RTK GPS setup requires: (1) a base station over a known control point broadcasting corrections, or a network RTK connection via cellular, (2) the rover achieving RTK Fixed status (not Float) before collecting any positions, (3) a site calibration to tie WGS84 GPS to the project coordinate system, and (4) a check shot on an independent control point to verify accuracy within 0.02 ft horizontal and 0.05 ft vertical.
RTK GPS from Trimble, Topcon, and Leica is the foundation of modern construction survey. Multi-constellation GNSS receivers tracking GPS, GLONASS, Galileo, and BeiDou achieve centimeter-level accuracy in less than 90 seconds from power-on. However, centimeter accuracy requires a correct setup procedure — incorrect base antenna height, a missing calibration, or a Float solution will produce meter-level errors that look identical to a correct setup on the data collector screen.
A GNSS receiver set over a control point that broadcasts corrections to rovers by radio. Provides corrections to unlimited rovers within radio range (typically 5 to 10 miles). Requires an operator to set up and monitor the base. Required for areas without cellular coverage. Common base receivers: Trimble R10, Topcon Hiper HR, Leica GS18.
A service that delivers RTK corrections via cellular internet from a network of permanent reference stations. No physical base station setup required. One subscription covers any location within the network coverage area. Requires cellular coverage at the job site. Services: Trimble RTX, Leica SmartNet, state CORS networks.
Multi-constellation receivers tracking GPS, GLONASS, Galileo, and BeiDou. Current-generation receivers: Trimble R12i, Topcon Hiper 6, Leica GS18 T. Tilt-compensated models allow measurements without bubbling the pole. Track 400 to 800 channels simultaneously for fast fix times and reliable performance in partially obstructed environments.
Trimble TSC7, Topcon FC-6000, or Leica CS20 running Trimble Access, Magnet Field, or Leica Captivate. The data collector manages satellite connections, calibration, point collection, and stakeout. Job files contain control coordinates, design points, calibration, and collected field data.
Center the base tripod over the control point using the plummet. Level the tribrach. Mount the base antenna and measure the antenna height from the control point monument to the ARP (Antenna Reference Point) mark on the antenna — measure slant height or vertical height per your antenna manufacturer specification. A 0.01 ft error in antenna height creates a 0.01 ft elevation error in every rover shot.
Enter the control point coordinates (Northing, Easting, Elevation), antenna height, and antenna type in the base controller. Power on and confirm the base status LED shows corrections broadcasting. Test the radio link by walking the rover to the far end of the site and confirming corrections are received.
Power on the rover. Open the survey job in the data collector and select the correction source (radio or NTRIP). Monitor the solution status: Autonomous (no corrections) → Float (corrections received, initializing) → Fixed (full RTK, centimeter accuracy).
If Fixed is not achieved within 5 minutes: confirm corrections are streaming, check sky visibility (minimum 15-degree elevation mask), and verify satellite count above 6 and PDOP below 3.0.
GPS positions are in WGS84 — a global 3D datum. Your project uses a local coordinate system (state plane, local arbitrary, or project-specific) and a vertical datum (NAVD88 or project benchmark). A site calibration computes the transformation between the two systems.
In RTK Fixed status, occupy each local control point. In the data collector, run the localization or calibration procedure. Use a minimum of three horizontal control points and two vertical control points. The data collector computes residuals at each point — residuals should be below 0.02 ft horizontal and 0.05 ft vertical.
Save the calibration to the job file. Every rover using this job file will automatically apply the calibration. If adding a second rover to the project, load the same job file with the saved calibration — do not re-run calibration independently on each rover.
Occupy a control point not used in the calibration. In the data collector, select "Check Shot" mode and measure the point. The data collector displays the difference between the measured and known coordinates.
If the check shot fails, do not begin layout. Recheck base antenna height, verify the correct control point coordinates are loaded, and confirm you are occupying the correct monument. Re-run calibration and re-verify before proceeding.
During survey work, keep the solution status display visible on the data collector. If the solution drops to Float or Autonomous — due to signal loss, multipath, or correction stream dropout — stop collecting immediately. Reacquire Fixed status before continuing.
After any solution interruption, re-occupy a known control point to verify accuracy before continuing layout or as-built collection. Log check shots in Gradelog throughout the survey day so you have a continuous record of solution quality. Use the elevation calculator to cross-check critical elevation points collected with RTK GPS.
Measure antenna height carefully per the manufacturer specification (slant vs vertical). A 0.05 ft antenna height error creates a 0.05 ft elevation error in every rover shot. Measure twice and confirm before starting the base.
Float is 0.1 to 0.5 meters — far too inaccurate for construction layout. The data collector shows the solution type clearly. Make it a rule: never press Store in Float.
WGS84 GPS coordinates do not match local project coordinates without calibration. The offset can be 1 to 3 feet. This is the most common cause of GPS layout errors on construction sites.
Physically confirm which monument you are standing on before collecting a calibration point. Occupying the wrong monument produces a large calibration residual or a calibration that appears to fit but is offset.
If the base station is moved or bumped, all rover positions collected after the disturbance are invalid. The base must remain stationary throughout the session. Never leave a base station unattended in an equipment traffic area.
Field Documentation
Use Gradelog to log and verify your grade shots digitally — free to start. Check shot verification records, calibration history, and as-built data in one place.
GPS is the US satellite system. GNSS covers all systems combined — GPS, GLONASS, Galileo, and BeiDou. Modern receivers track all constellations. RTK is a correction technique using carrier-phase measurements from a base station or network to achieve centimeter accuracy. A GNSS RTK receiver is the most capable type for construction survey.
Set the base over a control point, level and center the antenna, measure antenna height to the ARP mark per manufacturer spec, enter control coordinates and antenna height in the controller, power on, and confirm the base broadcasts corrections. The base must remain stationary throughout the survey session.
PDOP below 2.0 is ideal. Below 3.0 is generally acceptable for construction survey. Above 4.0 indicates poor satellite geometry and should be avoided for precision work. High PDOP typically occurs when satellites are clustered due to obstructions.
RTK GPS can establish and verify elevations for construction control, but vertical accuracy is typically 0.05 to 0.10 ft with a good calibration — two to three times worse than horizontal accuracy. For precise elevation control to plus or minus 0.02 ft (sewer inverts, pad elevations), a level or total station provides higher vertical accuracy than RTK GPS alone.
NTRIP is a protocol for streaming RTK correction data over the internet from a network of continuously operating reference stations. Your rover connects to an NTRIP caster via cellular and receives corrections without a physical base station. Services include Trimble RTX, Leica SmartNet, and state CORS networks.