2.2 GNSS/GPS Survey Modes, Datums, and Field Checks
Key Takeaways
- Static GNSS post-processes long occupations (often 15 min to 2+ hours by baseline length) for the highest accuracy; RTK delivers real-time centimeter positions from a base-to-rover correction.
- Network RTK / VRS uses a CORS network to synthesize corrections near the rover, extending reliable RTK beyond a single base's range.
- GNSS yields ellipsoid heights on NAD 83 (2011); a hybrid geoid model such as GEOID18 converts them to NAVD 88 orthometric heights (H = h − N).
- PDOP, satellite count, multipath, and a checked-into-a-known-point fixed solution are the core RTK field-quality checks.
Survey Modes: Static, RTK, and Network/VRS
GNSS (Global Navigation Satellite System) generalizes GPS to include GLONASS, Galileo, and BeiDou. The FS exam expects you to distinguish three field modes by accuracy and effort.
Static GNSS records simultaneous carrier-phase data at two or more receivers over a baseline, then post-processes to resolve integer ambiguities. Occupation time grows with baseline length and geometry: NGS specifies a minimum of about 15 minutes for short marks, roughly 45 min to 1 hour for baselines near 10 km, and 2 hours or more beyond 20 km. Static yields the highest GNSS accuracy and is the standard for establishing control.
Real-Time Kinematic (RTK) places a receiver on a known base broadcasting corrections to a moving rover, giving centimeter positions in real time. It is efficient for topographic pickup and stakeout but degrades as the base-to-rover distance grows.
Network RTK / Virtual Reference Station (VRS) uses a network of CORS (Continuously Operating Reference Stations) to model atmospheric and orbital errors and synthesize a correction as if a base sat right next to the rover. This extends reliable RTK well beyond a single base and removes the need to occupy a local base.
Heights, Datums, and the Geoid
GNSS measures position relative to an ellipsoid, so it returns ellipsoid height (h) on the NAD 83 (2011) horizontal datum, not the elevation a level would give. To obtain an orthometric height (H) on the NAVD 88 vertical datum, apply a hybrid geoid model such as GEOID18:
H = h − N
where N is the geoid height (the separation between ellipsoid and geoid; negative across most of the conterminous U.S.). GEOID18 is tuned so that NAD 83 (2011) ellipsoid heights produce orthometric heights consistent with published NAVD 88 datasheet values.
| Quantity | Symbol | Reference surface |
|---|---|---|
| Ellipsoid height | h | NAD 83 ellipsoid (from GNSS) |
| Geoid height | N | Ellipsoid-to-geoid separation (GEOID18) |
| Orthometric height | H | NAVD 88 geoid (“sea-level" elevation) |
NGS is modernizing the National Spatial Reference System: NAD 83 and NAVD 88 are being replaced by terrestrial reference frames such as NATRF2022 and the geopotential datum NAPGD2022, after which published coordinates and heights shift by decimeters or more. FS candidates should know which datum a project uses and that mixing datums produces large, systematic blunders.
GNSS Field Checks and Error Sources
A GNSS observation is only as good as its solution quality. Key field checks:
- Fixed (not float) solution — RTK is trustworthy only after integer ambiguities are fixed; a float solution can be off by decimeters.
- PDOP (Position Dilution of Precision) — a geometry index; lower is better. High PDOP means poor satellite spread and weaker positions.
- Satellite count and constellation — more satellites improve geometry and shorten occupation.
- Redundancy on a known point — occupying a published control mark or re-observing after re-initialization catches setup and antenna-height blunders.
- Multipath and obstructions — reflective surfaces and canopy corrupt the signal; GNSS struggles near buildings or under trees, where a total station may be the better tool.
A frequent FS trap is the antenna height (slant vs. vertical to the antenna reference point) — a measuring blunder there propagates directly into elevation. The exam rewards answers that pair the right mode (static for control, RTK/VRS for production) with the right validity checks and the correct datum/geoid handling.
A GNSS receiver reports an ellipsoid height of 312.40 m and the GEOID18 model gives a geoid height N = −29.80 m at the point. What is the approximate NAVD 88 orthometric height?
What does Network RTK / VRS provide that single-base RTK does not?
For establishing high-accuracy primary control, which GNSS approach is most appropriate?
Choosing a GNSS Mode in Practice
The FS exam frequently frames GNSS as a selection problem: given a site and a deliverable, which mode fits? Static is chosen for primary control and long baselines because post-processing and long occupations drive accuracy to the millimeter-to-centimeter level. RTK is chosen for high-volume topographic pickup and stakeout where centimeter accuracy in real time is enough and the work stays within a few kilometers of the base. Network RTK / VRS is chosen when the project spans a wide area or the crew wants to avoid setting and securing a local base.
| Mode | Typical use | Trade-off |
|---|---|---|
| Static (post-processed) | Primary control, long baselines | Long occupation; office processing |
| RTK (single base) | Topo, stakeout, near a base | Degrades with base-rover distance |
| Network RTK / VRS | Wide-area production | Needs network/cellular connection |
A related FS point is the role of OPUS (Online Positioning User Service): a single receiver can occupy a mark, submit its static file, and receive a solution tied to the CORS network and the NSRS — a way to establish a control point without a second receiver in the field. GNSS also blends with conventional work: a crew commonly sets two or more control points with GNSS, then runs a total-station traverse or topo from them, combining GNSS positioning with line-of-sight detail. Recognizing when GNSS is unsuitable — dense canopy, urban canyons, indoors — and a total station or leveling is required is itself a tested judgment.
Why Datum and Epoch Awareness Matters
GNSS positions are tied not only to a datum but to a realization and epoch (for example NAD 83 (2011) epoch 2010.0). In tectonically active regions the ground moves over time, so a position observed today differs from its published epoch value unless a velocity model is applied. The FS exam expects you to treat the realization, epoch, and geoid model as part of the answer, not afterthoughts — quoting a coordinate without its datum is incomplete and a frequent source of meters-level disagreement between datasets.