A surveyor’s job is to capture accurate data – something that was traditionally done manually, with boots on the ground. With time-consuming workflows, margin for human error, and inherently dangerous methods, it’s little wonder surveyors are turning to drone technology to conduct important surveying tasks, but how can we ensure drone-based projects are delivering the necessary level of absolute accuracy?

In this post, we’ll explain the various methods and technologies that make this possible.

The first (and most primitive) option is Ground Control Points (GCPs). Whilst this has been a long proven method to ensure good quality drone survey outputs, relatively new advances with drone technology – specifically their integration with GNSS technology – have seen the introduction of real-time kinematic (RTK) and post-processing kinematic (PPK) methods, which can make the process easier, safer and less time-consuming. 

There are pros and cons to all of these methods, and here we take a look at each option in more detail to help you make an informed decision on the workflow that’s right for your operation.

Ground Control Points (GCPs)

GCPs are a series of geo-referenced physical markers positioned on the ground and that are visible within a number of images captured by the drone during a flight. The centre of these GCPs are 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 50mm in all axes.

Pros:

  • The use of GCPs is already common practice for surveyors
  • Consistent production of accurate data becomes possible with inexpensive drone hardware, such as the DJI Phantom 4 Pro V2.0
  • Provides ground truthing of the drone project’s accuracy

Cons:

  • Longer pre-flight setup time; as much as four times longer to setup in comparison to RTK and PPK-based systems
  • Setting up ground points is labour-demanding – especially if the area being surveyed is large
  • Can be dangerous in certain environments
  • Final projects’ accuracy is wholly dependent on the quality and setup of the GCPs
  • Tagging the GCPs in the software creates an additional layer of manual intervention
  • GCPs are prone to damage, eg. being run over by site machinery

In summary, while GCPs have been a proven method of accuracy for years, there are now safer, faster and more robust methods available.

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 or interpolated virtual reference station and a drone with an on-board RTK receiver, such as the M300 RTK or Phantom 4 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

Workings of an RTK solution

This method works well in flat terrain where the connection signal can’t be interrupted by things like trees or buildings, or in the case of a virtual reference station, where reliable 4G connection is available. However, an RTK workflow should never be seen as a complete replacement of GCPs, even if these are only used as Checkpoints to validate the projects’ accuracy. 

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 is 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 Edge1, 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 the 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 (unless using the Skycatch Edge1)
  • Additional step in the workflow, so slightly more human intervention is required (unless using the Skycatch Edge1)

    Workings of a PPK solution

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.

Speak to us today to see which method we’d recommend for your needs.