Drone technology has come a long way and continues to innovate and improve work practices and efficiencies. Drastically increasing efficiencies of projects from the traditional time scale of days and weeks to hours, without sacrificing data quality.
Mapping vs Surveying
Mapping is creating a visual representation of geography and features. A survey is also a map but with the ability to accurately measure distance and or distanced between two and three dimensional points both are related, however with difference use cases and value.
Drone in mapping and surveying can:
- Provide volumetric calculation
- Quick deployment and cover large areas quickly
- Provide survey-grade accuracy and leverage RTK and PPK
A typical drone mapping capture is a process where a drone takes photos with GPS and other global satellite positioning constantly geo-tagging the photo’s location. The photogrammetry software matches pixels in overlapping photos, making assumptions of camera focal length and lens distortions, camera orientations, camera positions, and whether the pixel in one photo is the same point as a pixel in another photo.
This model can effectively be considered malleable – or flexible – and when we can lock in some parameters to the real world we can take that model and fit it, stretch it and bend it to fit the constraints we can find in the real world. Surveyors then need to be able to confirm that their measurements on the model equate to real-world values.
How accurate is the data?
The first way this is done, is to use GCPs (Ground Control Points) – targets on the ground, typically placed by a surveyor, with known coordinates in a known coordinate system.
Ground control points (GCP) are marked points on the ground with known geographical location within your survey location. When used in photogrammetry, operators usually lay large visible “markers/points” to make them easy for the camera to capture. Then a survey grade GPS receiver is used to collect precise GPS coordinates of the centre of that mark. A number of GCP’s are needed to be spread through the survey area, depending on the size of the area.
GCP’s can be used as “checkpoints” during data processing to verify and or correct the accuracy of projects. One stipulation of GCP’s is the time required set and create them, which can decrease workflow efficiency. GCP’s are not always necessary depending on the application the project output is being used in, for instances like creating marketing material or scouting.
This was time-consuming on-site, as it required traveling the entire site with survey equipment, or placing and collecting ‘smart’ GCPs. Processing time was also longer both for the operator to pick those GCPs and the processing to re-iterate camera positions over and over based on GCPs and tie points. Errors in the system due to lens distortions and surface matching would creep in between the GCPs.
Putting the control at the camera turns the workflow around. By knowing where the camera was for each photo, the AT (aerial triangulation or matching)process in photogrammetry is more reliable and much faster as we are no longer reverse-engineering the assumed camera positions. AT works better on difficult surfaces. The entire project is covered with a blanket of hundreds of control points.
Real-Time Kinematic (RTK) vs Post-Processing Kinematic (PPK)
There are two options for implementing differential correction techniques use to enhance the quality of the location data gathered.
Drone Real-Time kinematic (RTK) with drones utilise a standalone “base station” that actively sends GPS data to the drone. RTK computes accurate coordinates of the drone in real-time, using data transmitted from a base station (on a known location) to the drone. 5cm accuracy typically. It can provide data quicker, live and in real-time and no post-processing is required.
Post Processing Kinematic (PPK) is an index of GPS correction technology which applies correction of the location data after it is collected and uploaded.
PPK is similar to RTK, in that it uses multiple constellation dual-frequency GNSS carrier phase data from clocks on satellites and applying relative range data between a base and rover.
Drones utilising an RTK system requires a constant and consistent connection and if signals are lost, obstructed, disrupted even the range between the drone and the base station, then it can result in a loss of correction data with the potential for inaccurate gaps during flight. RTK works well in flat terrains without trees, obstructions and landmarks.
PPK (post-processed kinematic) is a more robust alternative to RTK. Rather than a team trying to compute the accurate coordinates in real-time, which typically isn’t necessary anyway, the GNSS data is stored on the drone and in the base station receiver. After the flight, the data is brought together and processed forward and backward to determine the most accurate and reliable results for each camera position. Inherently a PPK drone surveying or mapping will require more time after the flight to allow for processing the data gathered and to implement GPS corrections.
PPK systems allow for greater flexibility and can be flown in more challenging environments where signals might otherwise be blocked, obstructed or loss.
In Summary
There is much discussion as to whether reliable PPK camera coordinates eliminate the need for GCPs. They can. The main error remaining in the system is the camera lens, its actual focal length, and distortions. If the camera parameters are calibrated, known, and stable, then they can go into the photogrammetry solution to achieve excellent results without GCPs. However, as any surveyor would know, that is not where it ends. It is essential to have checks on the ground, whether targets or known features, that you can measure and compare, to know exactly how accurate your 3D model is.
Whichever the system is the best, drones use in terms of efficiency, the quality of data and imagery captured, and safety plays an important role.
Air Inspect Australia provides professional spatial and survey data services. Our team have experience in providing data suitable for use in many applications like asset inspection, volumetric calculations and topographic surveys.
For example, Air Inspect Australia, uses aerial LiDAR (Light Detection and Ranging) method coupled with RTK or PPL systems in order to capture high volumes of spatial information quickly, over large distances and hard to access terrain.
Want to know about accuracy, survey and aerial lidar read our blog on Drones, LiDAR and the scanning system.
