Real-time kinematic (RTK) positioning is a form of surveying that involves the determination of a location at high accuracy using one or more base stations that are connected to a satellite positioning network. The positional accuracy of RTK collections is generally at the centimeter or subcentimeter level. This capability has become easier to implement on a smaller scale, and more drone platform options are becoming available.
RTK drones tend to provide greater benefits over typical consumer model drone platforms. RTK drones are constantly connected to a base station or correction service that helps to increase positional accuracy. This means that even while flying, the image locations are highly accurate and ground control is rarely needed to obtain accurate imagery products. Since they are also more expensive than other models, RTK drones tend to have higher-quality cameras that can capture images at higher resolutions or wider angles.
Real-time kinematic versus post-processed kinematic
Real-time kinematic and post-processed kinematic (PPK) technologies are both used to obtain highly accurate GPS information. The main difference is in the name; RTK is applied in real time, meaning that a drone has a constant connection between the one or more base stations and a satellite constellation providing positional information. With PPK, a drone is also connected to one or more base stations and a satellite constellation. However, the correction information that is provided by any base station is applied to the data post flight, typically through specific software.
One benefit of RTK data collection is you eliminate the step of postprocessing. This can save time and money by not having to buy purpose-built software for the task. PPK, however, has advantages over RTK in some areas. One advantage is increased flexibility in terms of where the drone can be deployed. Since a strong signal is not always necessary between the drone and the base station, the drone can fly more varied terrain without a line of sight. RTK drones require a constant connection between the drone, base station, and satellite for best results. This means flights are typically line of sight and over relatively flat or unobstructed terrain.
Both methods provide highly accurate survey-grade positional information for imagery, and it's a matter of what works best for your scenario or organization. ArcGIS Drone2Map can process the corrected datasets from either collection method.
Best practices
While ArcGIS Drone2Map can process RTK or PPK corrected imagery, it is important to maintain good collection practices to achieve the highest accuracy and best-quality products. The following tips can help you achieve the best field collection results:
- When using a base station, if the base station is not set to a known position, you will have good relative accuracy between photos, but you will not have good absolute accuracy relative to the coordinate reference system used. Ensure that the base station matches a known point and those coordinates are in the correct coordinate system for the project.
- The drone, drone controller, and base station should be updated to the latest firmware to avoid potential connection issues.
- With PPK flights, both the drone and the base station need to record satellite data for postprocessing. Ensure that both are set to record that information and the correct transformation is used when outputting the GPS data from the PPK software.
- If using ground control points, ensure that they are equally spread throughout the flight area and that the markers remain in position when moving the base station. Do not use the Fix Image Location for High Accuracy GPS (RTK and PPK) processing option if you are using ground control points as it can distort accuracy values when you are adjusting with both.
- Ensure that the drone and base station are recording in the same coordinate systems. If you're using a correction service, verify the coordinate systems before capturing the flight.
RTK and ground control points
RTK drones are generally more accurate than a typical handheld GPS device. However, RTK positioning is not infallible and GPS coordinates may be inaccurate if poor flight conditions exist or connection issues occur. Collecting ground control points for each flight can mitigate these issues. Using ground control points as a checkpoint measurement can help provide validation of the drone's positional accuracy. If there is a significant shift in the imagery compared to the checkpoint, the drone collection configuration may be incorrect. This applies to both horizontal and vertical measurements.
The following are tips when working with ground control and RTK data:
- If you see a significant shift between the RTK imagery and checkpoints, first review the coordinate systems used. Ensure that the imagery coordinate system and project coordinate system are both correctly defined.
- Account for the height of the base station if you are using your own custom RTK network. Measure the height starting from the top of the GPS receiver. If necessary, you can adjust image altitude to account for this shift.
- In the processing options, under the Adjust Images section, check the Fix Image Location for High Accuracy GPS (RTK and PPK) check box. This stops the software from attempting to adjust GPS measurements and use the measurements directly from the imagery metadata instead.
Note:
The Fix Image Location for High Accuracy GPS (RTK and PPK) option should only be used with checkpoints and not ground control points. Having this option and ground control linked during the adjustment step can produce accuracy errors.