Kevin, you raise good points which generate lots of discussion. That is good.
I don't visit SurveyorConnect every day and my post may be too late - so be it.
Relative Positional Precision is easy if you understand it - duh!
I believe many surveyors have a better handle on relative positional precision than they give themselves credit for in that one needs to be familiar with the capabilities and limitations of the equipment being used. Precision relates to consistent repeat measurements. Consistent results are evidence, but not proof, of good work. Accuracy is more related to coming up with the correct answer as determined by comparision with a standard or higher order work. Accuracy and precision are both important.
Your question relates to precision - that's OK.
But the question I have trouble getting an answer to is "With respect to what?"
Understanding "with respect to what," using calibrated equipment (accuracy), and obtaining consistent results coupled with commonsense will, in many cases, obviate the need for formal application of statistics. But, statistics are also important!
Of course, if measurements do not agree with each other and/or if measurements do not agree with "higher order values" you have a problem that statistics can not fix.
Having said that, statistics does offer some powerful tools of which every surveyor should be aware. But, your are quite correct, learning those tools and concepts is not easy. It takes a lot of work and effort. As a retired educator, I will comment that the rewards are worth the investment in diligent study and developing personal capability.
Jumping off the deep end, I'll suggest the following:
The following paper discusses spatial data accuracy (includes both relative and absolute positioning) and is posted at http://www.globalcogo.com/fsdagsdm.pdf
Not so deep, perhaps is would be best to start with some overheads I've used in seminars and in the classroom over the past 30 years. Judge for yourself. Go to http://www.globalcogo.com/overhds1.html and pull up the following overheads:
PT002
SD001
SD002
SD003
SM001
SM002
SM003
SM004
Of course, there are many other items available as well.
Have fun browsing (you could spend lots of time doing it) and if/when you have any questions, let me know and I'll be happy to help as I can.
Regards always,
Earl F. Burkholder, PS, PE, F.ASCE
Global COGO, Inc.
Las Cruces, NM 88003
www.globalcogo.com
The
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.
I often apply the idea that accuracy is more akin to "true" or "correct" and precision is a measuring ability.
But as to your example as applied to statistics, if you have a 15 redundant measurements that have a mean of 102.89 (+/-0.02) that is related to a 'precision'. Let's say that the "true" distance at the time you measured it between the monuments you measured between is 103.40. You would not be "accurate" by 0.52' (regardless of the original call and how far off the original monuments were off from the original call).
You or your helper may be setting the glass over a rock, you may have tribrach error that doesn't put you over the point, you have inherent distance error in the machine (regardless of how large or small) you may not have found the original monument that is around 103.4' away from what you measured to (or from)....etc.
Okay; semantics. I see deciding which monument to use is your expertise more than the statistics of your measurements. But the statistics of your measurements is important so that you know how good your reports are.
Just some ramblings, I know most know this.
Tom
