Why does Photoscan rotate my images away from the ground plane during align?

[jeremyeastwood]

Hi guys,

I'm doing a simple stitch of 4 images with ground control data; the images were all taken with the camera pointing straight down, and line up as shown in the first attachment.

When I align the photos (high accuracy, ground control pair selection), the images match up nicely, but the resulting model has been rotated away from the ground plane by about 90deg (see 2nd attachment).  It looks like Photoscan has estimated that all the images have to be rotated about the vector [1,1,0] by about 90deg for proper alignment, so I was wondering why such a major adjustment has been made (which is clearly physically unrealistic)?

As a consequence of this, the georeferenced orthophoto comes out completely skewed (as it is the projection of the model on the xy plane), while the actual reconstructed scene is fairly accurate (albeit rotated) - see 4th attachment (note the axes).

Are there any settings I can select to fix the model plane / allowable image rotations during align, or is this an issue with image quality etc?

Thanks

[Alexey Pasumansky]

Hello jeremyeastwood,

It is common problem for single linear flight line. Since the reference points all lay in the single line there's a rotational degree of freedom around the flight direction. Using at least two flight lines or GCPs will fix that.

[jeremyeastwood]

Great, thanks Alexey.  If I do try to stitch a single flight line again in the future, is there a way of constraining the camera orientation / model plane to be in the geographic coordinate system (i.e. cameras facing down the z-axis, model in the x-y plane)?