This article covers how to capture good RTK data with Hovermap, what affects georeferencing accuracy, and how to verify or improve accuracy using Ground Control Points.
Check the receiver before starting
Before starting the mission, confirm the receiver is reporting a good signal. A good signal requires a clear view of the sky and appropriate configuration.
See Monitor RTK Status for how to check the receiver's signal in Commander.
Start outdoors, even if the mission goes inside
If the mission needs to go indoors, start the scan outside first and wait for a good GPS fix before moving indoors.
Missions that start indoors, with the receiver only reaching good signal partway through, will have lower georeferencing accuracy than missions that start with a good signal.
Expect some variation in signal during the mission
Even with a good start, the signal can briefly drop in difficult spots such as deep inside a building or under dense tree canopy.
For more detail on what affects signal quality, see Factors that affect georeferencing accuracy.
Factors that affect georeferencing accuracy
GPS quality is impacted by many factors, which can affect the georeferencing accuracy achieved through RTK.
Wildcat SLAM can compensate for poor-quality GPS signals to create a locally accurate point cloud, but a good GPS signal is needed to achieve a globally accurate (georeferenced) point cloud.
Some of the key factors are outlined below.
Factor | Description |
|---|---|
Clear line of sight | A clear line-of-sight view of the satellites is the minimum requirement for GPS to work effectively for position calculation and signal tracking. Vertical obstructions block signals, impact satellite geometry, and result in poor positional estimation. Avoid positioning the receiver near buildings, tall structures, tree canopy, towers, deep cuttings, site vehicles, or other vertical obstructions. |
Baseline length | Each additional kilometer of baseline length decreases georeferencing accuracy by approximately 1 mm. The recommended maximum baseline length is 10 km, though acceptable results can be achieved with a baseline of up to 25 km depending on project requirements. Select a closer correction network base station (if available) or use a local base station to reduce the baseline length. |
Receiver configuration | Configure the receiver appropriately to receive GPS corrections that can be used by Hovermap. |
Urban canyons | In addition to blocking satellite visibility, tall buildings can reflect GPS signals, causing errors when the GPS receiver calculates its position. |
Man-made interference | GPS signals can be interfered with by man-made sources such as GPS repeaters, TV broadcast stations, signal jamming devices, and personal privacy devices. |
Satellite arrangement | Satellites constantly orbit the Earth, and different configurations can cause slight variances in precision. |
Verify accuracy with Ground Control Points
Use Ground Control Points (GCPs) if accuracy requirements exceed what is obtained using RTK and the point cloud needs to be further constrained.
GCPs are also useful for verifying the georeferencing accuracy achieved through RTK. Comparing the coordinates of the GCPs in the point cloud against their known coordinates allows the operator to assess the accuracy of the RTK georeferencing process.
The table below sets out the expected accuracy for each GPS status to help decide whether GCPs are needed. Accuracy can vary for many reasons and the stated accuracy is not guaranteed.
GPS status | Approx. horizontal accuracy |
|---|---|
RTK Fix | 3 cm |
RTK Float | 30 cm |
GPS | 1.5–3 m |
No GPS fix | N/A |
For more on expected RTK accuracy in real-world conditions, see the Vehicle RTK accuracy report and Backpack RTK accuracy report.
Choose between a correction network and a base station
A GNSS correction network subscription is preferable if:
The extra cost of a base station is not justified for the work.
The local area has a good quality correction network.
Correction networks require less equipment and have a shorter setup and configuration time.
A base station is preferable if:
The highest accuracy georeferencing is required.
The local correction network is poor quality.
The closest correction network base is more than 10 km* away.
*Depending on georeferencing accuracy requirements. Up to 25 km can work for lower accuracy use cases.
The following third-party software can also be used with Hovermap RTK data: