Banana Effect and "Not Enough Reference Data" for GCPs

[Coral3D]

I'm trying to correct a model that has the "banana effect" using GCPs but keep getting the "Not enough reference data" message when I click the update button (see attached screenshot).

 I have more than 3 markers, each with more than 5 projections, and all of them are checked in the reference panel. The XY coordinates are unknown but the Z coordinates should all be 0 as they are on a flat level surface. I saw in another thread to put inflated accuracy numbers for the XY and a low error for the Z coordinates, so the markers all read 0,0,0 for their local coordinates and accuracy is set to 100,0.001. The pixel error is sub-zero for each marker, and the camera does not have GPS so there is no coordinate info for the images. I've tried resetting alignment and re-aligning the images with the GCPs checked, but still get curvature in the model.

Things I've tried
More GCPs
Estimating the approximate XY coordinates of three of the markers, relative to a fourth marker given coords 0,0,0, and keeping an inflated XY accuracy
Adding the marker directly to the images
Adding points to the model and then refining the projections in the photo pane
Adding scalebars

Help!

[olihar]

Why are you using local coordinates?

[James]

i had a similar issue with a single long strip of 200 images yesterday. i was able to add about 8 gcps but these were more or less colinear so not useful for orientation. i was also fortunate that all cameras were facing directly down the z axis with no appreciable roll/pitch/yaw, so setting rotation values to 0,0,0 for each camera (which i think corresponds to looking down z, with x and y coincident with the xy of the image plane), setting rotation accuracy to 0, and ticking the box next to rotation for each image, and optimising seemed to do the trick.

if your images are differently oriented then it might be tricky to work out the required rotation values, but not impossible.

might be worth a try anyway.

Why are you using local coordinates?

i too use local coordinates for almost everything, simply because my work is rarely georeferenced (or if it is then it's to a local projected system) and xy meters are easier to get my head around and record than lat long, especially as most of my work is related to buildings which either have their own local project coordinate systems or the specific coordinate system used isn't relevant to the project which is normally to just get a good image out at the end.

[SimonBrown]

Single long strips of images can struggle to robustly align and this method should be avoided.

Causes of curvature is discussed here:

https://www.mdpi.com/2077-1312/12/2/280

It's related to UW work but applies to terrestrial images too.

Finally, dont go down the rabbit hole of thinking camera calibration will help:

https://accupixel.co.uk/2023/09/06/course-update-manual-camera-calibration/

Recalibrated cameras can be an advantage in certain scenarios, but not on a texture-rich coral reef.