Section A: Horizontal Datum Compliance
Horizontal datum compliance is the foundational requirement before any survey submission. All items below must be confirmed:
- Datum Identification: The project datum is explicitly documented on all deliverables (e.g., NAD83(2011) Epoch 2010.0000)
- NSRS Alignment: For federally funded projects, the datum must be tied to the National Spatial Reference System (NSRS) via an NGS-approved method
- NADCON5 Transformation: If historical datums (NAD27, NAD83 earlier realizations) are involved, NADCON5 via NCAT is used — not 3-parameter or 7-parameter Helmert approximations
- WGS84 vs NAD83 Reconciliation: Raw GNSS output (which typically references WGS84/ITRF) has been transformed to the project NAD83 frame using time-aware parameters if sub-10cm accuracy is required
Section B: Vertical Datum Compliance
Vertical datum errors are the most common cause of flood zone compliance failures:
- NAVD88 Is Current Standard: All NFIP-related work uses NAVD 88 (EPSG:5703), not NGVD 29
- Conversion Applied: If existing benchmarks are in NGVD29, VERTCON 3.0 shift ΔH has been applied: H_NAVD88 = H_NGVD29 + ΔH
- Geoid Model: Ellipsoidal height from GNSS has been converted to orthometric height using the current GEOID model (GEOID18 or later)
- FEMA BFE Datum Check: The FIRM panel datum has been confirmed and elevations compared against it consistently
Section C: Grid-to-Ground (DOT/Engineering) Compliance
| Parameter | Formula | Acceptable Range |
|---|---|---|
| Elevation Factor | E = R/(R+H) | Computed at project centroid |
| Grid Scale Factor | k from SPCS tables | <1:10,000 for SPCS |
| Combined Factor | C = E × k | Applied to all measured distances |
| Traverse Closure | 1:50,000 (1st order) | Verified before submission |
Section D: ICAO/Aviation Compliance
For surveyors working on aerodrome and airspace data:
- WGS84 Mandatory: All aeronautical coordinates submitted to AIS/AIM must reference WGS84 (ICAO Annex 15 / PANS-AIM)
- Coordinate Resolution: Threshold and obstacle positions recorded to 1/100 of a second of arc minimum
- 90% Confidence Accuracy: Survey demonstrates ≤1 m horizontal accuracy at 90% confidence for runway-class features
- Datum Transformation Documented: If survey was performed in a national frame (e.g., ETRS89, GDA2020), the transformation to WGS84 is documented by method and epoch
✅ Pre-Submission Compliance Checklist
- Datum is explicitly identified on all survey documents and deliverables
- NSRS tie performed using NGS-approved control network for federal/FEMA projects
- NADCON5/VERTCON3 used for all datum transformations (no ad-hoc shifts)
- Elevation factor E and grid scale factor k computed for all DOT/engineering projects
- Combined factor C applied to all measured distances in State Plane projects
- NAVD88 used for all FEMA/NFIP elevation certificates
- NGVD29/NAVD88 discrepancy resolved via site-specific VERTCON 3.0 shift
- Aviation data formatted in WGS84 with required resolution and accuracy
- Traverse closure meets the required order for the project type
- All QA documentation retained for the professional statute of limitations period
Frequently Asked Questions
What is the current official US horizontal datum for NSRS work?
NAD83(2011) Epoch 2010.0000 is the current NSRS horizontal datum for most CONUS projects. NGS is transitioning toward NATRF2022 under the NSRS Modernization program, which will align with ITRF2014. Surveyors should confirm the applicable datum with the project authority.
When is a GNSS survey NSRS-qualified?
An NGS-approved GNSS survey must use Online Positioning User Service (OPUS) or a differential GNSS baseline to published, NGS-maintained CORS stations. The resulting coordinates are then traceable to the NSRS.
What geoid model should I use in 2026?
NGS recommends GEOID18 for CONUS projects pending the GEOID2022 release tied to NAPGD2022. Always verify the current recommended model at ngs.noaa.gov before project execution.