start looking at something other than a "normal distribution", but I've never had such a situation arise in my Surveying work.
> Attorneys should be challenging surveyors not on the math itself, but their use of math over boundary law principles. We should understand these concepts better than an attorney but as a whole I would say we don't.
Reminds me of an ongoing theme in lectures I've heard from Mr. Robillard... 😉
I tend to agree, as well. The math has nice and neat rules. The law does not - it's a chaotic jangling of contradictory concerns. And it's a rare luxury to be on a field crew with a Party Chief who really knows the law, so most of us don't learn that part in the field or through experience.
I agree with what your saying BSA.
I find it interesting that "they" (the royal they) simply took the 2005 wording and substituted the word "Precision" in place of the word "Accuracy".
If I were to venture a guess I would say that by using the word "accuracy" previously there was an implication that a surveyor had to locate a corner to less than 0.07 feet plus 50 PPM. So if I measured to where I think the corner should be and find a pipe or a rock or you name it 0.12 feet away, I needed to set something "closer", thus creating the dreaded "pin cushion"
Most of us would agree that's not correct. 😉
So "they" had to change the word to "precision", thus instructing that the surveyors "measurements" should fall within that tolerance, not the location of a corner.
Did I use enough quotes?
Thanks again for all the input. It is invaluable.
Kevin
Not So Kevin, We Find Many Corners Not Where They Should Be
and it does not affect the accuracy of our survey. Possibly it reflects against the accuracy of the original surveyor.
I surmise the switch from accuracy to precision has more to do with the legal aspects of surveying. As a trained professional I can testify to the precision of the work that leads to my opinions but not to the accuracy. Accuracy may be easily affected by monumentation that I knew nothing about or just could not find with reasonable effort.
Paul in PA
Not So Kevin, We Find Many Corners Not Where They Should Be
Accuracy could be do I have the correct monument or not or it could be the accuracy of my survey within itself. I have 15 redundant observations to the monument at 102.89 feet (+/-0.02') which is reported on the original survey to be at 100.00 feet. My measurement is accurate to 0.02' but the original survey is only accurate to 3' more or less.
I find a lot of monuments in the forest that are slightly leaning downhill. There is a punch mark or cross on top. Since the use of the land is to cut 2' plus diameter trees I don't worry about the couple of tenths difference in the leaning monument; for my purposes I measure to the punch mark in all cases. I could straighten every monument up vertical but maybe it was set that way in the first place besides it will just go back to where it was in short order anyway.
Can anyone tell me if the XL spreadsheet formula for confidence is a fair way to state uncertainty at the 95% confidence level? It's certainly easy to use. The result for redundant coordinate pair observations seems to be about 95% of the spread between min and max as a rule which seems about right.
Properly used, the Excel CONFIDENCE function is correct for one-dimensional gaussian normal (bell-curve) distributions. That would include leveling data and the easting (only) or northing (only) of horizontal measurements. See the help for that function to get more details.
However, it would be misapplied to the magnitude of a horizontal closure. That is always positive and has error interacting in two dimensions. Thus it has a different distribution, resulting in a different factor.
