Why do Surveyors use GCP or RTK or PPK?
A surveyor’s job is to capture accurate data With boots on the ground, this responsibility is almost entirely up to the surveyor and their level of expertise. However, developments in technology, Drone, 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)
GCPs are a series of geo-referenced physical markers positioned on the ground and that are visible within several images captured by the drone during a flight. The center of these GCPs is then tagged with their respective coordinates (X, Y, and Z axes) within the drone images. This process provides the data processing software with a series of anchor points that have known, real-world positions. It is these anchor points that all remaining imagery gets tied to during the processing stage.
Done correctly, this method can bring the entire project’s absolute accuracy to 20 to 50 mm in all axes.
Pros
- The original method: most widely known and trusted
- Consistently produces a high level of precision and overall accuracy
- Allows production of the defensible quality report to prove the validity of the technology
- Allows consistent ground truth of project’s accuracy
Cons
- Can take as much as four times longer to set up in comparison to RTK and PPK
- May need an entire crew, depending on the size of the 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, they should be used only when RTK and PPK are not possible.
Real Time Kinematic (RTK)
RTK is a technique used to enhance the positioning accuracy of imagery collected by the drone in real-time. An RTK workflow consists of a satellite-based positioning system, either a physical reference station, such as the D-RTK to the Mobile Base station, or an interpolated virtual reference station and a drone with an onboard RTK receiver, such as the M300 RTK or Phantom RTK from DJI. An RTK workflow enables the drone’s geotagged images to be corrected while in flight, therefore, removing the need for any post-flight corrections and reducing the reliance on GCPs.
Pros:
- Increased site safety – teams spend less time on foot setting up GCPs
- Reduces the reliance on the quality and setup of GCPs
- GNSS corrections happen without any manual intervention and in real-time
- No GNSS post-processing required
Cons:
- Requires either physical reference station hardware or virtual reference station subscription and mobile data plan
- Can only be used with an RTK-equipped drone – a slightly higher initial outlay than a GCP workflow
- Requires consistent connection between drone and reference station throughout the flight
- Any RTK signal outage during data capture will render the absolute accuracy of the project unreliable
- Checkpoints (similar to GCPs) should always be used to validate project accuracy, so doesn’t remove the downsides of a GCP workflow all together
Post Processing Kinematic (PPK)
An alternative method to RTK is PPK. This technique uses identical components to those used in an RTK workflow, but with one critical difference – the geotags of the drone imagery are corrected AFTER the flight has taken place.
In contrast to RTK, a PPK workflow does NOT involve a real-time connection between the drone and the reference station. As such, PPK introduces a critical layer of data capture redundancy.
PPK corrections have traditionally been made possible with cloud-based post-processing software. However, with a solution like the Skycatch edge, it is now possible to bring even greater automation to the PPK workflow. This device not only acts as the physical reference station; it also takes care of applying the post-flight GNSS corrections to the collected imagery and can even carry out local photogrammetric processing, all without any internet connectivity.
Pros:
- Increased site safety – the reliability of a PPK workflow means that there is no need for teams to lay GCPs
- The fastest and most reliable method of data collection
- No real-time connection with the drone, so completely removes the risks associated with RTK signal outage
- Can work happily in 4G deprived environments
Cons:
- Can only be used with an RTK-equipped drone – a slightly higher initial outlay than a GCP workflow.
- Requires additional post-processing software.
- Additional step in the workflow, so slightly more human intervention is required.
It’s clear that PPK has some similar pros and cons to RTK when comparing it to the traditional GCP method. The main difference between PPK and RTK is that the former provides a much more robust workflow, with its inherent failsafe of having no live connection between drone and reference station.
Each method has its pros and cons, but in all instances the integration of GNSS technology has dramatically improved the workflow of drone pilots, bringing accuracy, efficiency, cost-effectiveness, and, most importantly, safety.
