# Forum

### Author Topic: Best way to roughly constrain alignment/geometry?  (Read 4846 times)

#### andyroo

• Sr. Member
• Posts: 426
##### Best way to roughly constrain alignment/geometry?
« on: May 04, 2012, 09:51:23 PM »
I am generating surfaces from unreferenced aerial images, then assigning GCPs after geometry creation. On my first model it went very well. With my second one, I get weird results from a bad alignment calculation. Photoscan actually runs the pointcloud from one set of images right through the other (see attached image).

There is about 66 percent overlap between the images...

Since I haven't created geometry yet, I have to specify each GCP individually on each photo (then I was planning to re-run the alignment)

Is there a better way to do this? Or is there a way to better constrain the geometry that can be built? The photos represent 4-up, 4 down passes, so half of them are 180° from each other (and they are four different elevations).

#### JanS

• Newbie
• Posts: 18
##### Re: Best way to roughly constrain alignment/geometry?
« Reply #1 on: May 05, 2012, 09:42:35 AM »
Hi,

Since you don't seem to have that many photos, you could try to assign each photo relative coordinates e.g. (0,0) , (40,0),... if you have some idea on the distance between photos within and between flight lines. You can try using these coordinates as ground control to have PhotoScan better pre-order the images before point cloud construction.

#### Alexey Pasumansky

• Agisoft Technical Support
• Hero Member
• Posts: 14503
##### Re: Best way to roughly constrain alignment/geometry?
« Reply #2 on: May 05, 2012, 02:18:50 PM »
Hello andyroo,

If I understand right there are two groups of images that should be parallel? Is there enough overlap between these groups?
Best regards,
Alexey Pasumansky,
Agisoft LLC

#### andyroo

• Sr. Member
• Posts: 426
##### Re: Best way to roughly constrain alignment/geometry?
« Reply #3 on: May 06, 2012, 12:11:46 AM »
I figured out the issue, although I think I could have changed the processing order and fixed it even if I hadn't.

It turns out that I had accidentally removed six images from one of the flight lines, which I think was the first one being processed. This "broke" the imagery into two unreferenced sections. the three other flight lines then "stacked" imagery on each of these broken sections, the flight lines going up-valley tied to the first part of the broken set, and those going down-valley tied to the second part. When I added the six images back in, it properly aligned the images.

Re Alexey's question, there are four groups of images that are all partially overlapping (two passes up-valley and two down-valley). Along-track they have 65%-75% overlap depending on elevation. Across-track is more variable, since it's a hired pilot trying to keep straight and level over the river.

Attached another screenshot to show how it looks with proper alignment. Thanks for your help!