I am wondering how much precision (cm?dm?) can WebODM achieve using photos taken by RTK UAVs such as DJI phantom4 RTK.
Depends on the accuracy of your data, but cm precision is common with RTK. Just make sure your images are tagged with the appropriate
GPS DOP tags. Related: https://github.com/OpenDroneMap/ODM/issues/1092
The challenge here is also how the flight is done: with hover at each photo taken, you can get very good accuracy. If instead you are moving during each photo take, the accuracy is going to be limited by two factors: the speed at which you are moving multiplied by the 1 second increments of RTK.
Having done some work with a very famous and expensive fixed wing system with RTK (therefore hover at location is impossible), we are seeing 50cm errors in the horizontal.
I used to post process the images and insert the RTK positions into the images with 30ms accuracy (shutter speed and sync method issues) at a speed of 3m/s this is an error of 10cm. This was the best I could do with a Phantom 4 Advanced, rolling shutter and Reach RTK. BUT if you rely on the position inserted in realtime you will see larger errors as stated. You then have the issue of roll, pitch, yaw impacting the position of the camera vs the gps sensor. Fortunately unlike LIDAR this only has a positional impact within the frame of the aircraft, not projected all the way to the ground.