In recent years, unmanned aerial systems (UAS) – widely referred to as drones – have been increasingly used to conduct aerial surveys and generate 3D maps that measure a timeline of site changes across industries such as Construction, Mining and Agriculture.
With a constant need to reduce the time and resources used for traditional methods of ground surveys and manual measurements, drone technology has developed at a rate that allows professionals seamless integration into their everyday operations. However, some are of the opinion that former methods still provide them with more accurate and reliable results.
With the DJI Phantom 4 RTK (P4 RTK) having been released back in October 2018, DJI teamed up with leading drone software provider DroneDeploy to explore whether the P4 RTK can deliver accuracy to contend with real-world GPS-based methods in a report titled ‘Measurement Accuracy of the DJI Phantom 4 RTK & Photogrammetry’ written by Jay Mulakala, Enterprise Solutions Engineer at DroneDeploy. But before we explore their findings, here’s a quick overview of what Ground Control Points are and the difference between GPS, PPK and RTK methods:
Ground Control Points (GCPs) are defined points on the surface of the earth that have a known geo-referenced location. GCPs are marked across the project site (usually between 2-10 GCPs depending on area) so they can be positively identified in the aerial photographs. Accurately surveyed GCPs are used to achieve centimetre accurate results by pinning these points within an aerial map produced from UAS imagery, however they can be time-consuming to setup throughout the duration of the project.
Global Positioning System (GPS) can be used in surveying to achieve sub-centimetre accurate results when post-processing aerial photogrammetry with GCPs.
Real Time Kinematics (RTK) eliminates the requirement for post-processing with GCPs to achieve sub-centimetre accurate results with aerial photogrammetry, as you use a local or network RTK base station to broadcast its position to the RTK rover in real time. The DJI Phantom 4 RTK has a built-in RTK unit (base station + rover) so it can use RTK processing to produce highly accurate aerial maps for professional surveyors.
Post Processed Kinematics (PPK) follow a similar process to RTK however they are unable to correct the positioning data in real time. Therefore, a control point is required so that GPS data can be simultaneously collected in-flight by the drone and base station. This data can then be downloaded for post processing in GPS software.
How did they test the accuracy of the Phantom 4 RTK?
Firstly, DroneDeploy set up a control point system made up of 7 GCPs across a test site. The GCPs were configured at points with different elevations, latitudes and longitudes. Then an additional 4 checkpoints were created with both the distance between points measured and their elevation, latitude and longitude.
In order to directly compare the effectiveness of the P4 RTK, DroneDeploy also tested the DJI Phantom 4 Pro V2.0 (P4 PRO) in conjunction with GCPs as this was shown in a previous study to produce the most accurate photogrammetric maps compared to other DJI aircraft models. This allowed them to measure the accuracy of RTK versus GCPs using two DJI Phantom 4 Series aircraft.
How did they capture the aerial data?
Next, DroneDeploy operated 20 flights with the DJI P4 RTK and DJI GS RTK app and 12 flights with the DJI P4 V2.0 and DJI GS Pro app, both at 100 ft above the site’s surface level.
For the RTK controller they used a third-party corrections service called SmartNet along with a mobile hotspot.
How did they capture the ground data?
A Trimble Catalyst was used to tag the geographic center of the targets for the control. The targeted points were used to serve as control points when measuring the accuracy of the P4 RTK against GCP measurements.
“Trimble Catalyst is an RTK GNSS positioning ‘as a service’”, explains Jay Mulakala. “Rather than using a ‘hardware’ GPS chip, Catalyst uses an entirely ‘software-defined’ GNSS receiver; positions are calculated in software, using your smartphone’s processor – reducing the size, weight, and battery power demands of the overall system.”
As Mulakala further explains, “Due to its simplicity, high degree of accuracy, and acceptance amongst DroneDeploy’s customers base, we were able to use the Trimble Catalyst to confidently ensure the validation accuracy against the P4 RTK. We would recommend using a high accuracy GNSS receiver, such as Catalyst, when processing maps with DroneDeploy to verify the accuracy of the maps on your jobsite for auditing and assurance.”
What did they discover?
According to the report, the P4 RTK delivered 2cm relative vertical accuracy and 1.2cm relative horizontal accuracy when flown at 100ft.
These findings are extremely relevant to professionals within the construction and infrastructure industry who require accurate elevation measurements across various development projects including roads and bridges.
The table below shows the vertical accuracy of the checkpoints, both in the RTK processed map and the P4 Pro with GCPs map:
P4 RTK (cm) P4 PRO with GCP (cm) GPS Unit
Vertical Accuracy (cm) 2.00 1.95 2.26 (0.24%)
Horizontal Accuracy (cm) 1.20 0.90 0.59 (0.03%)
Table 1. Comparing P4 RTK vertical accuracy against P4 Pro with GCP measurements
Next, they found P4 RTK control distances within 0.1% of P4 PRO using GCP measurements. As shown in the table below, the accuracy of the P4 RTK is compared against the P4 Pro:
P4 PRO with GCP (cm) P4 RTK (cm) Difference (cm)
Distance 1 924.5 922.2 2.26 (0.24%)
Distance 2 1912.9 1912.3 0.59 (0.03%)
Distance 3 1220.5 1221.0 0.43 (0.04%)
Average 1.09 (0.10%)
Table 2. Comparing P4 RTK versus P4 Pro w/GCP Control Distance Measurements
As the report tells us, “On average, P4 RTK measurements are within 1.09 cm, or 0.10%, of P4 Pro measurements processed with GCPs. These results show that using a Phantom 4 RTK drone, a user can expect—on average—1.09 cm accuracy on RTK processed maps in DroneDeploy, when compared to similar maps flown with the Phantom 4 Pro and processed with GCPs.”
This point to point accuracy is a crucial element when looking at grade lines, drainage along a road, volumetrics of stockpiles and alike in construction projects and mining sites.
What does this mean for professionals operating the DJI Phantom 4 RTK?
This study is great news for professionals looking to use drone technology to speed up their work without compromising on accuracy, as proven in the study with the P4 RTK delivering 2cm relative vertical accuracy and 1.20 cm relative horizontal accuracy (when processed on DroneDeploy Map Engine). In addition to achieving accurate results, incorporating P4 RTK drone technology as your primary mapping tool could save valuable resources including time, costs and manpower.
To read the full report click here.
For more information about using the DJI Phantom 4 RTK for aerial mapping, photogrammetry and surveying please contact our sales team on 0330 111 8800 or submit your details below and we will be in touch ASAP: