GPS Accuracy for Construction FAQ: RTK, GNSS, and Tolerances

Understanding RTK Accuracy Specs

Published RTK accuracy specifications (e.g., "8mm + 1ppm horizontal") represent the 1-sigma (68% confidence) accuracy under ideal conditions. In practice, two-sigma (95% confidence) accuracy is about double the published spec. On a construction site with good satellite geometry, a stable correction source, and no obstructions, you can expect 95% of your measurements to fall within ±15-30mm horizontally and ±30-40mm vertically.

Vertical accuracy is inherently weaker than horizontal accuracy in GNSS — satellites are above the horizon, so vertical position determination is geometrically less precise. For grade-critical work, supplement GNSS with laser levels or total station shots at key control elevations.

When GPS Is Not Sufficient

GNSS is not the right tool for: work within 5-10 meters of structures (multipath), underground or covered areas, column layout to ±3mm tolerance, and any work where the required tolerance is tighter than ±1 inch vertical. For these applications, use a total station. See the total station guide for a comparison.

Frequently Asked Questions

What accuracy can I expect from RTK GPS on a construction site?

Under good conditions with a valid RTK fix, expect 15-25mm (0.05-0.08 ft) horizontal and 25-40mm (0.08-0.13 ft) vertical accuracy at 95% confidence. Published specs are 1-sigma values under ideal conditions. Near buildings, under trees, or with poor satellite geometry, actual accuracy can be 2-5x worse than published specs.

Why is GPS vertical accuracy worse than horizontal?

GNSS satellites orbit above the horizon, so there is always a gap in coverage directly below the antenna. This weakens vertical geometry compared to horizontal. The result is that vertical accuracy is typically 1.5-2.5x worse than horizontal accuracy in any GNSS system. For height-critical work, verify your GPS elevations against a leveled benchmark or use a total station or level for the final vertical control.

What is PDOP and why does it matter?

PDOP (Position Dilution of Precision) measures how well the available satellites are spread across the sky. A PDOP below 2 is excellent — tight, well-spread satellite geometry. PDOP of 2-4 is good; 4-6 is acceptable but expect degraded accuracy. Above 6, accuracy is significantly degraded and important layout work should wait for better geometry. Check PDOP on your data collector before beginning precision work.

What is the difference between GPS accuracy and precision?

Accuracy is how close the measured position is to the true position. Precision is how repeatable the measurements are — how tightly clustered multiple measurements of the same point are. High precision with low accuracy means you are consistently wrong in the same direction (systematic error, often from a bad control point or incorrect coordinate system). Always check both by comparing to multiple known control points.

How does obstruction affect GPS accuracy on a construction site?

Buildings, cranes, stockpiles, and heavy equipment block satellite signals on the obstructed side, reducing the satellite count and worsening geometry. This typically causes horizontal position errors and, in severe cases, a loss of RTK fix entirely. Work within 5-10 meters of a tall structure should be done with a total station rather than GPS. On open sites, GPS performs closest to its published specification.

What is multipath and how bad can it get?

Multipath is satellite signal reflection off nearby surfaces — building facades, metal structures, vehicles, or even standing water. Reflected signals arrive at the antenna delayed and combine with the direct signal, causing position errors. Multipath errors can be 0.1 to 1 meter in severe cases. Multipath is the dominant error source near structures and the main reason GPS cannot replace total stations for work close to buildings.

How do I know if my GPS reading is reliable on site?

Check: RTK fix status (fixed, not float), satellite count (at least 6, preferably 10+), PDOP (below 4 for important work), and correction age (latency under 5 seconds for network RTK). Check your position against a second known control point at the start of each shift. If the check shows error larger than your required tolerance, investigate before collecting layout data.

What is the difference between RTK fixed and RTK float?

RTK fixed means the receiver has resolved the integer ambiguity in the carrier phase measurement — this is the high-accuracy mode at 8-15mm horizontal. RTK float means the receiver has not resolved the integer ambiguity and is using a floating-point solution — accuracy is typically 0.3-1 meter, not suitable for construction layout. Always confirm you have an RTK fixed solution before collecting layout points.

How long does it take for GPS to achieve RTK fixed?

Initialization (achieving RTK fixed from cold start) typically takes 30 seconds to 3 minutes with a good correction source and sufficient satellites. Moving into a clear sky area after obstruction re-initializes in 30-60 seconds. If initialization takes more than 5 minutes, check your correction signal (base station or network RTK connection) and satellite count.

What accuracy does GPS machine control achieve on a dozer?

3D GPS machine control with RTK achieves ±1-2 inches vertical at the blade under good conditions. This is sufficient for rough and finish grading on most earthwork and subgrade applications. For concrete base and finish grade tolerances (±1/4 to ±1/2 inch), verify GPS-graded surfaces with a laser or total station before placing the next layer. See the machine control accuracy FAQ.

Can I use GPS for column layout?

No — column anchor bolt layout typically requires ±3-6mm (±1/8 to ±1/4 inch) positional accuracy. RTK GPS can achieve this under ideal conditions, but the risk of multipath near the building structure and the difficulty of verifying GPS accuracy at that tolerance level makes total stations the standard tool for column layout. Use GPS for approach stakeout and a total station for the final anchor bolt positions.

What accuracy does GPS stakeout achieve for utility layout?

For utility centerline stakeout in open areas, RTK GPS at ±0.05 ft horizontal is typically acceptable. For utility inverts and pipe centerlines at manholes where accuracy requirements are tighter, supplement with a total station or level. Always confirm stakeout accuracy against your project's design tolerance specification — different utilities have different accuracy requirements.

How does the correction source affect GPS accuracy?

A base station on a precisely known control point provides the most consistent corrections. Network RTK (VRS) corrections are typically comparable in accuracy to a well-set base station within the network coverage area. Both are significantly better than SBAS (WAAS) corrections, which provide sub-meter accuracy. For construction layout, RTK (base or network) is required — SBAS is not suitable for construction stakeout.

What is ionospheric error in GNSS and does it affect construction work?

The ionosphere (upper atmosphere) delays satellite signals and introduces position errors. RTK corrections largely cancel ionospheric error for baselines under 20 km. For longer baselines to the base station, ionospheric error can degrade accuracy. This is one reason network RTK (which uses nearby reference stations) outperforms a long baseline to a distant base station. On most construction sites, ionospheric error is not a significant factor if the base-rover baseline is under 20 km.

How should I document GPS accuracy on a construction project?

Document: the RTK correction source used (base station coordinates or network provider name), the control points used and their check residuals, daily setup checks against known points, and any periods where fix quality was degraded. This record demonstrates due diligence if accuracy questions arise during final inspection. Field documentation platforms like Gradelog can automate this logging.