RTK, PPK, and GCPs: three ways to put a drone map in the right place

The GPS receiver in a consumer drone knows where it is to within a metre or three — fine for flying home, hopeless for telling a surveyor where a pad corner is. Every survey-grade drone map therefore uses one of three correction strategies, often two. DroneDeploy's primer on the topic is a good reference; here is the practical version.

RTK — corrected while flying

Real-Time Kinematic positioning streams corrections to the drone during the flight from a base station on site or a network of reference stations (a CORS network). The drone tags every photo with a centimetre-level position on the spot, so the map is essentially georeferenced when the aircraft lands. DroneDeploy highlights the upside — immediate results and far less time placing targets — and the catch: the correction link has to stay up. Terrain, structures, and poor cell coverage can break it, and the practical distance to the base is limited (the article cites roughly 35 km).

PPK — corrected after landing

Post-Processed Kinematic flips the order. The drone logs raw satellite observations during the flight; afterwards, software matches them against the base station's log and computes corrected positions for every photo. Nothing is lost if a radio link drops, because there wasn't one. DroneDeploy notes PPK tolerates much longer baselines (up to around 100 km) and suits remote sites, mountainous terrain, dense urban areas, and beyond-line-of-sight flights. The costs are a processing step before you see results and software that someone has to know how to run.

GCPs — surveyed truth on the ground

Ground control points are high-contrast targets laid out across the site and measured with survey equipment before the flight. During processing the software finds them in the photos and pulls the model onto those known coordinates. They're the original method, they work with any drone, and they remain the most transparent: you can see exactly which points constrained the map. Their cost is labour — someone walks the site placing and shooting targets, which on a large or active site is real time and real risk.

RTK and PPK drastically reduce how many GCPs a job needs. DroneDeploy's advice — and ours — is that they don't remove the need to verify. Keep a few surveyed points out of processing and measure them on the finished map as checkpoints. That reported error is what makes a deliverable defensible when a quantity or a position is disputed.

Which one for your site?

• Weekly progress on a site with decent cell coverage → RTK, with a handful of checkpoints for verification.
• Remote quarry, long corridor, or anywhere the radio link is unreliable → PPK.
• Legal, pay-application, or as-built deliverables → RTK or PPK plus independent GCP checkpoints, with the accuracy report attached.
• One-off flight with a non-RTK drone → GCPs do the whole job; plan for more of them.