Understanding and Applying the Maryland Survey Accuracy Standards for Boundary Surveys

Introduction

Tom Manning is a Senior Associate and Field Survey Coordinator at Rodgers Consulting, Inc. Tom was preparing a series of educational emails about least squares for the survey team and mentioned in those emails that one of the primary reasons to use least squares to adjust field data was to be able to prove that it meets the State and ALTA/NSPS precision standards. He told former MSS President Aaron Worley that he considered breaking just that part out as a shorter article, and Aaron asked him to not only do so, but encouraged Tom to submit it for publication in the newsletter. In the time-honored tradition of having another set of eyes review your work for errors and omissions, Tom had it sent to Alan Dragoo, who returned it with only one tiny minor edit.  It is a good, comprehensive treatment of the subject and we are happy to have it to share with you.

Understanding and Applying the Maryland Survey Accuracy Standards for Boundary Surveys

Survey measurements made in the performance of boundary surveys in the State of Maryland need to meet certain measurement quality standards, per state code. It is important to understand what these standards are, what they do and do not address, and how to meet them. This article is an explanation of what the surveyor is required to do to prove that his measurements meet the required quality standards (based on the actual wording of the code), rather than a technical how-to. It presupposes a familiarity on the part of the reader with least squares adjustments, their correct weighting, and the interpretation of their output statistics. This article also does not address the myriad other factors affecting the“correctness” of a survey, including proper research, recovery and evaluation of evidence, and professional judgement and application of laws and boundary principles.

In a nutshell, in order to meet the measurement quality standards set forth in the Maryland code, a surveyor needs to ascertain that the length of the semi-major axis of the 95 percent confidence level error ellipse representing the uncertainty (due to random errors in measurements) in the location of the property evidence marking any corner of the surveyed property relative to the property evidence marking any other corner of the surveyed property, does not exceed 0.07 feet (or 2 centimeters) plus 50 parts per million, based on the direct distance between the two corners being tested.

The bold text above is excerpted from the wording of the actual code:

Maryland state standards:

“Title 09 DEPARTMENT OF LABOR, LICENSING, AND REGULATION

Subtitle 13 BOARD FOR PROFESSIONAL LAND SURVEYORS

Chapter 06 Minimum Standards of Practice

Authority: Business Occupations and Professions Article, §15-208(b)(4), Annotated Code of Maryland

.02 Definitions.

B. Terms Defined.

(11) “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 property evidence, marking any corner of the surveyed property relative to the property evidence, and marking any other corner of the surveyed property at the 95 percent confidence level (two standard deviations).

.03 Boundary Surveys.

E. Field Procedures.

(1) Field measurements shall be made by methods that will provide the precision required by this regulation.

.03 Boundary Surveys.

G. Accuracy Standards.

(1)  The maximum allowable relative positional precision for boundary surveys shall

be 0.07 feet (or 2 centimeters) plus 50 parts per million, based on the direct distance between the two corners being tested.

(2)  The surveyor shall as certain that the positional uncertainties resulting from the survey measurements do not exceed the allowable relative positional precision.

(3)  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 relative positional precision will exceed the allowable amount, the surveyor shall add a note to a survey explaining the site conditions that necessitated the deviation from the relative positional precision.

(4)  The surveyor shall, to the extent necessary to achieve the standards set forth in §G of this regulation, compensate or correct for systematic errors, including those associated with instrument calibration.

(5)  The surveyor shall use appropriate error propagation and other measurement design theory to select the proper instruments, field procedures, geometric layouts, and computational procedures to control and adjust random errors to achieve the allowable relative positional precision tolerance.”

It is clear from the above then, that in order to satisfy the precision requirements of the Maryland standards, relative error ellipses need to be computed between all of the located markers used as property evidence on the survey. In orderto compute these relative error ellipses, the amount of uncertainty in the positions of all the located evidence (corner) points must be determined. The standards leave it up to the surveyor how to go about this:

.03 Boundary Surveys.

G. Accuracy Standards.

(5) The surveyor shall use appropriate error propagation and other measurement

design theory to select the proper instruments, field procedures, geometric layouts, and computational procedures to control and adjust random errors to achieve the allowable relative positional precision tolerance.”

The generally accepted method is to use results of a least-squares adjustment of the survey data done with one of thecommercially available software packages. It is important to note that the computed uncertainties in the relative positionsof points must be due to random errors, after blunders have been removed, and systematic errors compensated for. The least squares adjustment must be correctly weighted.

Relative Error Ellipses

Two types of error ellipses can be computed from the results of a least squares adjustment of survey data – point error ellipses, and relative error ellipses.

Point, or “absolute” error ellipses are the elliptical areas within which you are certain the point actually falls, in relation to the network’s control, at a given confidence level. Essentially, it is the positional uncertainty of that point relative to the fixed control.

The “relative” error ellipse between a pair of points on the survey represents the amount of uncertainty in the spatialrelationship (bearing & distance) between those points, at a given confidence-level. For the Maryland standards, the required confidence level is 95%. The standard error ellipse is at about the 35% confidence level - different software packages use different strategies to scale them to 95%, but a commonly used multiplier is ≈ 2.45. Here is an illustration of error ellipse types and components (scale is exaggerated):

If, as is typically the case, boundary evidence is located as side-shots from a traverse, the software used needs to be able to compute relative error ellipses between points not directly connected by observations.

Here is a small survey network, with boundary evidence (blue) located from traverse points:

Point error ellipses are shown in red, relative ellipses between points directly connected by survey observations in magenta.

To prove the survey meets the Maryland measurement quality standards, relative ellipses need to be computed between all of the corner points - in this example between 500-501, 500-502, 500-503, 500- 504, 501-502, 501-503, 501-504,502-503, 502-504, and 503-504. In this illustration, needed connections are shown in green, and relative ellipses in blue. Note that the green connections do not represent direct field measurements between the points.

Once the survey network has been adjusted and the relative error ellipses between located corner points computed, the ellipses need to be checked against the standard.

Per the standards, the semi-major axis of the relative error ellipses cannot be longer than .07’ [constant part] PLUS 50 parts per million [proportional part] of the distance between the 2 corners whose relative positional precision is being tested. Here is how to check if the connection passes:

50ppm = 50/1000000 = 1/20000

If the 2 points being checked are 1000’ apart, allowable RPP is .07 + (1000/20000) = 0.12’

If the dimension of the semi-major axis of the 95% confidence-level relative error ellipse between the 2 points is 0.12’ or less, the connection passes.

Here are the results from the example network above:

(output from MicroSurvey’s Star*Net software)

“Allowed” semi-major axis dimensions are computed per the .07’ + 50ppm standard, for example between 500 &501:

0.07 + (687.0609/20000) = 0.1044’

Note that the measurement precision standards only address local accuracy, and not network accuracy – that is, they address the precision of the relative positions of the located boundary points shown on the survey, and not the accuracy of the surveyed points relative to a known datum. The datum information required to be shown on the plat of survey isspecified in section .03 F. (g):

“(g) A statement indicating the origin and method of determination of the bearings or coordinate system shall be made on a plat, and shall include one of the following:

(i)  A reference to true north, as determined by astronomic observation;

(ii) A reference to the Maryland Coordinate System with the controlling stations and a combination factor comprised of an elevation factor and a scale factor noted;

(iii)  A reference to a local coordinate system with the controlling stations listed;

(iv)  A reference to the record bearing of a well-established line found monumented on the ground, as called for in a relevant deed or plat; or

(v) If the above alternatives in this paragraph are not practical, a dated magnetic bearing may be used;”

The accuracy of the “method of determination of the bearings or coordinate system” is not mentioned.

Also not included in the standards are outdated metrics like minimum angle, distance, and closure requirements . This allows for different types of survey equipment and data to be used in the performance of a boundary survey. RTK or static GPS vectors from a local base can be combined with terrestrial observations, for example, as long as they can be adjusted together in a single network, and relative ellipses can be computed between the points located using thiscombination. As to whether or not network RTK could be used to tie in property evidence, it would depend on what typeof data is generated, and if it can be incorporated into a least squares adjustment for analysis of Relative Positional Precision. Going back to the standards:

.03 Boundary Surveys.

G. Accuracy Standards.

(5) The surveyor shall use appropriate error propagation and other measurement

design theory to select the proper instruments, field procedures, geometric layouts, and computational procedures to control and adjust random errors to achieve the allowable relative positional precision tolerance.”

Some field software will record vectors from the network base station to the GPS rover. These vectors can be included in a least squares adjustment in the same way vectors from a local base can. If this constitutes the use of “proper instruments, field procedures, geometric layouts, and computational procedures” would depend on the results of the adjustment and the size of the relative error ellipses computed from that adjustment. The survey data would need to be collected using appropriate field procedures, and would need to include enough redundancy to test the validity of the software-supplied vector weighting.

I did not address the ALTA/NSPS measurement standards in this article, because they are essentially identical, with one difference only – instead of requiring the surveyor to check the relative positional precision between each corner and EVERY OTHER corner located on the surveyed property, it only requires that the relative positional precision betweenADJACENT corners be checked:

MINIMUM STANDARD DETAIL REǪUIREMENTS FOR ALTA/NSPS LAND TITLE SURVEYS

(Effective February 23, 202c)

3. Surveying Standards and Standards of Care

E. 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” is the accepted indicator of measurement quality on an ALTA/NSPSLand Title Survey. It is defined as the length of the semi-major axis, expressed in meters or feet, of the error ellipse of the line connecting the monuments or

witnesses marking adjacent boundary corners of the surveyed property at the 95 percent confidence level.

If you are meeting the Maryland measurement quality standards, you are by default then meeting the ALTA measurement quality standards.

About the Author

Tom Manning is a Senior Associate and Field Survey Coordinator at Rodgers Consulting.

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