DJI Phantom 4 & 3Dsurvey - Can a consumer drone deliver data needed for professional land surveying results?
With consumer drone market booming and drones becoming more powerful, easier to use and equipped with rolling shutter cameras, there is an opportunity for professionals to use consumer drones for mapping work. We decided to put the latest top-end consumer drone DJI Phantom 4 to the test to find out the quality of captured data and its usability in professional land surveying projects.
Be it common land surveying, environmental monitoring, change detection, agriculture or construction, geospatial data acquisition and processing no longer rests on classical measuring and processing techniques. Drone-based mapping paired with image processing software is becoming an extremely powerful tool, mostly due to its time efficiency, processing speed, improved quality and quantity of acquired information, and cost effectiveness.
As most commercial UAVs can still be fairly expensive we decided to try DJI’s Phantom 4, one of the best value-for-money drones out there, to see if its image resolution, integrated GPS, embedded rolling shutter camera and flying stability are sufficient enough to be used for professional mapping projects where survey-grade results are of the highest importance for further processing in photogrammetry software.
A simple small-size landfill was chosen for testing purposes to recreate the conditions we think Phantom 4 is most suitable for. With a flight time of 28 minutes it is not meant to be used on large-scale projects but proves ideal for small and mid-size land surveying tasks (up to 10ha). Additional batteries can certainly improve the ability to map even bigger areas, however in this case the focus was on the quality of acquired data.
What really interested us was the image quality from its integrated camera. Three flights at different heights (40m, 60m, 80m) were conducted to find out whether image resolution proves sufficient for photogrammetric processing and ensuring the highest accuracy. 80% overlap in both directions was applied just to be sure enough details got captured for testing purposes. 7x GCP and 2x independent validation points were set up for GCP based orientation and quality control.
Data sets from all three flights were imported and processed separately. Phantom 4 supports writing GPS coordinates to EXIF data of every individual image on the fly. It means no manual geotagging is needed when importing images to 3Dsurvey for processing. Further quality control was achieved by placing GCPs and establishing independent validation points.
Upon automatic orientation and dense point cloud generation we calculated orthophotos and achieved the following resolution: 1.67 cm (pixel size) at 40m (flight height above ground), 2.5 cm at 60m, 3.5 cm at 80m.
There was no problem generating the whole spectrum of outputs from any of the three point clouds. The accuracy and quality depended on the source data, meaning we got the highest-resolution orthophotos from the flight performed 40m above ground. No surprise there but the point of our test was to see how accurate and of what quality the results would be at different heights. How high one decides to fly should always depend on the specific needs of one’s project, expected accuracy and required level of details.
In our case the 60m flight proved the most usable as it had the right balance between image sizes, interpolation and flying height. It produced results with a sub-CM accuracy on all GCPs, however our independent validation points showed realistic geospatial orientation accuracy values of 4 -7 cm.
The more compact and detailed the project requirements the closer to the ground Phantom needs to fly as the camera is integrated and image pixel size naturally increases with height.
Find below the results and screenshots from 3Dsurvey that show generated outputs as well as comparisons between orientation accuracy and orthophotos calculated from each of the three data sets (each of the three flights) and a zoomed-in orthophotos from 60- and 80-meter flights for a detail quality comparison.
Advantages of using a Consumer Drone for Professional Land Surveying Purposes
It is ideal for small and mid-size projects where the surveyed area is not bigger than 10-12ha and allows for flights fairly close to the ground.
As a consumer rather than commercial drone, a DJI Phantom 4 is extremely capable and very affordable.
If used properly (close enough to the ground) it produces up to 1cm resolution which in turn enables results that meet the requirements of any professional land surveying project.
Project Tools & Specs
Drone: DJI Phantom 4 (single 5-minute flight)
Camera: Integrated DJI w/ embedded rolling shutter
Positioning & Orientation: Integrated GPS (EXIF data), 7x ground control points (GCP) and 2x independent validation points used for geospatial orientation accuracy testing purposes
Photogrammetric processing & Outputs: 3Dsurvey software
Number of Images: 139
Area size: 110 x 105 m