A GPS rover uses RTK corrections from a base station or network to achieve centimeter-level accuracy. To use it for construction layout: initialize the rover to RTK Fixed status, run a site calibration to local control points, verify with a check shot within 0.02 ft, then navigate to design points for stakeout or collect existing feature positions for as-built records.
GPS rovers from Trimble, Topcon, and Leica allow one-person layout and as-built survey across large sites without line-of-sight constraints. The key to accurate results is understanding the difference between RTK Fixed and Float status, and performing a proper site calibration to local control points before any layout work begins.
High-accuracy GNSS receivers: Trimble R12i, Topcon Hiper 6, Leica GS18 T. Multi-constellation receivers (GPS, GLONASS, Galileo, BeiDou) achieve Fix faster and perform better in obstructed skies. Tilt-compensated models (Leica GS18 T, Trimble R12i) allow pole-tilt measurement without bubbling up.
A base station (Trimble R10, Topcon Hiper HR) broadcasts corrections to the rover via radio. Network RTK services (Trimble RTX, Leica SmartNet, state CORS networks) provide corrections via cellular internet — no physical base needed. Network RTK requires cellular coverage at the job site.
Trimble TSC7, Topcon FC-6000, or equivalent running Trimble Access, Magnet Field, or Leica Captivate. The data collector manages satellite connections, site calibration, stakeout guidance, and data recording. Job files carry your control coordinates, design points, and calibration.
Aluminum or carbon fiber pole, typically 2 meters. A bipod maintains pole plumb while you operate the data collector. Pole tip centering must be exact — a bent or worn tip introduces a systematic position offset equal to the tip displacement.
If using a base station: set the base over a known control point, measure the antenna height, power on, and confirm it is broadcasting on the correct radio channel or Bluetooth frequency. The rover must receive base corrections before it can achieve RTK Fixed status.
If using network RTK: open your data collector, connect to cellular, and configure the NTRIP or RTX connection with your subscription credentials. Confirm the correction stream is active before powering on the rover. For projects in areas with spotty cellular coverage, bring a portable hotspot as a backup.
Power on the rover and open the survey project in the data collector. The status bar shows the solution type: Autonomous (no corrections), Float (corrections received, ambiguities not resolved), or Fixed (full RTK, centimeter accuracy). Wait for Fixed before collecting any positions.
Fixed status typically takes 30 to 90 seconds in open sky. If Fixed is not achieved after 5 minutes, check: correction source is active and received by the rover, satellite count is above 6, PDOP is below 3.0, and there are no obstructions blocking the sky. Avoid operating near tall buildings or under tree canopy.
GPS coordinates are in WGS84 — a global datum. Your project coordinates are in a local grid (state plane, local arbitrary, or project-specific). A site calibration collects GPS observations on local control points and computes the mathematical transformation between the two systems.
Occupy two or more local control points with the rover in RTK Fixed status. In the data collector, run the localization or calibration routine. A minimum of two horizontal and two vertical control points is required; three or more is recommended for reliability.
Without a site calibration, your layout positions may be off from project coordinates by 1 to 3 feet even with RTK Fixed — a very common source of layout errors on GPS-equipped crews.
After calibration, occupy a control point not used in the calibration and compare the measured position to the known coordinates. Horizontal residual should be within 0.02 ft and vertical within 0.05 ft for construction layout. If residuals are larger, check that you occupied the correct monument and that the control coordinates are in the correct coordinate system.
Log your calibration check shots in Gradelog — timestamped verification records protect you if the inspector questions layout accuracy later.
For stakeout: select the target point. The data collector shows directional arrows and a running distance to the point. Walk toward the point until the horizontal distance reads less than your stakeout tolerance (typically 0.02 ft). Mark with a hub nail or lath stake.
For as-built: hold the rover pole plumb on each feature (pipe rim, manhole cover, grade hub, corner) and collect the position. The data collector records coordinates, elevation, and a comparison to design elevation. Use the elevation calculator to cross-check critical elevations before accepting a feature as-built.
Float solution accuracy is 0.1 to 0.3 meters — never acceptable for layout. Always confirm Fixed status in the data collector before collecting any points.
GPS positions in WGS84 do not match local project coordinates without calibration. The offset can be 1 to 3 feet. Run a site calibration at the start of every new job or after moving to a new area of a large project.
PDOP (Position Dilution of Precision) above 3.0 indicates poor satellite geometry. Avoid collecting layout points when PDOP is above 3. Check PDOP in the satellite status screen of your data collector.
Reflected GPS signals from metal surfaces, buildings, and overhead power lines degrade accuracy. Keep the rover antenna away from these surfaces when collecting or staking points.
If using a base station, an incorrect antenna height entry biases all rover elevations by a fixed amount. Measure and record the base antenna height precisely. A 0.1 ft error in base HI means a 0.1 ft elevation error in every rover shot.
Field Documentation
Use Gradelog to log and verify your grade shots digitally — free to start. As-built reports, control verification records, and inspection-ready data on every job.
With RTK Fixed solution and a site calibration to local control, a GPS rover achieves horizontal accuracy of 0.02 to 0.03 ft and vertical accuracy of 0.05 to 0.08 ft for construction work. Without calibration, GPS positions may be off from project coordinates by 1 to 3 feet. Always calibrate to local control before layout.
RTK GPS uses a base station or correction network to broadcast satellite carrier-phase corrections to the rover. The rover uses these corrections to resolve carrier-phase ambiguities and compute a precise position in real time. Without RTK corrections, a standalone GPS receiver is accurate to only 1 to 3 meters. With RTK Fixed, accuracy improves to 1 to 2 centimeters.
RTK Fixed means the receiver has resolved carrier-phase ambiguities and is computing a precise centimeter-level position. RTK Float means ambiguities are not fully resolved — accuracy is 0.1 to 0.3 meters, which is not acceptable for construction layout. Never stake or record as-built positions in Float status.
Network RTK (VRS/CORS services) eliminates the need for a physical base station by providing corrections from a network of reference stations. It is convenient for large projects and multiple rovers. A physical base station is required in areas with poor cellular coverage or when network RTK service is unavailable.