Investigating the geometric accuracy of Propeller AeroPoints and Propeller cloud processing

Report compiled by Luke Wijnberg and Jonathan Swart from 3DroneMapping 14 June 2017

The Propeller AeroPoint is a smart ground control point (GCP) which aims to make the gathering of accurate data simple and affordable during drone surveys. These lightweight and durable units repeatedly record positional data during UAV flights, with each AeroPoint working collaboratively against each other; therefore the more AeroPoints used, the greater the accuracy of each unit.

Propeller claims that their AeroPoints will work anywhere in the world with geometric accuracies of 20mm (X), 20mm (Y) and 50mm (Z). This statement was put the test in Cato Ridge, South Africa, with a pre-planned AOI which would be the testing ground for various geometric comparisons to be conducted against 3DroneMapping’s (3DDM) tried, tested and proven drone surveying methods.

To successfully investigate the geometric accuracy of the AeroPoint technology, two tests were conducted in order to analyse and compare the resulting data between. The two tests were as follows:

Test 1 -AeroPoints versus traditional survey-grade GPS control measurements

10 AeroPoints were roughly distributed around the perimeter of the site and were activated to log raw GPS data. The exact location of the AeroPoints were measured using a survey grade Emlid Reach RS GPS in RTK mode as well as logging raw data for post processing. Some points were purposefully placed in technically challenging areas such as under power lines in order to test the robustness of the final coordinate. After two hours of data collection, the AeroPoints were removed and the raw data automatically uploaded to the Propeller processing servers. The coordinate for the “LZ” was determined prior to the exercise and was issued to Propeller to coordinate or reference itself too. This was done in order to find commonality between the survey tests.

A reach RS was set with a 2m vertical offset over “LZ”. This was set to broadcast raw RTCM over a LoRa radio to a rover. The base also recorded raw measurements at 1HZ for later post-processing. The RTK results were exported and compared to the results issued from Propeller. Both sets of coordinates were transformed from WGS84 (EPSG:4326) to Hartebeeshoek Lo31 (EPSG:2054) for ease of coordinate comparisons.

Test 2 – Propeller Platform processed data versus locally processed PPK survey

843 images were collected from a fixed wing platform. On-board sensors included a high resolution camera and an Emlid Reach for PPK work. As a test for the AeroPoints and their performance with the Propeller cloud photogrammetry processing software, images were geotagged using only the uncorrected navigational GPS (+-5m accuracy). The 8.29GB of data was uploaded to Propeller and set to process; no further options were made available. The resultant orthophotos and DEM were downloaded, re-projected to Hartebeeshoek Lo31 (EPSG:2054) and compressed for later comparison.

The same images were also geotagged in PPK fashion from the Reach and its camera event sync system. The events and track-log were further post processed using “LZ” as a reference or base for the survey. A Reach RS was set with a 2m vertical offset over “LZ” and recorded raw measurements at 1HZ for post processing. The on-board Reach GPS recorded raw GPS data at 14HZ. Post processing was done in RTKLIB V2.4.3 b27 with 100% of the events being resolved to 1cm accuracy. Photogrammetry was undertaken locally with Pix4D (V3.2) with appropriate settings for high accuracy geotagging. No additional ground control was used to reference the model. Exported othophotos and DTM were compressed and used in the final comparisons.

Results

Following a successful day in the field, both parties began processing their gathered data and running the necessary tasks using the preferred specialised software. The results provided informative data from the different systems used and were ideal for investigating the accuracy of the AeroPoints.

Comparisons of recorded XYZ data (meters)

The above table compares 3DDM’s control GPS which was collected using a ReachRS device, against digitised PPK data (orthophoto and DEM). This process was included as it shows a low level of variance of 0.001m between the GPS and PPK systems, proving that the GPS data is a true reference to use for determining the accuracy of the AeroPoint technology.

The above table was part of the first conducted test and contains the XYZ raw data captured by the AeroPoints. This raw data was compared to the findings of 3DDM’s ReachRS GPS (which have been proved to be extremely accurate in table 1). The comparisons drawn here have concluded excellent results between the two systems, with a calculated variance of 0.002m.

For the second test, the processed AeroPoint data (using Propeller’s own software) has been compared to the PPK digitized survey by 3DDM. Once again, the AeroPoints have gathered accurate results as this processed data has resulted in a comparison variance of 0.003m.

Cross sections (meters)

The variance of the recorded comparisons from the three tables above can also be observed in the form of cross sections. Cross sections were drawn in five different locations within the AOI over sections of different surface characteristics (ie. a concrete parking area, rough field, etc). Sections such as tarred roads and concrete parking zones are usually extremely flat and shouldn’t show much variance between the different systems, however uneven surfaces such as within the rough field, could provide a harder test for accurate readings.

GPS (red) against PPK (blue)

GPS (red) against the AeroPoint’s processed data (green)

Final thoughts

Using two different methods to determine geometric accuracy, there is sufficient evidence to conclude that the Propeller AeroPoints and accompanying cloud processing are truly capable of delivering consistent, high-quality data. Although the AeroPoints are not as accurate as the more traditional surveying methods such as a PPK system, the added benefits of quick to deploy technology which is durable and user-friendly, certainly compensates for the small amount of geometric variance. Propeller’s claim regarding their AeroPoints recording accurate data in any region of the world has also been confirmed in this case, as Australian designed GCP technology has captured admirable results in the Southernmost tip of Africa.

However, with high geometric accuracy and such an easy to use solution for drone survey GCPs, one can expect costs to be high on acquiring such technology (6000USD for x10 AeroPoints, compared to 1600USD for 2 ReachRS RTK/PostProcessing units capable of placing unlimited GCPs). With the costing overlooked, Propeller’s AeroPoints can save drone survey companies time and money while not having to be concerned with negatively affected geometric accuracy.