2.2 GNSS/GPS Survey Modes, Datums, and Field Checks

Use GNSS/GPS as a Survey Method, Not a Magic Coordinate Source

GNSS/GPS can be a powerful surveying method, but the FS exam expects candidates to understand its limits. A receiver can produce coordinates quickly, yet those coordinates are only useful when the occupation method, correction source, datum, coordinate system, and quality checks fit the job. Treat GNSS/GPS as a surveying workflow that requires planning and verification.

Common field decisions include whether to use static, rapid static, real-time kinematic, network-based corrections, or mapping-grade collection. The exact terminology in a problem may vary, but the decision logic is stable. Higher precision control generally needs stronger procedures, longer or better-planned observations, reliable corrections, and independent checks. Lower precision mapping may tolerate faster collection, but it still needs correct metadata and quality awareness.

Datum awareness is a major practical issue. GNSS/GPS observations relate to geodetic reference frames, while project deliverables may require a state plane coordinate system, local grid, assumed datum, or specified vertical datum. If a problem says coordinates do not match record control, one possible cause is that the crew mixed datums, transformations, units, or grid-ground assumptions.

Vertical results deserve special caution. GNSS/GPS ellipsoid heights are not the same as orthometric elevations used in many surveying deliverables. A workflow may require a geoid model or other vertical transformation. FS questions may not ask for advanced geodesy in this chapter, but they can test whether a candidate recognizes that GNSS/GPS vertical output needs correct reference handling.

Field checks should be designed before collection starts. Occupy a known control point, revisit points later, compare with total station or level observations where appropriate, and inspect residuals or quality indicators. Do not wait until office processing to discover that a canopy, wrong rod height, or wrong coordinate system affected the entire session.

GNSS/GPS also affects field records. Notes should identify receiver setup, antenna height, point name, observation method, correction source, time, coordinate system, datum, and unusual site conditions. A coordinate without metadata can become difficult to defend.

For FS preparation, practice reading scenarios for the failure mode. If the site is under heavy tree cover, sky visibility may be the issue. If repeated occupations disagree near a metal building, multipath may be involved. If all points are consistently shifted, project settings or datum transformation may be suspect. The exam often rewards that diagnostic reasoning.