GCPs vs RTK vs PPK: When to use what and why

GCPs, RTK, PPK – Why are they so important?

A surveyor’s job is to capture accurate data. With boots on the ground, this responsibility is almost entirely up to surveyor and their level of expertise. However, developments in technology, like drones, have made the surveying process faster, safer and more efficient.

While laying ground control points (GCPs) has been a necessary step in drone surveying workflows from the beginning, advancements in GNSS technology have led to the evolution of real-time kinematic (RTK) and post-processing kinematic (PPK) methods. But like everything in life, there are pros and cons to each of these processes.

Ground Control Points (GCPs)

AgEagle’s Drone Dictionary defines a ground control point as “a location or object on the ground that has precisely known coordinates”. GCPs are used to accurately geo-reference and align projects from relative accuracy—one to two meters—to absolute accuracy—two to five centimeters.

Pros

• The original method: most widely known and trusted
• Consistently produces a high level of precision and overall accuracy
• Allows production of defensible quality report to prove validity of technology
• Allows consistent ground truth of project’s accuracy

Cons

• Can take as much as four times longer to setup in comparison to RTK and PPK
• May need an entire crew, depending on size of working area, to set points
• Can be dangerous in certain environments
• Requires additional equipment, including GPS rover, base, VRS network license, spray paint and targets

GCPs have been a proven method of accuracy for years, but with safer and faster methods available, it should be used only when RTK and PPK are not possible.

Real-Time Kinematic (RTK)

Real-time kinematic is a technique used to enhance the precision of position data derived from satellite-based positioning systems, which relies on a single reference station or interpolated virtual station to correct geotagged locations while in flight. In other words, RTK is a correction method that enhances GNSS precision.

Pros

• Increases safety because it does not require teams to maneuver through dangerous terrain setting GCPs
• Provides time-savings in comparison to GCPs, making the process more productive and efficient
• Provides real-time corrections to the drone onsite
• Ideal for geo-tagging in absolute accuracy throughout flights in real-time
• No GNSS post-processing necessary with real-time correction

Cons

• New concept to drones, not adopted as easily, especially since drone technology itself is still considered new
• Requires base station, special equipment and consistent connection to process data in real-time
• Cannot retain new or old data to prove accountability
• Does not operate the same on-site as offsite
• Moderate possibility of malfunctioning

The RTK methods works well in flat terrain where trees or mountains won’t get in the way of the communication signal. RTK is restricted by the power of ground and air communication with the drone. If there’s more than three kilometers between the drone and the ground station, or if there are obstructions such as trees or mountains, there’s a chance it will lose signal.

To use RTK, flights in open terrain and within two or three kilometers of the ground station is ideal.

Post-Processing Kinematic (PPK)

An alternative method to RTK is post-processing kinematic. This kinematic technique corrects geotag accuracy after capturing and uploading drone data. The data is corrected in the flight data manager (FDM) and then processed in the cloud.

Pros

• Increases safety because it does not require teams to maneuver through dangerous terrain setting GCPs
• Provides even more time-savings than RTK-to-GCP comparison because less time is spent onsite preparing RTK connection
• Easier base station setup because no cable or Bluetooth connection is required
• More dependable than RTK because it does not depend on signal strength or GNSS information from the base station in real time
• Allows for more flexibility in flight since connection isn’t required for data capture

Cons

• New concept to drones, not adopted as easily, especially since drone technology itself is still considered new
• Additional time needed to process position data after flights have been conducted
• Easy to make mistakes in the workflow, which can cause a georeferenced shift or poor precision in the overall project output

PPK has some similar pros and cons to RTK in comparison to GCPs because of the safety and efficiency these methods provide. The major difference is that the PPK method is ideal for longer flights, especially BVLOS missions. The longer the flight, the greater chance there is of losing signal to the connection needed for RTK.While it’s clear each method has its share of pros and cons, the big picture is the same: the integration of GNSS technology with the drone industry has helped improve the workflow of drone pilots. Its made their mapping missions more accurate, efficient, cost-effective and, most importantly, safe.