L1 20c Calibration

The Mavic 2 pro camera is known to be photogrammetrically capable. The constraints of the electronic shutter are known and the GSD is covered here: https://billboyheritagesurvey.wordpress.com/2019/05/01/mavic-pro-2-gsd8mm/

Getting optimum modelling from it requires a calibration. Below find the results of a close range image set with auto focus. Further experiments with a bigger target board at a more realistic stand off may give different (and more relevant) results. Using the Mavic as a hand held camera is interesting, the gimbal and vibration damping work well giving sharp shots at 1/30th.

3840 x 2160 px 8bit SDR LCD panel
The calibration set
Exif shows f 2.8, ISO 250, 1/30th
The radial distortion chart suggests a ‘sweet spot’ of about 2000 px diameter in the centre of the image where distortion is under 2 px. With an image area of 3640 x 5460px, roughy a fifth of the image area.
ParametervalueSDUnit
f10.26mm
Exif mm
fx4479.942.26513
f in px
fy4374.512.15679
cx2727.480.813274principal point px
cy1811.920.719281
skew-1.916570.0843506
k1-0.01969480.00126486Radial distortion
k20.006521190.010933
k30.09092950.0368218
p10.0002373542.27565e-005Tangential distortion
p20.001084943.73953e-005
Agisoft Lens shows slight pin cushion distortion (K1)

About billboyheritagesurvey

Heritage worker
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4 Responses to L1 20c Calibration

  1. Robert says:

    Dear Bill,

    Congratulations on a fine blog and thanks especially for this very interesting post. May I ask what software you used to perform the calibration? I eagerly look forward to additional calibration data for the Mavic 2 Pro using a distant target. And if it is not asking too much, would you have any interest in calibrating the Mavic Pro as well?

    Thanks!

    • The calibration was done in Agisoft Lens.

      The next effort will be with the camera focussed to infinity at f8 or f11. I don’t have the Mavic Pro and the results for it will be very different.

      B

      • Robert says:

        Thanks in advance for making the next effort. I await the results of calibration at infinity.
        Since you are an expert on such matters, would you happen to know whether the aircraft yaw and gimbal yaw given in DJI flight telemetry data and in the EXIF data of photos taken with DJI cameras (in particular the Mavic Pro and Mavic Pro 2) is a true bearing or a magnetic bearing?
        I appreciate your help.

  2. Robert,
    I only have 20 hours on the Mavic2 Pro so I’m afraid I’m hardly an expert!
    From the Exif of this image:
    DJI_0423

    Absolute Altitude – +84.54
    Flight Pitch Degree – +13.70
    Flight Roll Degree – -3.4
    Flight Yaw Degree – +3.20
    Gimbal Pitch Degree – -32.4
    Gimbal Roll Degree – +0.00
    Gimbal Yaw Degree – +7.10
    Relative Altitude – +80.80

    I assume:
    Altitude is probably barometric difference from RTH GPS value.
    Flight roll is from the IMU
    Flight yaw is from the compass or IMU
    Gimbal pitch from x axis gimbal motor feedback
    Gimbal roll from the y axis gimbal motor feedback
    Gimbal yaw from z axis gimbal motor feedback
    Relative altitude barometric

    based on the likely sensors for each element. The compass deals with the aircraft yaw, the Exif states gimbal deflections so the angles relate to the gimbal independent to the aircraft.

    This is all guess work based on how I understand the Mavic2 systems behave, an inquiry to DJI is more likely to give more precise information on this.

    All the best,
    B

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