ZED 2i long object measurement accuracy issue underwater

Hardware
1

Hi,

I am using a ZED 2i camera for underwater coral garden measurement and I am facing an issue with measurement accuracy for large objects.

Small object measurements are very accurate and stable, but when measuring longer objects or larger coral structures, the measurements become inconsistent and less accurate. I observed this behavior both underwater and above water, so it does not seem to be a simple calibration issue.

Current setup:

  • Camera: ZED 2i

  • SDK Version: 5.3.0

  • Resolution: HD720

  • Depth Mode: NEURAL PLUS

  • Depth Min: 0.30 m

  • Depth Max: 10.0 m

  • Depth Stabilization: 35%

  • Confidence: 85%

  • Texture Confidence: 100%

I would like to ask:

  1. Is this expected behavior for stereo depth when measuring long objects or larger scenes?

  2. Does underwater refraction significantly affect large-scale measurements with the ZED 2i?

  3. Are there recommended settings for improving long-distance or large-object measurement accuracy?

  4. Would using point cloud measurements provide more stable results than direct depth measurements?

  5. Is there a recommended underwater calibration or correction method for accurate large-scale measurements?

The issue mainly appears when the measured object becomes longer or farther from the camera. Small nearby objects remain highly accurate.

Thank you

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2

Hi @RodinaGhazaly
Welcome to the StereoLabs community.

The ZED cameras are factory-calibrated in the air. When you use them underwater, you must perform a manual calibration to obtain precise measurements because the water acts like a lens, and the camera parameters are no longer valid.

How do you perform your measurements? The depth map provides a “depth” measurement, the perpendicular distance from the camera plane. If you need a 3D distance, you must calculate it using the Euclidean Distance formula: d = sqrt(X²+Y²+Z²).

You can use our OpenCV-based tool with a waterproof calibration board: