8.3 Datums, Geoid Models & NSRS Modernization (NATRF2022 / NAPGD2022)

8.3 Vertical Datums, Geoid Models & NSRS Modernization

Height determination is one of the most technically complex aspects of geodetic and land surveying. Unlike horizontal positions, which are referenced to purely geometric ellipsoids, elevations must account for the Earth's irregular gravitational field to predict the direction of fluid flow, floodplain boundaries, and drainage paths.

With the National Geodetic Survey (NGS) replacing legacy datums (NAD83 and NAVD88) with the modernized NSRS of 2022 (NATRF2022 and NAPGD2022), North Carolina land surveyors must master the relationship between ellipsoids, geoids, and orthometric elevations.

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1. The Fundamental Geodetic Height Relationship

To understand modern three-dimensional positioning, a surveyor must distinguish between the three primary surfaces of the Earth:

                    THE THREE GEODETIC HEIGHT SURFACES

                                  ▲
                                 / \  Topographic Surface (Physical Earth)
                                /   \
  -----------------------------*-----*-------------------------
                               │     │
                               │     │  Orthometric Height (H)
                               │     │  (Elevation above Geoid)
                               │     ▼
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~│~~~~~*~~~~~~~~~~~~~~~~~~~~~~~~ Geoid (NAVD88 / MSL)
                               │    / 
        Ellipsoidal Height (h) │   /    Geoid Height (N)
        (Height above GRS80)   │  /     (Negative in NC: -28m to -34m)
                               ▼ ▼
  =============================*============================== GRS80 Reference Ellipsoid

The Three Core Geodetic Surfaces:

1. The Topographic Surface: The actual, physical ground surface where survey monuments, structures, and terrain features exist.
2. The Reference Ellipsoid (GRS80): A mathematically defined, smooth oblate spheroid representing the geometric size and flattening of the Earth ($a = 6,378,137.0\text{ m}$, $f = 1/298.257222101$). GNSS satellites calculate coordinates and heights relative strictly to this geometric ellipsoid.
3. The Geoid: An equipotential surface of the Earth's gravity field that best coincides with global Mean Sea Level (MSL) in the open oceans. Because gravity varies with subsurface mass density, the geoid is undulated and irregular. Plumb lines and spirit bubble levels align perpendicularly to the geoid.

The Fundamental Geodetic Height Equation:

$h = H + N \iff H = h - N$

where:

• $h$ = Ellipsoidal Height (geometric height measured from the GRS80 ellipsoid to the ground point, derived directly from GNSS observations).
• $H$ = Orthometric Height (physical elevation above the geoid, corresponding to NAVD88 elevation, measured along the curved plumb line).
• $N$ = Geoid Height (or Geoid Undulation) (the vertical separation between the GRS80 ellipsoid and the geoid at that specific location).

Geoid Undulation Behavior in North Carolina:

In North Carolina, the GRS80 ellipsoid sits above the geoid across the entire state. Consequently, the geoid height ($N$) is always negative in North Carolina, ranging from approximately $-28.0\text{ m}$ ($-91.9\text{ ft}$) in the Coastal Plain (Manteo/Cape Hatteras) to $-34.5\text{ m}$ ($-113.2\text{ ft}$) in the Blue Ridge Mountains (Cherokee/Murphy).

+-----------------------------------------------------------------------------+
|                 WORKED CALCULATION: ORTHOMETRIC ELEVATION                   |
|                                                                             |
|   A GNSS receiver on a control monument in Raleigh measures:                |
|   • Ellipsoidal Height (h) = +98.450 meters (+323.00 ft)                    |
|   • GEOID18 Model Geoid Height (N) = -31.850 meters (-104.50 ft)            |
|                                                                             |
|   Calculate the NAVD88 Orthometric Elevation (H):                           |
|   H = h - N                                                                 |
|   H = (+98.450 m) - (-31.850 m) = 98.450 + 31.850 = +130.300 meters         |
|   H = 130.300 m * (3937 / 1200) = 427.49 US Survey Feet                     |
+-----------------------------------------------------------------------------+

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2. Evolution of Vertical Datums: NGVD29 vs. NAVD88

+-----------------------------------------------------------------------------------------+
|                         COMPARISON OF U.S. VERTICAL DATUMS                              |
+-----------------------+----------------------------------+------------------------------+
| Feature               | NGVD 1929                        | NAVD 1988                    |
+-----------------------+----------------------------------+------------------------------+
| Original Name         | Sea Level Datum of 1929          | North American Vertical      |
|                       |                                  | Datum of 1988                |
+-----------------------+----------------------------------+------------------------------+
| Datum Definition      | Fixed Mean Sea Level at 26 tide  | Single primary tidal bench   |
|                       | gauges (21 in US, 5 in Canada).  | mark: Father Point/Rimouski. |
+-----------------------+----------------------------------+------------------------------+
| Gravity Correction    | Normal orthometric correction    | True geopotential numbers    |
|                       | (did not use observed gravity).  | using actual gravity data.   |
+-----------------------+----------------------------------+------------------------------+
| Network Distortion    | High (forced MSL to zero at all  | Minimal (single-point        |
|                       | gauges despite sea topography).  | minimum-constraint adjust).  |
+-----------------------+----------------------------------+------------------------------+
| NC Datum Offset       | Base reference level.            | NAVD88 is approx 0.7 to 1.2  |
| (H_NAVD88 - H_NGVD29) |                                  | ft LOWER in numerical value. |
+-----------------------+----------------------------------+------------------------------+

Why NGVD29 Was Flawed:

NGVD29 assumed that Mean Sea Level was at zero dynamic elevation across all 26 tide gauges simultaneously. In reality, ocean currents (like the Gulf Stream off the North Carolina coast), salinity variations, and wind patterns create sea surface topography, causing true mean sea level to vary by up to $1\text{ meter}$ between the Atlantic, Gulf, and Pacific coasts. Forcing all gauges to zero introduced internal mathematical warping across the national leveling network.

Vertical Datum Shifts in North Carolina:

When converting between NGVD29 and NAVD88 in North Carolina using the NGS VERTCON transformation tool (integrated into the NCAT - NGS Coordinate Conversion and Transformation Tool):

$\text{Elevation}_{\text{NAVD88}} \approx \text{Elevation}_{\text{NGVD29}} - 0.85\text{ ft} \quad (\text{Varies from }-0.65\text{ ft in Outer Banks to }-1.25\text{ ft in Cherokee})$

[!CAUTION] When evaluating FEMA Flood Insurance Rate Maps (FIRMs), surveyors must verify the vertical datum. Older FIRM panels cite NGVD29, while newer Digital FIRMs (DFIRMs) use NAVD88. Applying an elevation from an NGVD29 benchmark to an NAVD88 flood certificate without converting datums creates a disastrous 0.85-foot error, potentially placing a structure below base flood elevation (BFE)!

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3. Hybrid Geoid Models (GEOID12B to GEOID18)

Because the raw gravimetric geoid does not perfectly fit the leveling network due to historic benchmark settling and regional gravity anomalies, NGS produces Hybrid Geoid Models for surveying applications.

+-----------------------------------------------------------------------------+
|                     HYBRID GEOID MODEL CREATION PROCESS                     |
|                                                                             |
|   [Gravimetric Geoid Model (e.g., xGEOID)]                                  |
|   • Derived purely from airborne gravity (GRAV-D), satellite, & surface data|
|                                │                                            |
|                                ▼ Warped using Residual Interpolation        |
|   [GPS on Benchmarks (GPSonBM) Dataset]                                     |
|   • Thousands of high-precision GNSS observations on published NAVD88       |
|     first- and second-order spirit-leveled bench marks                      |
|                                │                                            |
|                                ▼                                            |
|   [Final Hybrid Geoid Model: GEOID18]                                       |
|   • Enables direct conversion: H_NAVD88 = h_NAD83(2011) - N_GEOID18         |
|   • Absolute accuracy across North Carolina: 1.0 to 1.5 cm (0.03 to 0.05 ft)|
+-----------------------------------------------------------------------------+

GEOID18 represents the pinnacle and final hybrid geoid model supporting NAD83(2011) and NAVD88. It incorporates improved airborne gravity data and extensive GPSonBM campaigns conducted by NCGS across North Carolina.

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4. NSRS Modernization: NATRF2022 & NAPGD2022

To overcome fundamental geometric and physical limitations in NAD83 and NAVD88, the National Geodetic Survey is completing the Modernized National Spatial Reference System (NSRS Modernization).

+-----------------------------------------------------------------------------------------+
|                    LEGACY VS. MODERNIZED NSRS REFERENCE FRAMES                          |
+-----------------------+----------------------------------+------------------------------+
| Geodetic Component    | Legacy System                    | Modernized NSRS (2022)       |
+-----------------------+----------------------------------+------------------------------+
| Geometric Datum       | NAD83 (2011) Epoch 2010.00       | NATRF2022 (North American    |
| (Horizontal & 3D)     | (Non-geocentric; 2.2m offset).   | Terrestrial Reference Frame) |
+-----------------------+----------------------------------+------------------------------+
| Geopotential Datum    | NAVD88                           | NAPGD2022 (North American-   |
| (Vertical & Height)   | (Passive brass bench marks).     | Pacific Geopotential Datum)  |
+-----------------------+----------------------------------+------------------------------+
| Geoid Model           | GEOID18 (Hybrid model).          | GEOID2022 (Purely gravimetric|
|                       |                                  | geoid model from GRAV-D).    |
+-----------------------+----------------------------------+------------------------------+
| Coordinate Dynamics   | Static coordinates tied to       | Time-dependent coordinates   |
|                       | fixed epoch dates.               | with Intra-Frame Velocities. |
+-----------------------+----------------------------------+------------------------------+

1. NATRF2022 (North American Terrestrial Reference Frame of 2022)

• True Geocentric Origin: The origin of NAD83 is offset from the Earth's true center of mass (geocenter) by approximately 2.2 meters (7.2 ft). NATRF2022 is aligned with the International Terrestrial Reference Frame (ITRF), placing the coordinate origin precisely at the Earth's center of mass.
• Horizontal Coordinate Shift in NC: Moving from NAD83(2011) to NATRF2022 shifts horizontal coordinates in North Carolina by approximately 1.1 to 1.4 meters (3.6 to 4.6 ft) in a west-northwesterly direction.

2. NAPGD2022 (North American-Pacific Geopotential Datum of 2022)

• Pure Gravimetric Datum: Replaces passive, decaying bench marks with a dynamic geopotential surface defined by the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project.
• Vertical Elevation Shift in NC: Elevations in North Carolina change by $-0.20\text{ m to }-0.45\text{ m}$ ($-0.66\text{ ft to }-1.48\text{ ft}$) relative to NAVD88.

3. Time-Dependent Coordinates & Intra-Frame Velocity Models (IFVM)

Because tectonic plates drift and crustal subsidence/uplift occurs continuously, coordinates in NATRF2022 will be expressed with reference epochs and updated using the Intra-Frame Velocity Model (IFVM), accounting for localized crustal motion in coastal and mountain zones of North Carolina.

4. SPCS2022 in North Carolina

Under the modernized State Plane Coordinate System of 2022 (SPCS2022), North Carolina maintains:

• A Statewide Single-Zone Lambert Conformal Conic layer (similar to the legacy system, retaining statewide consistency for GIS and transportation).
• Low Distortion Projections (LDPs): Regional multi-zone coordinate layers designed specifically to place the projection grid directly at ground elevation in high-relief areas (such as the Blue Ridge Mountains), reducing ground-to-grid scale distortion to $\le 20\text{ ppm}$ ($0.02\text{ ft}$ per $1,000\text{ ft}$).