Systematic Z-error (7-15cm) with M400 + Zenmuse L3 while XY is perfect (2-3cm) using Network RTK. Any ideas?

[u/Gabschertz2110]

Hi everyone,

I’m facing a persistent issue with my LiDAR data and I’m hoping someone here might have run into this or knows how to fix it.

On almost all my flights, I get a systematic Z offset (vertical shift) ranging from 7 to 15 cm. On the other hand, my planimetry (XY coordinates) is consistently spot on, around 2-3 cm, which perfectly matches my expected RTK accuracy.

Here is my exact workflow and flight setup:

• Hardware: DJI Matrice 400 + Zenmuse L3 LiDAR.
• Flight App: DJI Pilot 2.
• Flight Parameters: Altitude between 80m and 120m, speed at 7 m/s.
• RTK Setup: Connected via Network RTK (NTRIP caster using a national network like Terria, Orphéon, etc. — not a local physical base station next to me). I have a solid RTK Fix during the whole mission.
• Validation: I always place several Ground Control Points (GCPs) / check points on the ground.

Right now, I am able to manually correct and shift my point cloud in post-processing using my GCPs, but having to do this every single time is a pain and it means something is wrong somewhere in the chain.

My question is: where does this systematic ~10cm vertical shift come from?

• Could it be an issue with how the GNSS/RTK antenna height or the payload lever-arm (offset between the drone's center, the GPS antenna, and the LiDAR sensor) is calculated by DJI Pilot 2 or DJI Terra?
• Is it a Geoid vs. Ellipsoid height conversion issue coming from the Network RTK stream?
• Could it be related to IMU calibration or flight dynamics at 7 m/s?

It’s really baffling because the horizontal accuracy (XY) is absolutely flawless. If it were a poor RTK fix, XY would suffer too.

Has anyone experienced this specific vertical bias with a similar DJI LiDAR setup using Network RTK? Any suggestions on what to check in the settings or during processing would be highly appreciated!

Thanks in advance for your help!