Over the past year or more the Phantom 4 RTK has become the popular entry-level mapping drone. But now with its older brother, the Phantom 4 Pro, back on the block, it’s worth taking a good look at how these drones go head to head on surveying and mapping. To bring the P4Pro up to speed for this comparison we have installed the basic KlauPPK system.This is a high quality, high accuracy, survey-grade PPK mapping system that was very popular on the P4Pro prior to its initial demise.
Ease of Use
The Phantom 4, no matter which version, is a super-easy drone, light, convenient to travel with, a very accessible entry-level drone.
The P4RTK has a dedicated RTK base station and a built-in display, the mission planning and flight app is fully integrated, so it is very self-contained and simple to use. All of the GNSS and camera connectivity is internally integrated. Couldn’t be simpler. If RTK works for you then it’s all you need, but you should be aware of the limitations and how you can do better.
The P4Pro with KlauPPK is effectively similar but the PPK payload and camera connectivity is external. It does require some additional effort. The P4Pro will connect to your mobile device and a large selection of mission planning and flight apps, from the simplest popular apps to sophisticated systems such as UgCS.
This is the essential place to start when evaluating any aerial mapping system.
Both drones have the same camera, very similar in spec to the X4s which was sadly deprecated. This camera is good for mapping considering its size, weight and cost. It has a leaf shutter and no moving lens parts, so it is a decent little camera on either drone.
The P4RTK camera logs centre of exposure timing directly with the GNSS raw data and photos. Pretty convenient.
The P4Pro requires a camera modification and external wiring to send the timing marks to the KlauPPK system.
The advantage in the modified camera is that it is supplied by Klau Geomatics with a lab calibration, with precisely determined lens parameters to go into your photogrammetry software. With a good calibration, you can get high absolute accuracy on your ground mapping without GCPs.
Which brings us to the discussion of ground targets, GCPs and checkpoints. You should always have some checks on the ground to know how accurate your work is. These are not GCPs, they are not used in the creation of your model but are points to compare, to ground truth your mapping.
The KlauPPK system on the P4Pro enables you to use your drone to capture ground targets for use as GCPs or checks, then extract ground coordinates through the KlauPPK software.
With the P4RTK you will need to have existing control points, or use GNSS equipment to survey marks in, or use “smart” GCPs. You will need at least one GCP to help manage lens errors.
Data from the KlauPPK on the P4Pro is processed with the KlauPPK software. This software processes the GNSS trajectory, synchronises camera events and produces coordinates in any published coordinate system with geoids, and can create a site localization.
While you can run the DJI base station to transmit data for an RTK solution, this is limited in range, suffers from link data losses and the processing is less robust. There is also a better PPK processing option.
Klau Geomatics has released a version of its software that accepts data from the Phantom 4 RTK (and M210 RTK) for PPK processing to get the best possible results. Using the KlauPPK software you can now improve your efficiency, reliability and accuracy, even with DJI P4RTK hardware.
The KlauPPK unit on the P4Pro can be taken off the drone and used on other drones, even manned aircraft. So, if you crash your drone (come on, it does happen) the KlauPPK in its carbon fibre enclosure will typically live to fly another day. Add more drones to your fleet or upgrade to another drone and you can leverage the Klau PPK technology with just another mounting kit.
With the P4RTK, everything is in the one unit, which will one day sit on the shelf with your other old drones.
PPP is a precise positioning system that uses corrected satellite data. Unlike RTK or PPK, it is not tied to a GPS base station. No matter where you are, you don’t need to run a base, find CORS data (if it is even available) or have ground-based network corrections.
Real-Time PPP is available on the Phantom 4 Pro with the latest KlauPPK hardware. This enables high accuracy in real-time, like RTK, but with no base station requirements, no range limitations, high accuracy anywhere in the world.
The other PPP option is Near Real-Time. Data from the P4Pro/KlauPPK is supported by the Klau Geomatics desktop software for Near Real-Time PPP processing. Processing happens soon after the flight to achieve high accuracy camera positions.
Accuracy is where it all shakes out, and you get what you pay for.
The KlauPPK system has military-grade GNSS hardware at its core, with the highest quality components and advanced interference mitigation, capturing data at 20Hz (measurements per second). The DJI hardware captures at 5Hz and the data quality is not as good, particularly in harsh environments such as around buildings or cell towers.
Accuracies of both systems, using PPK processing, will be similar in ideal conditions at low flight speeds, around 2-3cm XYZ at the camera. As environmental conditions deteriorate or flight speeds increase, the accuracy of the P4RTK will get worse. Typically, without a base station, any drone, even the P4RTK, will achieve only 3-10m accuracy.
That’s where PPP post-processing does its magic to achieve high accuracy autonomously, globally, with no base station. The data from the P4Pro/KlauPPK achieves PPP accuracies of 2.5 to 4cm XYZ.
Accuracy on the ground is a function of the camera position accuracy, lens calibration accuracy and factors in the imagery such as GSD, homogenous surfaces and so on.