With the Emlid Reach RTK GPS Receivers now available, we’ve been conducting tests to determine their accuracy and working on the integration into the E384 and E386. The goal was to determine relative, or scale, accuracy as well as absolute accuracy verified with a survey grade Trimble R6 Model 4.
We post-processed the data in three different ways to explore the effect each would have on the resulting data. For PPK GPS processing, there is a receiver onboard the aircraft and another stationary receiver on the ground. The ground receiver (base station) is used to calculate corrections to refine the position of the airborne receiver. The base station also calculates a precise GPS coordinate for itself, with the option of writing in another, more accurate coordinate if desired. We constructed orthomosaics on the Drone Data Management System™ using geotags calculated from the Reach base station and the Trimble base station, using either the Reach base coordinate or the Trimble base coordinate. The combinations for each test are listed below.
|Base Station Corrections||Base Station Coordinate|
|Reach||Reach + CORS|
|Reach||Trimble R6-4 + ODOT VRS|
|Trimble R6-4||Trimble R6-4 + ODOT VRS|
It was clear straight away that there was an offset between the Reach and Trimble coordinates, so we focused on scale accuracy for this test. The offset is clearly visible in the image below, where emp is the Reach base station coordinate and 6 is the same coordinate shot by the Trimble R6. To measure the scale accuracy of the Reach-only orthomosaic, we measured distances between several pairs of GCPs in different directions. The error was 3cm in each case.
|GCP Pair||Reach Orthomosaic (m)||Trimble R6-4 (m)|
Processing the geotags using the Emlid Reach base station but using a coordinate shot by the Trimble R6-4 resulted in very good accuracy relative to the Trimble shot GCPs, with an RMSE of 3.36cm.
Finally, processing using the trimble base station for both corrections and the base coordinate yielded similar results to those obtained with the Reach corrections, RMSE 3.54cm.
These results should be considered very preliminary, as there were a number of factors that could have adversely affected the accuracy. The Reach coordinate may improve once we are able to calculate it with a closer VRS. The mission was collected with a relatively high GSD of 3.5cm/pixel, so it is difficult to pick the GCPs accurately.
Still, there are some conclusions we can draw from this data. Even without a good base station coordinate, the Reach system can produce very good scale accurate results. When paired with a higher quality coordinate, the Reach can produce very good absolute GCP coordinates. If a fixed position can be marked once by a survey grade GPS, then it can be used as a reference point for all missions in the same area, forever. It may also still be possible to obtain similar results with the Reach alone using the VRS network or Precise Point Positioning.
We’ll run more tests to verify the accuracy, but initial results are very good. We’re now making the first deliveries of the Reach system to select clients before a wider release in the very near future.