Since we have run tests and proven 1 cm (0.4 in) drone survey accuracy, we’re sharing the steps that we find mandatory for you to get closer to these results yourself. Keep in mind that this article does not cover all conditions for every survey. Some unmentioned factors may influence results. Nevertheless, the following pointers are a solid step in the right direction, which set the stage given any other local considerations.
Fly with a high-quality, PPK-enabled drone and payload
To achieve this level of absolute horizontal accuracy with a drone, you’ll need to fly with a high-quality, multi-frequency GNSS receiver on board. It must at least handle both L1 and L2 frequencies. We recommend post-processed kinematic (PPK) for more dependable location reading over many surveys. You’ll also need a payload with a high pixel count so that your images are a good enough resolution to cover a decent amount of ground per flight while still allowing a lower (better) GSD value. Absolute accuracy can range from 1-3x your GSD (lower being better). The quality of your drone’s camera lens and sensor determine how close to 1x you get.
Manage the distance between flight plan and base station
A 1 cm (0.4 in) level of absolute accuracy depends on the correction data from static base station logging. The closer a base station is to the flight location, the better the correction data will be for the points that are logged by your drone’s onboard GNSS receiver.
When using a continuously measuring GNSS station, make sure it is close enough. The accuracy of your results will reduce gradually the further you are from the station. Rule of thumb—every 10 km (6.2 mi) adds 1 cm (0.4 in) to the RMS error.
In case the GNSS station is further away, use your own base station.
Always use a high-quality, survey-grade base station
When setting up a new base station on an unknown point, let the GNSS receiver log the GPS data for a couple of hours or, even better, overnight. Logging GPS data for longer periods will help ensure higher accuracy results.
Note that if a new base station is established on a known point, the results depend on how accurately the point was measured before.
Important! Don’t forget to check minimum base station requirements, which are:
- Possibility of continuous logging with interval of 15s or faster (1s is recommended for highest accuracy)
- Logging at least L1 and L2 frequencies
- Receiving constellations: GPS + GLONASS (optional for high precision)
Be aware of elevation influence
A long baseline affects the vertical accuracy most of all. In case of a height difference between the base station and surveying area of more than 500 m (1640 ft), accuracy becomes significantly worse. Take that into consideration when planning your projects.
Establish checkpoints to prove the accuracy of your results
When measuring your checkpoints, make sure to use an RTK or PPK GPS receiver. Common brands include Trimble or Leica. Using any device other than an RTK or PPK GPS receiver will compromise the accuracy.
Remember to place the tip of your GPS receiver directly on the center of the control point marker. Make sure to calibrate your GPS receiver when it’s level with the ground. Follow the instructions on the system provided by the measurement device manufacturer. Use a tripod to make sure that the receiver is stable and does not move during the measurement process.
Be careful with different coordinate systems
Get to know what coordinate system your drone’s data is captured in. In case you need a local projected coordinate system as an output, you’ll need to transform your data. Be aware that the final results in local coordinate systems are only as good as the provided conversion tools for those systems.
Avoid environmental obstacles
GNSS satellite signals can be blocked by big obstacles such as tall buildings, mountains or trees. When using a drone to capture images with absolute levels of accuracy, carefully choose locations where the surrounding environment does not shelter your receiver from the satellite signals.