Hi need help.

I have a Phantom 4 RTK with the base station DRTK2, i flew a 12 km long highway with 50 m wide. I had to make 8 flights and about 3500 images. The thing is that between flights in the overlap images i got different z values so after processing data in pix4d blim i have no continious data. Can anyone tell me why this happen i mean with the rtk base station the data must be really precise and i got differences between flights about 4-5 meters in z?, x and y are ok, and ortho is fine, I want to know if my equipment is correct or if there is a problem with hardware. I attach a profile elevation. Thanks for your help.

All flights were at 72 m with a 2cm gsd and all time RTK was FIX ok thanks

Hi Cabeko,

I have just got one Phantom 4 RTK + D-RTK 2 Mobile Station and I still have to takeoff the first time, but I will try to give you some clues.

The D-RTK 2 Mobile Station can find its absolute position averaging multiple Single Point (Pseudorange) mesurements. The declared absolute acuracy is  H.acc. 1.5m - V.acc. 3.0m (RMS).
To get centimeter absolute accuracy (Horizontal 1cm+ 1ppm - Vertical 2cm+ 1ppm  RMS) the D-RTK 2 Mobile Station needs to be connected to a NRTK via the 4G Dongle + SIM.
If the D-RTK 2 Mobile Station is positioned exactly on a survey point of precisely known coordinates, you can set them manually in the DJI GS RTK app.

The RTK correction sent by the D-RTK 2 Mobile Station (Base receiver) to the Phantom 4 RTK (Rover receiver) is "relative" to the calculated / inputted position of the Base. Consequently, even in the FIX status the absolute accuracy associated to each image principal point is dependent on the absolute accuracy of the calculated / inputted position of the Base.

Due to the large differences in Z between adjacent flight lines, I suspect you repositioned the BASE GNSS receiver for each of the 8 flights and did not use neither a NRTK correction nor precise coordinate manual input. Moreover a Single Point (Pseudorange) position can change during the day in relation to GNSS satellite geometry and visibility, even surveying the same point.

In any case you usually need some GCP to check and describe the final accuracy of your survey, even using RTK.

Take a look of the following links:

Accuracy and Precision of P4P RTK
https://forum.dji.com/thread-187711-1-1.html

P4P RTK Base Station Coordinates
https://forum.dji.com/thread-181111-1-1.html

https://www.pix4d.com/blog/rtk-ppk-drones-gcp-comparison
https://www.pix4d.com/blog/phantom-4-RTK-drone-Pix4Dmapper

https://www.aerotas.com/general-rtk-ppk-processing
https://www.aerotas.com/phantom-4-rtk-ppk-processing-workflow



I hope this could help.

Cheers,
Maurizio

mbuonanno Posted at 9-22 00:49
Hi Cabeko,

I have just got one Phantom 4 RTK + D-RTK 2 Mobile Station and I still have to takeoff the first time, but I will try to give you some clues.

Thank you very much. You help me a lot in my research.

Im now trying to fix data with known gcp, im running process to correct images i hope it works. You were right i didnt use the dongle neither exact coordinates for the mobile station, i will consider that on my next proyect.

Cheers
Mario

Cabeko Posted at 9-22 14:53
Thank you very much. You help me a lot in my research.

Im now trying to fix data with known gcp, im running process to correct images i hope it works. You were right i didnt use the dongle neither exact coordinates for the mobile station, i will consider that on my next proyect.

Hi Cabeko,

given that you own both the P4RTK and the D-RTK2 base station, instead of measuring a nunber of GCPs it would be better to use the RTK protocol suggested and tested by AEROTAS. You just need to connect the D-RTK2 to the P4RTK radiocontroller and to measure precisely a minimum of 1 GCP with any GNSS receiver capable of PPK postprocessing.

AEROTAS - Phantom 4 RTK Full Workflow
https://www.aerotas.com/dji-d-rtk-2-survey-workflow
https://www.aerotas.com/phantom-4-rtk-field-workflow

Maurizio
;-)