Drone Surveying — Accuracy and RTK

"±2 cm accuracy" appears on almost every drone survey quote — but the number alone says little if you don't know how it's achieved. This article goes one level deeper than our drone surveying fundamentals article and explains what technically distinguishes RTK from PPK, when ground control points are worth it anyway, and which levers most strongly affect the accuracy actually achieved in the field.

RTK vs. PPK — the difference

Both methods use a fixed reference station with a known position to correct the drone's GPS measurement down to centimetre level. The difference lies in when the correction happens:

• RTK (Real-Time Kinematic): The correction runs during the flight via a radio or mobile link to the reference station. The result is available immediately after the flight, but it depends on a stable radio connection.
• PPK (Post-Processed Kinematic): The drone and reference station each record raw data independently. Correction happens after the flight, on a computer. That makes PPK resilient to radio dropouts during flight, but it costs time in post-processing.

In practice, RTK is the more common standard for survey flights in the DACH region because the result can be checked directly on site. PPK gains importance for very large sites or in areas with poor radio coverage.

Why ground control points still matter

Ground control points (GCPs) are fixed, surveyed reference markers placed on site whose exact coordinates are known independently of the drone. Even though RTK already determines the drone's position to centimetre accuracy during flight, GCPs solve a different problem: they provide independent verification and further improve the georeferencing of the finished model.

GCPs make the most sense for long, narrow sites (e.g. corridors, embankments), for especially high accuracy requirements, or when the RTK correction may have briefly dropped out during flight. For most standard projects — company premises, construction sites, solar farms — RTK alone without additional GCPs is sufficient.

What influences real-world accuracy

RTK is a necessary but not a sufficient condition for an accurate result. In practice, these factors determine the accuracy actually achieved:

• Flight altitude and ground sampling distance (GSD): The lower the flight, the finer the ground sampling distance and the higher the potential detail accuracy — at the cost of flight time and battery life.
• Image overlap: Insufficient forward or side overlap between photos creates gaps or weak tie points in the photogrammetry computation.
• Camera calibration: An imprecisely calibrated camera systematically distorts the computed geometry — regardless of how good the GPS position was.
• Wind and vibration: Motion blur from wind or vibration degrades image quality and, with it, the matching accuracy between overlapping shots.
• Lighting conditions: Hard shadows or changing light during the flight make photogrammetric matching between overlapping images more difficult.
• Distribution of control points: Even with RTK: without control points near the edges of the site, systematic distortion at the boundaries is harder to detect.

Horizontal vs. vertical accuracy

One point that quotes often gloss over: horizontal positional accuracy (X/Y) and vertical elevation accuracy (Z) are not equally good with photogrammetric drone methods. Vertical accuracy is inherently worse than horizontal accuracy — a well-known, method-inherent characteristic of image-based measurement that holds regardless of the RTK system used. For projects where elevation matters most (earthworks volume calculations, flood simulation, roof pitch), it's worth looking explicitly at vertical accuracy rather than relying on horizontal accuracy alone.

When RTK is enough, when you need more

A practical breakdown:

• RTK alone is enough: Compact, well-shaped sites (company premises, construction sites, solar farms), standard accuracy for planning and documentation.
• RTK plus ground control points: Long, narrow sites, especially high accuracy requirements, independent quality control required (e.g. for regulatory sign-off).
• Classical surveying also needed: Statutory positional reference, land-registry relevance or millimetre-level tolerances — here the drone complements classical geodesy but doesn't replace it.

For more on the output formats a drone flight produces and what they're suited for, see Orthophoto, DTM, DSM, Point Cloud — Which Output for Which Purpose.

Frequently asked questions about accuracy and RTK

Conclusion

RTK is the foundation for accurate drone surveying, but it's not a free pass. Flight altitude, overlap, calibration, light and wind matter at least as much for the final result as the positioning system itself. Anyone who needs especially high accuracy or independent verification adds ground control points to RTK — and keeps horizontal and vertical accuracy in view separately.

Planning a project with clear accuracy requirements? Request a consultation — we'll help you work out which setup fits your site.