Rotate and align model with distance and altitude

[stefan_fvt]

Hi,
I'm exploring photoscan in order to make decent DEMs and ortophotos. I have photos taken from a camera on a drone with GPS EXIF data from the camera. I don't have access to accurate GCP since we don't have equipment for taking this. However we can measure distance between points and also we know the altitude (Z-value) of certain points throughout the model.

When we input the photos to PS it most often create a bent and tilted result. I found the method of using the scale bar to set length between points but. But it doesn't seem to be possible to only use Z-value for GCP omitting the X and Y value. When I try this and optimize alignment the model disapears. 

I will need to create contour lines in a 3D party program and when the model is tilted in random direction this isn't possible. Does anyone have any idea if this is possible to achieve?

/Stefan

[Alexey Pasumansky]

Hello Stefan,

After scaling the model using scale bars, you can rotate the model using Rotate Object tool, giving it more or less proper orientation.

In current version of PhotoScan it is not possible to use only one or two coordinates from XYZ triplet. So if any coordinate is missing for marker or camera position, the corresponding instance should be unchecked in the Ground Control pane prior to optimization.

[stefan_fvt]

But rotating the model only gives it a rough alignment. Is it possible to confirm the alignment for the model. If for example I know that the two points have the same Z-value?

[Alexey Pasumansky]

Hello Stefan,

By two points you can't define the orientation of three axis, however, using Python scripting you can make Z-axis parallel to the vector connecting two points with the arbitrary orientation of X and Y axis in plane.

Maybe you can provide us the project file (only with aligned cameras) where a pair of markers is set up in space to be used as vertical direction? And we'll try to supply you with the script sample.

[stefan_fvt]

I will try to arrange a data set for you. Would it help PS if I fly at a known and fixed altitude and provide this data in the Exif for altitude?

[Alexey Pasumansky]

Hello Stefan,

If there are X, Y and Z coordinates for the camera positions, you can georeference the model in the given coordinate system.

[stefan_fvt]

Yes Alexey,
I understand that it gets georeferenced since it uses the cameras X and Y coordinates from the camera. But the camera doesn't provide any information about the flying altitude. I can however add the altitude of each image since I know the height I am flying at.

Question 1:
If I specify the altitude for each image and fly at the same altitude for all images. Will this help PS to align the ground plane more accurate?

Question 2:
If I provide images taken from different altitudes and don't specify a altitude value for the images. Will this make it more probable PS produce a model with a tilted ground plane?

[stefan_fvt]

No one that would know?