RTK Fixed vs Float: What's the Difference and Why It Matters
Fixed and Float are two different RTK solution states with very different accuracy levels. A Fixed solution means the receiver has resolved the carrier-phase integer ambiguity, delivering centimeter-level accuracy (typically ±8mm horizontal). A Float solution means that ambiguity has not yet been resolved, delivering only decimeter-level accuracy (roughly 20-50cm) — still better than standard GPS, but not precise enough for boundary staking, construction layout, or cadastral work. Understanding which state your survey is actually in is critical: many field errors trace back to recording points while still in Float, mistaking it for Fixed.
1. The Four RTK Solution States
A GNSS receiver calculates its position through distinct computational stages depending on signal availability and differential correction inputs:
| Solution State | Accuracy Level | Technical Description |
|---|---|---|
| Single (Autonomous) | Meter-level (1.5m to 3.0m) | Standard standalone positioning using uncorrected satellite broadcast ephemeris. |
| DGPS | Sub-meter (0.5m to 1.0m) | Code-phase pseudorange differential positioning without carrier-phase ambiguity resolution. |
| Float | Decimeter-level (20cm to 50cm) | Carrier-phase differential positioning where integer ambiguities remain unresolved statistical estimates. |
| Fixed | Centimeter-level (±8mm horizontal) | Full carrier-phase integer ambiguity resolution delivering survey-grade precision. |
👉 Review foundational terminology in our comprehensive reference: GNSS & RTK Glossary: 40+ Survey Terms Explained
2. Fixed: What It Means and How Accurate It Is
A Fixed solution represents the target operational state in high-precision RTK surveying. It indicates that the positioning engine has mathematically solved the exact integer number of radio carrier wavelengths (carrier-phase integer ambiguities) between every tracked satellite and the receiver antenna.
Resolving integer ambiguities eliminates the primary atmospheric (ionospheric and tropospheric) and clock timing error sources, unlocking true centimeter-level positioning (typically ±8mm horizontal and ±15mm vertical accuracy).
Under open sky conditions with a modern multi-constellation receiver, acquiring a Fixed solution typically takes less than 30 seconds.
A Fixed solution is mandatory for legal cadastral boundary demarcation, construction stakeout, structural deformation monitoring, and earthwork grading where engineering tolerances must be legally and structurally certified.
3. Float: What It Means and How Accurate It Is
A Float solution indicates that the receiver is actively ingesting differential corrections and tracking carrier-phase signals, but the positioning algorithm has not yet resolved the ambiguities into definite integer whole numbers. Instead, ambiguities are estimated as continuous decimal (floating) variables.
Float accuracy sits in the decimeter range (typically 20cm to 50cm horizontal and 30cm to 80cm vertical). While far superior to autonomous consumer GPS, it lacks the precision required for boundary pins or structural alignment.
Common causes for remaining in Float include initial convergence time, partial physical obstructions (dense tree canopy, building facades), high GDOP, radio link dropouts, or high correction latency.
👉 If your receiver is repeatedly dropping between states, explore our troubleshooting guide: RTK Fixed Solution Keeps Dropping to Float: Diagnosis & Fix
4. Accuracy Comparison: Fixed vs Float Side by Side
The operational gap between Fixed and Float directly impacts project compliance and field efficiency:
| Comparison Dimension | Fixed Solution | Float Solution |
|---|---|---|
| Horizontal Accuracy | ±8mm (0.008m) | 20cm to 50cm (0.2m to 0.5m) |
| Vertical Accuracy | ±15mm (0.015m) | 30cm to 80cm (0.3m to 0.8m) |
| Ambiguity Resolution | Fully resolved whole integers | Statistical floating-point estimates |
| Suitable for Legal Cadastral Work | Yes (Mandatory standard) | No (Exceeds statutory error limits) |
| Suitable for Civil Construction Stakeout | Yes (Meets engineering tolerances) | No (Risk of foundation/curb misalignment) |
| Suitable for Preliminary Reconnaissance | Yes (Optimal precision) | Yes (Sufficient for rough spatial scouting) |
| Typical Time to Acquire | <30 seconds (Clear sky) | Instantaneous / Intermediate stage |
5. When Float Is Acceptable (And When It Isn't)
For cadastral boundary pegging, foundation bolt layout, road curbing, drainage invert grading, and geodetic control networks, a Fixed solution is strictly required. Storing Float points in these applications introduces 20cm to 50cm discrepancies that lead to costly reconstruction or legal boundary disputes.
Float accuracy is acceptable for route scouting, preliminary site reconnaissance, asset mapping in forestry management, and rough agricultural land classification where decimeter approximation satisfies project deliverables without slowing down field crews.
Regardless of the survey type, ensure your data collector records the exact solution state (Fixed vs Float) and corresponding horizontal/vertical RMS values alongside point coordinates. This maintains full traceability and prevents office engineers from processing Float points as verified engineering baselines.
6. How to Tell Which State You're In
Field data collection software (such as ApekSurv) provides continuous real-time telemetry on the status bar of your data controller, displaying explicit indicators (e.g., green "FIX" vs orange/yellow "FLOAT").
Before triggering the point store command on your controller, always verify that the status indicator displays "FIX" and that the estimated horizontal RMS is within ±10mm.
Field software allows administrators to enforce quality control thresholds (e.g., enabling "Store on Fixed Only"), which automatically blocks the controller from logging points when the solution drops into Float.
7. FAQ
Is Float accuracy good enough for construction stakeout?
No. Construction stakeout requires millimeter-to-centimeter precision (typically within ±10mm to ±20mm) to align structural foundations, utilities, and grade elevations. Float accuracy (20cm–50cm) introduces substantial positional errors that exceed engineering building tolerances.
Why does my RTK receiver show Float instead of Fixed?
A receiver displays Float when it has not resolved carrier-phase integer ambiguities. Common causes include insufficient initialization time, overhead foliage or building multipath, weak satellite geometry (high PDOP), or high latency in base station radio/NTRIP correction streams.
How long should I wait for Fixed before recording a point?
Under open sky with good satellite coverage, a modern multi-constellation receiver achieves Fixed status within 10 to 30 seconds. If the receiver remains in Float for several minutes, relocate the rover to a more open vantage point or reset the RTK engine in your controller software.
Can I use Float solution data for legal cadastral boundaries?
No. Cadastral and land title surveys legally require certified boundary coordinates with centimeter-level precision. Float solution data carries an uncertainty of 20cm to 50cm, which fails statutory accuracy standards and can lead to boundary overlaps or legal liability.
KNOW YOUR SOLUTION STATE. TRUST YOUR DATA.
APEKS RTK GNSS displays real-time solution status on every field controller — so you always know if you're recording Fixed-quality data. ±8mm accuracy when it matters most.
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