What are Ground Control Points, Real Time Kinematics, Post Processing Kinematics?
With boots on the ground, a surveyor’s job is to capture accurate data. This responsibility is almost entirely up to the surveyor and their level of expertise. However the surveying process has become faster, safer and more efficient with development in drone technology.
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)
Ground Control Points are defined as 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 method 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 on site 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. It's made their mapping missions more accurate, efficient, cost-effective and, most importantly, safe.
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