3.4 Bearing, Distance, Elevation & Improvement Tolerances; GNSS Quality Control

The rest of the Rule 7.5 table

Distance accuracy: constant plus proportional

Each distance tolerance has two parts:

• a constant (0.03, 0.05, or 0.07 ft), covering centering, prism-constant, and reading errors that do not grow with length; and
• a proportional part in parts per million, covering errors that scale with distance, such as atmospheric and scale errors.

One ppm is 1/1,000,000 of the distance, or 0.001 ft per 1,000 ft. The rule's own remark gives a quick check: 100 ppm = 1:10,000.

Allowable error = constant + (ppm × 10⁻⁶ × distance)

Example. On a 2,000-ft line in a Class B survey, the tolerance is 0.05 + (100 × 10⁻⁶ × 2,000) = 0.05 + 0.20 = 0.25 ft. Common wrong answers drop the constant (0.20 ft) or treat 100 ppm as 0.1 ft per foot.

Bearing accuracy relative to the source

Rule 7.3 requires every survey to be referenced to true north, grid north, or the record bearing of a well-defined, monumented line (Section 4.1). Rule 7.5 then requires bearings to be accurate relative to that source within ±1′ (A), ±2′ (B), or ±3′ (C). If the basis of bearings is a monumented subdivision line, the surveyor's bearing between those monuments must agree with the record within the class tolerance, and the rest of the survey is rotated consistently.

Elevations for water- and contour-controlled boundaries

Some boundaries are elevations, not lines on the ground: a tidal boundary at mean high water, a lake contour, or a reservoir take line. For these, Rule 7.5 requires the controlling elevation to be established to ±0.05 ft (A), ±0.10 ft (B), or ±0.20 ft (C), based on NGVD or NAVD. On flat marsh or a gently sloping lake shore, a small vertical error moves the boundary a long way horizontally. On a 1% slope, a 0.10-ft vertical error moves the contour about 10 ft. Mississippi's Public Trust Tidelands Act separately defines mean high water from a nineteen-year tidal epoch (Section 10.3).

Tie measurements to improvements

Rule 7.3 requires the plat to show visible encroachments with their extent and the location of permanent improvements relevant to the survey's purpose (Section 4.3). Rule 7.5 sets how accurately those ties must be measured: ±0.1 ft (A), ±0.2 ft (B), ±1.0 ft (C). A downtown building corner 0.15 ft from a line needs a Class A tie to report reliably which side of the line it is on.

GNSS and other radial methods (Rule 7.5(2))

"If radial survey methods such as global positioning systems (GPS) or other acceptable technologies or procedures are used to locate or establish points on the boundary survey, the Professional Surveyor shall apply acceptable surveying procedures and perform adequate quality control measures to ensure that the allowable positional accuracy and/or positional tolerance of such points are not exceeded."

The rule does not list specific PDOP limits, session lengths, or re-occupation times. It sets a performance standard, and the surveyor must be able to show how it was met. Accepted practice, drawn from National Geodetic Survey guidance for real-time and static GNSS, includes:

• Checking into published control or a known monument before and after the work.
• Taking redundant, independent observations of boundary points under different satellite geometry, with fixed solutions only.
• Comparing repeat positions against the class tolerances above.
• Tying to at least two control points so rotation and scale can be verified.
• Keeping the field and processing records needed to show that quality control was performed.

(Section 12.2 covers the network and field practice in more depth.)

When State Plane Coordinates appear on a plat (Rule 7.3, item 18)

Whenever State Plane Coordinates are used in the process of conducting a survey, the plat must show:

1. the State Plane Coordinate System zone;
2. the horizontal or vertical datum used;
3. the method used to derive the information (GNSS or conventional), including any published horizontal and vertical control points used;
4. a combined or correction factor;
5. the convergence angle; and
6. the coordinates of at least two reference points relevant to the survey.

If State Plane Coordinates are part of a written description, the description must include the same items (Rule 7.3, item 13). The Mississippi Coordinate System statute (§§ 89-6-1 to -19) adds its own conditions for recording coordinates (Section 12.1).