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On the Level: Uncertainty about positional uncertainty.

July 29, 2003
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I recently attended a seminar in which a local attorney spoke on legal requirements for the minimum standards promulgated by the licensure board for that state. It was an excellent seminar, effectively presented by a professional with first-hand knowledge of the standards. The standards set forth three terms for expressing survey accuracy: linear closure, closure ratio and positional error (or positional accuracy).

Linear closure is defined in that state’s standards as: “a measure of the horizontal linear error without regard to direction, between the computed location of the first and last points of a traverse...” Closure ratio is defined as: “the ratio between the [horizontal] linear error of closure to the total [horizontal] distance traversed...” Positional error is defined as: “the linear [horizontal] distance without regard to direction by which a [measured] position of a monumented survey marker differs from its computed location.”

The minimum accuracy standards as required in that state are arranged according to location of the survey, whether in urban, suburban or rural locations, and according to property type in each location. What captured many attendees’ attention (including mine) in this seminar was the table of minimum allowable error. Closure error as an expression of accuracy is allowed only where parcels of land have all sides 100 ft or longer or those having a perimeter of 500 ft or more. In the case of small lots where buildings may be erected along property lines or where land values warrant high accuracy, positional error (positional accuracy) must be used as an expression of accuracy.

A lively discussion ensued in which attendees offered their understanding of how to compute positional error/ positional accuracy. When making a comparison of the measured position of a monumented point to its computed location, is it a matter of relativity of adjacent points in a survey? Or is it a matter of a monumented point’s measured position compared to its record location related to some external base, such as a coordinate in a grid system, or a corner in the Public Land System? Five or six people debated this and other questions, offering their own explanations for the solution. The rest of the 40 or so attendees seemed to have nothing to say. My observation from this experience is that we have a terminology problem in surveying when there is such confusion over something as critical as a mandatory statement of accuracy. I can’t help wondering how one of those surveyors would offer a defense in a disciplinary action brought against him or her by his or her state board of licensure, if that surveyor was unable to describe how the state-mandated standard was to be met. In fact, I got the impression that most or all the surveyors in that seminar use closure ratio exclusively as an expression of survey accuracy even though the state-mandated standard calls for something else in many situations.

The Accuracy Standards for ALTA-ACSM Land Title Surveys were revised in 1997 with the addition of positional uncertainty and positional tolerance as an expression of accuracy. Positional uncertainty is defined in these Standards as: “the uncertainty in location, due to random errors in measurement, of any physical point on a property survey, based on the 95 percent confidence level.” Positional tolerance is defined as: “the maximum acceptable amount of Positional Uncertainty for any physical point on a property survey relative to any other physical point on the survey, including lead-in courses.” The ALTA-ACSM document introduces the subject this way:

“These Accuracy Standards address Positional Uncertainty and Minimum Angle, Distance and Closure Requirements for ALTA-ACSM Land Title Surveys. In order to meet these standards, the Surveyor must assure that the Positional Uncertainties resulting from the survey measurements made on the survey do not exceed the allowable Positional Tolerance. If the size or configuration of the property to be surveyed or the relief, vegetation, or improvements on the property will result in survey measurements for which the Positional Uncertainty will exceed the allowable Positional Tolerance, the surveyor must alternatively apply the within table of “Minimum Angle, Distance and Closure Requirements for Survey Measurements Which Control Land Boundaries for ALTA-ACSM Land Title Surveys” to the measurements made on the survey or employ, in his or her judgment, proper field procedures, instrumentation and adequate survey personnel in order to achieve comparable results.”

The Move to Modern Systems

Surveyors who have graduated from some of our fine four- and five-year university programs are comfortable working with the statistics-based systems of least squares and positional uncertainty, but there are at least two generations of surveyors still at work who learned to express the accuracy of their surveys by the closure ratio method, and few of them seem prepared to change to more modern systems.

Though the ALTA-ACSM Standards document defines the terms, it does not and cannot teach the practitioner how to make the required calculations. The final words of the introductory statement quoted above, beginning with the words “or employ, in his or her judgment…” allow the surveyor/ practitioner off the positional-uncertainty hook to use the standard with which he or she is familiar.

But getting back to those state standards that were the subject of debate in the seminar I attended, I suggest that surveyors be more aware of the requirements placed upon them by their licensing boards. They should demand full definition of terms, and where mandatory standards are seen to be unrealistic, surveyors should, through the action of their state societies, reason with their boards for change. Or else they must all go back to the classroom to learn these methods, more of which will be required as we convert to more modern methods of surveying, like GPS and radial surveys.

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