Why aren't my GCP points working? Off 5-30m in Mosaic

[aggieair]

Hello all,

I have a PhotoScan Pro project where I've added about 20 GCP on a 5 mile stretch of river.  The GCP are just rocks and log features, not official air photo targets.  I have XYZ coordinates (picked from a basemap, not in the field).

NORMALLY, after I've add GCP to an existing project, I redo the optimization (thus clearing the previous non-GCP solution), realign photos, then redo the point cloud, then the mesh and finally texture - this usually updates the mosaic with the GCP an everything is good.  New rectified mosaic done.

The project I am on right now is not dong that.  The observed GCP locations are about 5-30 meters off from the actual GCP locations when comparing GCP to the Agisoft Mosaic.  I have check and rechecked that I am picking the right points in my mosaic - even the model shows them all in the right place.  But the exported mosaic is not correct.  I even used a couple of those points on mosaics upstream and downstream with success.

What is going on?  I have not started over completely (which sometimes is one of my methods).  I'm trying to avoid doing that.  So far I've always been realigning photos within the same existing project.

[andyroo]

The couple times I've had anything like that happen it's been due to human (my) error. In one case I erroneously applied swapped a GCP on one image to a similar feature in another. it might be worth filtering by GCP and clicking through all the images for each GCP (sort by error first for GCP and start with the ones with highest error).

Another issue I've encountered that affects the final orthoimage is setting the wrong coordinate system in either the GCP settings or the output. I am guessing you've checked those though.

As a last resort, I had something similar happen early in my project where I didn't have GCP markers and was using objects, some of which moved. I was able to find and fix the problem by removing (unchecking) every other GCP (sorted by latitude since I have a N-S flowing river) then re-optimizing and generating a mosaic, then swapping the disabled GCPs and doing the same. That helped me pin down the troublesome GCP.

Good luck.

[aggieair]

THank you for your tips, Andy!

I paid more attention to the error of each point, sorted them, revisited them, removed/edited points until my overall error seemed good, and then reran the optimization followed by the point cloud.

It worked!  Thanks!