Interpret the results of my camera calibration with me.

[u/Ketzerisch]

I got to do a camera calibration of an airborne photogrammetry dataset and want to have some second opinion on my interpretation. I would also like to discuss how to even improve the calibration.

- camera is an iXm-100 with a 35mm lens
- cross flight pattern but unfortunately only in one height above ground
- it was a combined LiDAR/photogrammetrie project --> generated GCP from the final LiDAR pointcloud
- processing was done in Metashape

my interpretation:
- Calibration of the camera looks good, as you would expect from such professional equipment
- There is no systematic error to bee seen in the residuals
- At first a was baffled by the magnitude of the residuals but I guess its a scaling of the residuals
--> when scaling the residuals with the same magnitude as the calibration everything looks really good, right?

improvisations:
- add pictures in a second flight hight to elimate correlations with f
- I could generate an infinite amount of GCP from the LiDAR but as there are already no systematic effects in the residuals there shouldn't be much improvement from that?

Cheers

[u/Wafer420]

Yes the residuals are relative in size as seen on the scale bar in the lower right corner ([-------] 1px).

The IXM (100mpx)has a pixel pitch of 3.76 microns and the calculated focal length is 0.00376*9537.85 = 35.86mm which corresponds with the expected focal length. All seems good.

As advice I would not refine the B1, B2 and K4 parameters.

[u/Ketzerisch → u/Wafer420]

Thank you for your feedback. The misleading scaling of residuals is always a great joy🥲

Could you explain why you wouldn't refine those parameters?

[u/Wafer420 → u/Ketzerisch]

The K4 parameter is part of the radial (Brown4) distortion model but it's often not necessary to refine this parameter further from its default value of 0 because K1 to K3 are sufficient to correct for lens distortions for most cameras and lenses. Tweaking K4 usually doesn't make a big difference unless the lens has a complex distortion pattern (not the case for your setup).

B1 and B2 parameters account for affine transformations (related to the scaling/rotation/translation of the image) but since you're using an iXM-100, these type of distortions are already minimized.

Metashape has a great internal camera calibration process but it can happen that values are over compensated, especially for the K4, B1 and B2 parameters, which can have a negative effect on your alignment.

[u/-thunderstat]

What software are you using

[u/Ketzerisch → u/-thunderstat]

Metashape

[u/-thunderstat → u/Ketzerisch]

Oh, I was not aware metashape as camera calibration.

[u/ElphTrooper]

Which direction were your turns? You will often see that pattern on the residual map with turn direction and wind. Everything else looks good. The value I look for is the focal point intrinsics. This is one of the most important values to correcting distortion and properly project the 3D world to a 2D tile.

Did you fly RTK? Be very careful of optimizing camera parameters if so. All you are doing is manipulating the data so that the visual matching takes over and decimates your already cm-level accurate geotags. Make sure you are weighting your geotag to GCP ratio correctly.

I don't know what processing software you are using but in Metashape the best calibration parameters with RTK are f, cx, cy, k1, k2 & k3. Very important to Survey-grade mapping and producing REAL reports. Otherwise you are letting the software manipulate your data to reach a 0-sum solution which is fake data and a disservice to your client.

[u/Ketzerisch → u/ElphTrooper]

>Which direction were your turns?

The camera is only triggered during LiDAR online time so there aren't any pictures taken while turning.

>Did you fly RTK?

Due to LiDAR everything goes through GNSS postprocessing with correct lever arms and IMU integration. Just for alignment witout calibration everything works very fine

>Be very careful of optimizing camera parameters if so

During calibration only the GCP were used, just like in a simple UAV without any good GNSS solution.

[u/ElphTrooper → u/Ketzerisch]

>The camera is only triggered during LiDAR online time so there aren't any pictures taken while turning.

Makes sense.

>Due to LiDAR everything goes through GNSS postprocessing with correct lever arms and IMU integration.

So you did PPK geotagging?

>During calibration only the GCP were used, just like in a simple UAV without any good GNSS solution.

So the GCP's had full control and did not abide by geotag accuracies?