Any tips or tricks for low overlap alignment?

[lkline97]

Hello all!

First and foremost, I am new to the forum and a relatively new user of Agisoft Metashape. Apologies if I have any unusual wording in my questions! I appreciate any help or advice.

I am attempting to stitch overlapping imagery taken by a UAV into an orthomosaic. We are surveying roughly 20m over the tops of pine trees that stand ~40m tall. These photos were taken as we were learning all of the subtleties of UAV surveying, so unfortunately the side overlap on these photos is 60%. We've since changed our protocol to have side overlap be 85% over these trees, but we'd like to see if we can mosaic the older photos as well.

The issue I'm facing is that I am losing data in the center of the cluster of trees for the overlap 60% photos, and I would like to attempt to have that render. For one of my 60m flights, I have 130 cameras and only 114 are aligning, with the 16 unaligned in the center of the pine trees. This leads to a blank space in the orthomosaic. Here are the steps I am following:

Align Photos
- Accuracy high, generic preselection, reference preselection, Source, key point limit of 0 and tie point limit of 0*, adaptive camera model fitting

Optimize Cameras
- Fit k1, k2, k3, k4, p1, p2, b1, b2

Build Mesh
- Sparse cloud, Arbitrary (3D), face count high

*Key point and tie point limit of 0 provided the best results but still a loss of data

And then the orthomosaic is built from the mesh. I was advised to try this when the process of building a dense cloud was computationally expensive and produced worse results anyway. I have attached the resulting mosaic here to provide a reference.

I am happy to send along the dataset I am using if I could submit that to an e-mail. I also understand that I just might not be able to rectify the problem due to the lack of overlap, but I figured reaching out was worth a try. Thank you!

[Paulo]

hi lkline97,

You can try using medium or even low accuracy, to see if you can get more aligned photos...

in the gap area, if you are patient (given you have 16 non aligned and in each you need to place at least 4 manuel markers with corresponding measurements in adjacent), then you could try to place manual markers between the non aligned and aligned photos then do a new alignment (without reset current alignment).

As an example I have a subset of images over forested area with lines between images representing tiept matches (the darker and thicker the lines are, the greater the matches are). Originally, IMG_5994 did not align so I had a gap over this area. After placing 4 manual markers (point 1 to point 4) in image and measuring them in adjacent images, I was able to align it....Of course this corresponds to only 1 non aligned image and you have 16 so work load would be much greater. See following screen capture....

Another way to obtain a gap less ortho (but probably with some  image artefacts) is to use https://github.com/agisoft-llc/metashape-scripts/blob/master/src/quick_layout.py script to align unaligned images according to camera reference location and rotation. Then use Tools/Mesh > Close Holes (100%) to close central gap in your mesh.
Then generate orthomosaic from closed mesh and you should get now an ortho without hole in central area....

[lkline97]

Good morning,

Thank you so much for your suggestions. I used the quick_layout script and the tool for closing holes in the mesh, and this largely solved the problems I was having. I apologize for taking so long to let you know; I am a grad student with a busy field season and just got around to trying this out :-)

Best,
lkline97