This article addresses the key factors to consider for georeferencing accuracy, a CORS networks vs. a base station, using Ground Control Points (GCPs), and how to merge and correct RTK data with GCPs.
Maximizing Georeferencing Accuracy
GPS quality is impacted by many factors, which can affect the georeferencing accuracy. While Emesent’s Wildcat SLAM can compensate for poor-quality GPS signals to create a locally accurate point cloud, it cannot improve georeferencing accuracy. Accordingly, the GPS quality you can achieve at the site will determine the georeferencing quality.
Some of the key factors to consider 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. Buildings, tall structures, trees, etc., can block signals, impact satellite geometry, and result in poor positional estimation. |
|
Baseline length |
Generally, each additional kilometer of baseline length will decrease georeferencing accuracy by 1mm. The recommended maximum baseline length is 10km, though acceptable results can be achieved with a baseline of up to 25km depending on the project requirements. Select a closer CORS base station (if available) or use a local base station setup to reduce the baseline length. |
|
Receiver configuration |
It is critical to configure your receiver appropriately to receive GPS corrections that can be used by Hovermap. |
|
Urban canyons |
In addition to blocking out the view of satellites, tall buildings can reflect GPS signals which can cause errors when the GPS receiver is calculating 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. |
Choosing between CORS networks and a base station
A CORS network subscription is preferable if:
-
You do not want the extra expense of a base station.
-
Your local area has a good quality CORS network.
CORS networks have the additional benefit of requiring less equipment and a shorter setup and configuration.
A base station is preferable if:
-
You require the highest accuracy georeferencing.
-
Your local CORS network is poor quality.
-
Your closest CORS network base is more than 10km* away.
*Depending on georeferencing accuracy requirements. Up to 25km away can work for lower-accuracy use cases.
Using Ground Control Points
Using GCPs is necessary if your accuracy requirements are greater than what is obtained using RTK, and there is a need to further constrain the point cloud.
In addition, use GCPs if you want to verify the georeferencing accuracy achieved through RTK. Comparing the coordinates of the GCPs in the point cloud against their known coordinates allows you to assess the accuracy of the RTK georeferencing process.
The table below sets out the expected accuracy for each GPS status to help you decide whether GCPs are needed. The accuracy can vary for many reasons and the stated accuracy is not guaranteed.
|
GPS Status |
Approx. horizontal accuracy |
|---|---|
|
RTK Fix |
3cm |
|
RTK Float |
30cm |
|
GPS |
1.5-3m |
|
No GPS Fix |
N/A |
To learn more about expected vehicle RTK accuracy, please see the Vehicle RTK accuracy report which tests RTK accuracy in a real-world environment.
Merging RTK Data and Correcting RTK Data with GCP
Merging RTK data or further constraining RTK data with GCPs are currently not supported in Emesent Aura. The following third-party software can be used with Hovermap RTK data to achieve this.