How to determine which method is right for your application.
Surveyors gather and establish critical geospatial information. This data is used to accurately construct infrastructure, predict the resources necessary to complete a job, and understand the threats of a changing landscape. Until recently, satellite positioning systems were the best tool to increase the speed and efficiency of surveying work. Drones for surveying allows surveyors to cover even more ground in less time, and with better resolution. While most drones rely on the same satellite systems, GNSS accuracy can suffer from distortions caused by atmospheric variations. The overall accuracy of the system is improved by using one of the techniques described below.
These variations can be calibrated out by either visible ground control points (GCP), post-flight calibration through a recorded base station or similar network (PPK), or from a known position on the ground sending corrections in real-time (RTK). So what is the difference between GCPs, PPK, and RTK? This article outlines the pros and cons of each method to help you determine which is best for your unique application.
Ground Control Points (GCPs)
Until recently, the go-to method for aligning aerial images to real-world positions was through the use of Ground Control Points (GCPs). GCPs are physical targets positioned on the earth in precise geolocations. Post-processing software ties the point locations to the georeferenced image to ensure accuracy.
GCPs provide consistent, accurate data. The technology is simple, well-understood, and widely used. However, you’ll need a significant amount of time to set up GCPs, depending on the terrain and the size of the survey area. It can take much longer to perform a drone survey with CGPs than with other methods, due to the complex setup. Additional crew members may also be needed to help set up the targets. In difficult terrain, it may not be possible to safely or easily set up GCPs. Special equipment like a GPS rover may also be needed to establish the precise locations of the GCPs.
Post-Processing Kinematic (PPK)
Another method of correcting GPS data is Post-Processing Kinematic (PPK). This technique uses post-processing software to marry satellite data received during flight with the aerial images. The aircraft and base-station log their respective GNSS position data throughout the entire flight. In some areas, a service like the CORS network provides similar functionality without the need for a separate base station. The two data sets are married after the flight to produce very accurate positioning information for the final deliverables.
PPK has the advantage of not requiring a real-time radio connection to the aircraft from the base station. Also, if the flight is in an area covered by networks like CORS, the base station isn’t needed, so setup is quick and easy. The disadvantage of this system is the time required in post-processing.
Real-Time Kinematic (RTK)
The third method that provides real-time corrections to location data while the survey drone is capturing photos is called RTK. This system collects position data from satellites and references a base station with a GNSS receiver on the ground. It also requires a reliable data connection between the base station and the flight data.
An advantage of RTK is that it’s safer than using GCPs because the team does not have to traverse difficult terrain to establish the target locations. It’s faster because the only setup required is the base station and it doesn’t require time for post-processing. RTK has the added advantage of improving positioning data for the vehicle itself. This can be particularly valuable if flying in close proximity to objects like trees or guy wires.
RTK Has a Troubled History. It’s Better Now.
You may have heard some grumbling about RTK. In the past, poor quality telemetry radios were fairly prevalent in the industry and occasionally lost their link with the base station. The surveying team wouldn’t realize there was a problem until after the flight when they were back in the office. This wasted their whole day out in the field. Many who have experienced this problem prefer PPK because data collection is not vulnerable to this failure mode.
The reason for the problems with the traditional method is because the RTK communicated from rover to drone, which increased the risk of a momentary loss of signal. The newer method relays RTK corrections through the base station. This provides a far more reliable connection. One caveat is that the pilot should remain within 30 meters of the base station.
Comparison Between GPS Correction Methods
Each GPS correction method has advantages and disadvantages. Depending on your application, one may stand out from the others as a clear winner. Here’s a breakdown of the information:
| Pros | Cons | |
|---|---|---|
| GCPs | • Trusted and well known • Consistently accurate | • Additional time and crew needed for setup • Requires boots on the ground, possibly in hazardous environments • GPS rover and physical targets are required |
| RTK | • Safer: doesn’t require boots on the ground to set up GCPs • Real-time GPS corrections for both the data and the vehicle • No post-processing | • Requires digital connection with the base station • No data retention to cross-reference • Loss of link was a cause of malfunctions in the past (See above) |
| PPK | • Safer: doesn’t require boots on the ground to set up GCPs • Can eliminate the need for a base station if flying in an area with calibration services | • Requires post-processing • Errors introduced in the workflow may compromise project accuracy • Longer workflow timeline to final data product |
Why Vision Aerial Prefers the New RTK System
With the pros and cons outlined above, Vision Aerial prefers to use the new RTK for the following reasons:
- RTK increases data accuracy and the accuracy of the vehicle position.
- It’s incredibly quick and easy to set up.
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