8.2 NCGS Real-Time Network & GNSS Survey Standards (21 NCAC 56 .1607)

8.2 NCGS Real-Time Network (RTN) & GNSS Standards (21 NCAC 56 .1607)

Global Navigation Satellite Systems (GNSS) have revolutionized geodetic positioning and boundary surveying across North Carolina. However, satellite measurement is subject to atmospheric refraction, orbit errors, multipath interference, and geometric dilution of precision. To ensure that satellite-derived positions meet strict legal and engineering tolerances, the North Carolina Board of Examiners for Engineers and Surveyors promulgates mandatory technical standards under 21 NCAC 56 .1607 (Global Navigation Satellite Systems Surveys), while the North Carolina Geodetic Survey (NCGS) maintains one of the nation's most sophisticated statewide reference networks.

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1. The NCGS Statewide Real-Time Network (RTN / CORS)

Under G.S. 102-1.5, the North Carolina Geodetic Survey is statutorily tasked with establishing, maintaining, and densifying the geodetic control infrastructure of the state. A core component of this infrastructure is the NCGS Continuous Operating Reference Station (CORS) Network.

+-----------------------------------------------------------------------------+
|                   NCGS STATEWIDE RTN ARCHITECTURE & DATA FLOW               |
|                                                                             |
|   [Statewide CORS Infrastructure]                                           |
|   • >100 permanent GNSS tracking stations across all 100 NC counties       |
|   • Multi-constellation: GPS (USA), GLONASS (Rus), Galileo (EU), BeiDou     |
|   • Choke-ring antennas on deep-drilled bedrock or stable structures       |
|                                │                                            |
|                                ▼ Stream raw carrier-phase data @ 1 Hz       |
|   [NCGS Central Network Server (Raleigh)]                                   |
|   • Computes ionospheric, tropospheric, and orbital error models           |
|   • Synthesizes Virtual Reference Stations (VRS) or Master-Auxiliary (MAC)  |
|                                │                                            |
|                                ▼ Cellular NTRIP Stream (RTCM 3.x)           |
|   [Field GNSS Rover (PLS in Field)]                                         |
|   • Transmits NMEA GGA position to NCGS Caster                             |
|   • Receives real-time vector corrections customized to exact location     |
|   • Achieves real-time fixed baseline accuracy: 1-2 cm Horiz / 2-3 cm Vert |
+-----------------------------------------------------------------------------+

Benefits of Network RTK (VRS) over Single-Base RTK:

1. Distance-Dependent Error Elimination: In traditional single-base RTK, ionospheric and tropospheric error decorrelation limits reliable baseline lengths to $\le 10\text{–}15\text{ km}$ ($6\text{–}9\text{ miles}$). Network RTK models atmospheric gradients across the entire network array, allowing cm-level accuracy anywhere within the network envelope.
2. Direct NSRS Datum Tie: Every NCGS CORS coordinate is rigidly tied to the National Spatial Reference System (NSRS) on the current realization of NAD83 (2011) Epoch 2010.00 and NAVD88. Positions derived from the RTN automatically share a seamless, statewide coordinate datum without requiring field setup over passive monuments.

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2. Regulatory Requirements: 21 NCAC 56 .1607

Rule 21 NCAC 56 .1607 governs GNSS surveying in North Carolina, and it is worth being precise about what it does and does not regulate. Paragraph (a) defines GNSS to include GPS (United States, originally Navstar), GLONASS (Russia), Galileo (Europe), BDS/BeiDou (China), and any other global satellite positioning system. Paragraph (b) prescribes the certificate. Paragraph (c) sets the accuracy standard for control networks. Paragraph (d) ties parcel-level work back to .1603 or .1606. Paragraph (e) requires the fixed stations to be shown.

What the rule does not contain is any numeric threshold for PDOP or GDOP, any minimum occupation time, any required number of redundant observations, or any satellite-count minimum. Those are professional-practice standards drawn from NGS guidance, manufacturer specifications, and the accuracy the rule demands — real obligations in the sense that failing them will blow the .1603 or .1606 tolerance, but not rule text you can cite by paragraph.

What .1607 actually requires

• (b) The nine-item certificate. The PLS in responsible charge shall certify all prepared documents; where a map or document has more than one sheet, only one sheet must contain the certificate and all others must be certified. The certificate or metadata notes shall contain: (1) class of GPS survey as defined in the Standards of Practice; (2) type of GPS field procedure — static, kinematic, pseudo-kinematic, real-time kinematic, real-time kinematic networks, or Online Position User Service; (3) positional accuracy; (4) dates of survey; (5) what datum and epoch coordinates or geographic positions are based on; (6) designation of fixed-control stations and their positional data; (7) geoid model used; (8) combined grid factor(s); and (9) units.
• (c) Control networks must meet Class AA. "GPS surveys to provide control networks shall be performed in such a manner that it meets a 95 percent confidence level of the positional accuracy of each point relative to the published positions of the control points used and shall meet the accuracy standards of a Class AA survey as set out in Rule .1603." Recall from Section 3.2 that Class AA positional accuracy is 0.05 feet (0.015 m) plus 30 ppm on either axis of the 95 percent error ellipse.
• (d) Parcel-level grid control. GNSS surveys establishing local horizontal or vertical grid control on a parcel whose boundary or topography will be shown relative to NC grid datum shall use techniques providing the standards of accuracy for the class of survey being performed, as set out in Rule .1603 or Rule .1606 as applicable.
• (e) Fixed stations on the face of the map. The fixed station(s) used for the project shall appear on the map, plat, or report, with minimum data of station name, horizontal position (northing and easting) or latitude and longitude, elevation (ellipsoid or orthometric), and datum and epoch.

Remember the companion limit from Section 3.4: under 21 NCAC 56 .1605(a), global positioning surveys shall only be used to obtain Class C vertical surveys, with GNSS vertical error not exceeding five centimeters relative to the referenced benchmarks at the 95 percent confidence level.

+-----------------------------------------------------------------------------------------+
|              21 NCAC 56 .1607(b) - THE NINE CERTIFICATE ITEMS                            |
+-----------------------+-----------------------------------------------------------------+
| Required Item         | Statutory & Rule Mandate                                        |
+-----------------------+-----------------------------------------------------------------+
| Horizontal Datum      | State exact datum and realization (e.g., NAD83 (2011) epoch     |
| & Realization         | 2010.00). Merely stating "NAD83" is legally insufficient.       |
+-----------------------+-----------------------------------------------------------------+
| Vertical Datum &      | State vertical datum (e.g., NAVD88) and the specific hybrid     |
| Geoid Model           | geoid model used (e.g., GEOID18 or GEOID12B).                   |
+-----------------------+-----------------------------------------------------------------+
| Coordinate Type       | Explicitly identify whether coordinates and distances shown are |
| (Grid vs. Ground)     | "Grid Coordinates" or "Ground Coordinates".                     |
+-----------------------+-----------------------------------------------------------------+
| Scale Factors         | List the Combined Grid Factor (CGF) and Grid Scale Factor (k)   |
|                       | applied, along with the project base elevation.                 |
+-----------------------+-----------------------------------------------------------------+
| Base Stations Used    | Identify specific NCGS CORS stations or physical control base   |
|                       | monuments used to control the network.                          |
+-----------------------+-----------------------------------------------------------------+
| Positional Accuracy   | Certify that positional accuracy satisfies Class A, B, or AA    |
| Certification         | linear precision ratios or 95% positional confidence limits.    |
+-----------------------+-----------------------------------------------------------------+

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3. Observational Methodologies: Static vs. RTK vs. Network RTK

Depending on project requirements, baseline lengths, and required precision, the surveyor must select an appropriate GNSS observation methodology.

Static GNSS & NGS OPUS Processing

Static GNSS remains the gold standard for establishing primary project control. Raw observation files (RINEX format) collected over 2+ hours can be submitted to the NGS Online Positioning User Service (OPUS):

• OPUS-Static (OPUS-S): Requires $\ge 2\text{ hours}$ of data; uses 3 distinct CORS stations to compute an unconstrained least-squares vector solution with sub-centimeter accuracy.
• OPUS-Rapid Static (OPUS-RS): Requires 15 minutes to 2 hours of data; uses $\ge 3$ nearby CORS enclosing the site within a tight Delaunay triangulation network.

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4. Quality Control: Satellite Geometry, Dilution of Precision & Redundancy

GNSS accuracy is critically dependent on the geometric distribution of satellites in the sky relative to the receiver antenna.

                      DILUTION OF PRECISION (DOP) GEOMETRY

            POOR GEOMETRY (High DOP)               IDEAL GEOMETRY (Low DOP)
           Satellites Clustered Together             Satellites Widely Dispersed
                   *   *   *
                     *   *                       *                     *
                                                      *           *
                                                             *
                     |                                       |
                     V                                       V
               [GNSS Rover]                            [GNSS Rover]
          High Positional Uncertainty              Sharp Geometric Intersection
           (PDOP > 4.0 - REJECT)                    (PDOP <= 2.0 - OPTIMAL)

Dilution of Precision (DOP) — professional practice, not rule text:

• PDOP (Position Dilution of Precision): 3D positional geometry. Maximum allowable: $\text{PDOP} \le 3.0$ (never exceed 4.0 for boundary determinations).
• GDOP (Geometric Dilution of Precision): Overall 3D position + receiver clock bias. Maximum allowable: $\text{GDOP} \le 4.0$.
• HDOP / VDOP: Horizontal and Vertical dilution of precision. $\text{HDOP} \le 2.0$, $\text{VDOP} \le 2.5$.
• Satellite Count & Mask: Minimum of 6 satellites tracked (multi-constellation typically provides 15–25 satellites). Minimum elevation mask of $12^\circ\text{ to }15^\circ$ to eliminate noisy low-horizon signals prone to severe tropospheric delay.

Redundant Occupation Requirements for Boundary Surveys:

Rule .1607 imposes no explicit redundancy requirement, but .1607(c) and (d) impose accuracy standards that a single unverified RTN shot cannot demonstrate. Redundancy is how you show compliance:

+-----------------------------------------------------------------------------+
|              MANDATORY RTK/RTN REDUNDANT OCCUPATION PROTOCOL                 |
|                                                                             |
|   [First Occupation (Shot 1)]                                               |
|   • Verify fixed integer ambiguity solution                                 |
|   • Record 30-epoch average (PDOP <= 3.0, >=6 SVs)                          |
|                                │                                            |
|                                ▼ MANDATORY TIME SEPARATION (20-30+ Minutes)  |
|   [Constellation Geometry Decorrelation]                                    |
|   • Satellites travel across sky (>15° angular displacement)                |
|   • Multipath reflections decorrelate                                       |
|   • Force receiver to dump ephemeris and re-initialize ambiguity resolution |
|                                │                                            |
|                                ▼                                            |
|   [Second Occupation (Shot 2)]                                              |
|   • Record second 30-epoch average                                          |
|   • Coordinate Delta Comparison:                                            |
|     - Horizontal Tolerance: |ΔN| <= 0.05 ft, |ΔE| <= 0.05 ft                |
|     - Vertical Tolerance:   |ΔElev| <= 0.08 ft                              |
|   • Average the two independent fixed positions                             |
+-----------------------------------------------------------------------------+

[!CAUTION] Taking two successive 5-second shots back-to-back on the same monument without breaking the satellite lock or waiting for constellation rotation does not constitute an independent redundant observation! If the initial shot resolved an incorrect integer carrier-phase cycle ambiguity, the second back-to-back shot will replicate the exact same blunder.

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5. Site Calibrations (Localizations) & Potential Mathematical Pitfalls

A Site Calibration (or Localization) is a mathematical coordinate transformation (typically a 3D Helmert transformation or 2D Conformal Polynomial + inclined plane) that translates, rotates, and scales WGS84/NAD83 geodetic coordinates onto an assumed local ground coordinate system.

$\begin{bmatrix} X_{\text{local}} \\ Y_{\text{local}} \end{bmatrix} = \begin{bmatrix} \Delta X \\ \Delta Y \end{bmatrix} + s \begin{bmatrix} \cos\theta & -\sin\theta \\ \sin\theta & \cos\theta \end{bmatrix} \begin{bmatrix} X_{\text{grid}} \\ Y_{\text{grid}} \end{bmatrix}$

The Severe Risks and Pitfalls of Site Calibrations:

1. Extrapolation Blunders: A site calibration is only mathematically valid inside the perimeter (convex hull) formed by the calibrated control monuments. Measuring points outside this perimeter causes geometric errors to explode exponentially due to uncontrolled angular rotation and scale warping.
2. Scale Factor Distortion: If one of the physical local control points has a damaged or erroneous position, the calibration software will artificially force a fit by expanding or contracting the scale factor ($s$). A true geodetic scale factor should remain near $1.000000 \pm 20\text{ ppm}$. If the calibration outputs a scale factor of $1.000450$, the surveyor is warping all property distances by $4.5\text{ ft}$ per $10,000\text{ ft}$!
3. Inclined Plane Vertical Tilt: In 3D calibrations, fitting elevations to three or four non-planar local bench marks tilts the vertical reference plane. GNSS elevations taken outside the control core will exhibit severe linear elevation gradients.

[!IMPORTANT] NC Best Practice: Whenever possible, avoid arbitrary local calibrations. Perform all boundary and control surveys directly in the North Carolina State Plane Coordinate System (NAD83 2011) and publish true Grid coordinates and ground-to-grid combined factors directly on the plat.