Per the Minimum Standard Detail Requirements for ALTA/ACSM Land Title Surveys:
3E. Measurement Standards - The following measurement standards address Relative Positional Precision for the monuments or witnesses marking the corners of the surveyed property.
i. “Relative Positional Precision” means the length of the semi-major axis, expressed in feet or meters, of the error ellipse representing the uncertainty due to random errors in measurements in the location of the monument, or witness, marking any corner of the surveyed property relative to the monument, or witness, marking any other corner of the surveyed property at the 95 percent confidence level (two standard deviations). Relative Positional Precision is estimated by the results of a correctly weighted least squares adjustment of the survey.
v. The maximum allowable Relative Positional Precision for an ALTA/ACSM Land Title Survey is 2 cm (0.07 feet) plus 50 parts per million (based on the direct distance between the two corners being tested). It is recognized that in certain circumstances, the size or configuration of the surveyed property, or the relief, vegetation or improvements on the surveyed property will result in survey measurements for which the maximum allowable Relative Positional Precision may be exceeded. If the maximum allowable Relative Positional Precision is exceeded, the surveyor shall note the reason as explained in Section 6.B.ix below.
My question is, how do you test that you have met the standard on your ALTA surveys? I have noticed that Carlson SurvNET has a option where you can run the test on specified monument pairs that you can input. But, is this standard on other least square adjustment programs? I don't suppose that there is an easy way of doing this without running a least squares adjustment, is there?
>I have noticed that Carlson SurvNET has a option where you can run the test on specified monument pairs that you can input. But, is this standard on other least square adjustment programs? I don't suppose that there is an easy way of doing this without running a least squares adjustment, is there?
No, the only way that I can think of would be if you used a positioning method so accurate that the uncertainties in every point positioned would be so small that the relative uncertainties between any pair of points would be less than the dimension of the semi-major axis of the 95% confidence relative error ellipse specified. This would, of course be true during all RTK demos, but never thereafter. :>
The last version of StarNet that I used allowed you to query the relative error ellipse between unconnected pairs of points using the .REL command. If you did that between what you knew to be your least well positioned points you'd have a good feel for how close you came to the ALTA spec. If your work is not real tight and you have a lot of points, there could be a lot of querying and analysis to do. Kent tells how to avoid all that. Since this spec came out I've wondered too what non least squares folks were doing to test their work against the spec.
Chris
Example of Carlson SurvNET ALTA Test Output
Carlson's SurvNet has an .ALT file that can be created to test monument pairs for the ALTA specification. On a recent job I did, here is what it contained:
RPT_ALL_MAX_PTS 20
REL_ERR_ALL
REL_ERR_PAIRS 190-161,161-121,121-119,119-113,113-240,240-214,214-209,113-112,112-121,112-106,106-161
The output in the Least Squares Adjustment report was this:
>Since this spec came out I've wondered too what non least squares folks were doing to test their work against the spec.
Nothing, they ignore it. 🙁
Do you know of any court cases where a surveyor was sued over not meeting the relative positional precision standards?
> Do you know of any court cases where a surveyor was sued over not meeting the relative positional precision standards?
I can't possibly believe one exists.
I despise the concept of statutorily or contractually mandated "universal one size fit's all" technical standards. No other "profession" puts up with this crap and until we're allowed to use our professional judgment on a case by case basis we're a profession in name only.
FWIW - Only two licensed and regulated "professions" in Maryland have statutory minimum standards of practice: surveyors and home inspectors. You're known by the company you keep
Texas recently dropped the precision standard from the MTS. I think it was a good move for two reasons: its practically unenforceable and the standards (which were pretty low) might be two stringent for certain applications.
> The last version of StarNet that I used allowed you to query the relative error ellipse between unconnected pairs of points using the .REL command.
Star*Net v6 (and, presumably, subsequent versions) also has positional tolerance checking built-in. You specify the tolerance values (base value + ppm) and either check all observed connections, or check for observed connection tolerance failures. You can also have the software check tolerance between all adjusted points (as opposed to only those connected by direct observations), or between a specified subset of adjusted points, via the inline PTOLERANCE option.
:good: :good:
If you get in the habit of reviewing your data by running a least squares analysis it becomes second nature. The time spent is nil compared to fixing projects or scratching your head trying to quantify results down the road.
The issue I see is simple. Some folks believe you have to ignore the law to learn math. Apparently the portion of the human brain that does math also evaluates survey evidence. Having limited space you have to dump one to learn the other...
All kidding aside, the ALTA standard is easy to achieve with sound practice and good habits. Knowing how to quantify and communicate the expected accuracy and precision is part of that package. That is a good bit of why I support education AND experience requirements.
My .02, Tom
:good: :good:
> ...This would, of course be true during all RTK demos, but never thereafter. :>
😀 :good:
