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The Nature of Measurement: Part XI: Suggested Measurement Standards for Property Surveys

Professional Surveyor Magazine - October 1998

Dr. Ben Buckner, LS, PE, CP

The following suggested standards evolved during 1996 and 1997. They were written after studying several sets of state standards, considering the various proposed ACSM/ALTA standards publicized during this period and drawing from my own knowledge of measurement science. These standards are similar to those prepared by a committee of the Northeast Chapter of the Tennessee Association of Professional Land Surveyors and suggested to the Tennessee state licensing board to replace the current standards.

I believe that these standards, if adopted by licensing boards and professional surveying associations, will overcome some misunderstanding about measurement accuracy and ease concern over positional tolerance. The standards are theoretically sound and easily attainable with modern instrumentation. The standards do not dictate methods. They are not specifications. The surveyor must read and understand them, then derive specifications designed to yield the accuracies dictated by the standards. Of course, the user must have a professional level knowledge of the science of measurement to interpret and apply the standards properly.

The standards are presented in a familiar "legal" format that can be easily adopted. I would caution any association or board against too much modification of any suggested standards. I have seen drastic changes made to well-considered suggestions where the surgeons simply lacked understanding and didn't bother to ask the author what was meant. The result is usually something impossible to apply. If something is not understood, the clarification will probably be found somewhere in Parts 1 through 10 of this series, which form the background of the standards.

Material in italics below is explanatory and is not part of the standards.

The Standards

Calibration of Instruments & Error Control
1. Basic Definitions
  1. Precision is relative agreement among duplicate measurements of the same quantity or the repeatability of a measurement. Precision is a function of random errors and their control in measurements. Relative error of closure of a survey checks the precision. Precision is also sometimes used to denote the significant figures expressed in a measured number.
  2. Accuracy is the degree of conformity of a final measured value, with respect to the true value as defined by accepted, legal standards on weights and measures, to geometric laws, or to accepted datums. Distance accuracy relates to standards established by the United States government defining the meter. Angle accuracy relates to geometric divisions of the circle and to geometric laws on closed figures. Accuracy is achieved by compensating or correcting for systematic errors and avoiding or removing mistakes from measurements and, to a lesser extent, by controlling random errors.
2. Instrument Calibration and Adjustment
  
  Distance measuring instruments, retroprisms, and associated instrumentation shall by calibrated using calibration base lines and other methods to assure they are operating within acceptable tolerances and to determine instrument constants for the purpose of correcting measured data. Angle measuring and leveling instruments shall be checked periodically for maladjustment of axes and general functioning. Optical plumbing devices, such as the optical plummets in tribrachs and prism pole bubbles shall likewise by periodically checked for proper adjustment and adjusted if necessary.
3. Compensation for Systematic Errors
  
  The surveyor shall, to the extent necessary to achieve the standards herein, compensate or correct for systematic errors associated with the instruments, natural phenomena, calibration and maladjustment of instruments, and other sources.
4. Control and Management of Random Errors
  
  The Surveyor shall control random errors in measurements by using due care andand specified procedures designed to achieve the required results. The surveyor shall use appropriate error propagation and other measurement design theory for selection of instruments, field procedures, geometric layouts, and computational procedures to control and adjust random errors in order to achieve the required relative positional tolerances specified herein.
5. Avoiding and Eliminating Mistakes
  
  The surveyor shall take measurements to avoid, detect, and remove mistakes (blunders) from field measurements and calculations. Mistakes shall not be adjusted, mathematically, but removed prior to adjustment of closing errors.
Accuracy Standards For Property Line Surveys
1. Basic Definitions
  1. Definitions Related to Property Line Evidence
    1. A corner is a point where a property line changes direction, or the point of intersection of two or more boundary lines. A monument or evidence of a monument may or may not exist at a corner.
    2. A monument, as used in property line surveys, is a physical marker, marking, or natural feature identifying the location of a property corner, found in place, or set or otherwise marked to preserve and perpetuate the position of the described and surveyed corner.
  2. Definitions Pertinent to Positional Tolerance and Errors
    1. Reference Point. A reference point may be a monument at the corner of a recorded subdivision or a prominent corner of a metes and bounds survey (generally being the point of beginning, points of commencement, or other such point). Generally, one reference point will be selected for any particular survey.
      
      Author's Note: Positional tolerance must relate to something identifiable, be it a physical monument or a geodetic reference system. Stating accuracy without "respect to what" is incomplete. A monumented "reference Point" is used here. This reference could also be a geodetic control monument. It could even be an ellipsoid-based coordinate system, with no particular physical monument mentioned, but I was afraid too much misunderstanding would spring from this suggestion. If all standards required referencing property surveys to such systems, then both the title identity and the coordinate reference would be accomplished with the reference point monument. In my opinion, requiring ties to systems such as state plane coordinates is overdue, with the densification and accuracy of control now realized by the high-accuracy reference networks. However, that is a separate issue.
    2. Relative Positional Tolerance. Relative positional tolerance or accuracy is the maximum acceptable error in position of a corner monument with respect to the monument identifying the selected reference point in a surveyed tract. Relative positional tolerance is not related to differences between measured and recorded distances or to uncertainty of the reference point or its identifying monument with respect to geodetic, cadastral, or other references. It is a standard for the accuracy of survey measurements made directly or indirectly between physical monuments. Where one or both of the monuments are set during the survey, the tolerance refers to the final monument position, not to temporary points set prior to final monumentation. In all cases, monuments shall have distinct markings to which measurements refer.
    3. Relative Positional Error. Relative positional error or uncertainty is the computed error or uncertainty of one physical point (i.e., monumented corner) with respect to another. In these standards, it is specifically the uncertainty of any corner monument with respect to the selected reference point monument. It is computed using either analysis of component distance and direction errors between points or from a correctly weighted least squares analysis. Computed positional errors shall be less than the allowable tolerances.
      
      Author's Note: Positional tolerance or accuracy is what is expected. This is the "standard". Positional error is what the surveyor estimates, using calculations involving random errors, with error propagation, to calculate a defensible positional error. This error would then be compared with the standard. In practice, this is a very important step. Surveyors must know error theory, how to test instruments and methods for statistically determined random errors and how to perform the error propagation and other procedures to evaluate the survey.
2. Application
  1. Application Relative to Accuracy and Monuments
    1. The following standards on relative positional tolerance and calculated positional error apply to accuracy, not to precision of measurements.
    2. These standards apply only to accuracy of measurements between monuments marking analyzed corner positions, not to the best location of the corner position as analyzed from the evidence with embodies procedures involving case law related to retracing boundaries. Thus, a corner shall be properly set from evidence first, then monumented, in order to apply these standards. The surveyor shall use instruments and methods designed to yield the minimum standards.
  2. Application Relative to Survey Classes
    1. The relative positional accuracy appropriate for a particular property survey shall be based on the potential use of the land, as related to its capability or suitability for development, not to existing use.
    2. Survey Classes
      1. Class 1: Developed land and land which has a high potential to be developed, either because of its physical attributes or because of its location or possible future demand. This class typically includes what is generally considered urban and suburban land, but can also include some rural land.
      2. Class 2: Land which is unlikely to be developed because of its physical attributes or remote location. This class includes mountains, marshes, and other areas unlikely to be intensely developed.
      Author's Note: Because land uses change over time, particularly in regard to "developable land", there is value in anticipating this and making the accuracy of the surveys match what would be desirable in the future. The dimension of time is important in any planning scheme. Suggesting placing surveys into only two classes will, of course, go against tradition, but I hope reasonable surveyors will see the merit of two classes. To put this into perspective, with the use of modern instruments such as total stations, there is probably no significant difference between what surveyors are achieving in "Class 1" and "Class 2" areas anyway. In fact, the "Class 2" areas may even be surveyed more accurately. This is because they are usually rural, with longer lines, as opposed to the shorter sight lines typical of urban surveys, which works in favor of increasing relative accuracy. For this reason there is actually merit in having no differentiation between "classes" based on land. However, I figured some would never accept this, so two classes are suggested.
    3. Required Relative Positional Tolerance (Accuracy).
      1. Class1: ± (0.07 feet + 1:20,000) or ±(20 mm. + 50 ppm)
      2. Class2: ± (0.26 feet + 1:50,000) or ±(80 mm. + 200 ppm)
      Author's Note: The fixed distance with a PPM should be familiar to most surveyors. The exact tolerance is thus variable and depends on the distance between the reference point and the monument being considered. Note that "error of closure" is not in the standards as related to accuracy because it relates to precision. Unless systematic errors have been compensated or corrected to the extent dictated by the standards, the desired accuracy will not result. Unless surveyors reject error of closure concepts in relation to accuracy, the professional will always suffer from its embarassing misunderstanding of basic measurement science.