height field, red side of bounding box & coordinate system

[jan]

Hello,

in the manual is stated the following (with regards to the building geometry step of the model):

"If the height field reconstruction method is applied, the red side of the bounding box will define the reconstruction plane. In this case make sure that the bounding box is correctly oriented."

I guess 'correctly oriented' means for aerial images: the horizontal plane?

But how can one know the horizontal plane at this stage. It is only known after the reference system is set by placing markers with known 3D-coordinates on the 3D model? 

Does "correctly oriented" here merely means "more or less correctly oriented"? What could go wrong if the bounding box is not precisely horizontal?

another question: If I choose top XY when producing an orthophoto in Photoscan, will it project the 3D model on the red side of the bounding box, or on the z=0 plane from the reference system which is calculated from the coordinates of the markers? (probably the latter, but just asking to be sure.)

kind regards,
Jan

[Alexey Pasumansky]

Hello Jan,

The red-colored side of the Bounding Box should be "more or less" parallel to the XY-plane. The exact correspondence is not obligatory.
Usually for aerial data processing there is GPS data for cameras that helps to determine the correct plane. And even there is not GPS data at Align Photos stage the sparse point cloud (more more or less planar areas) and calculated camera positions gives information about ground plane - since aerial vehicles usually have the constant altitude for flight plane.

"Top XY" (as well as others similar projections) uses the same coordinate system that is used in chunk and does not take into account Bounding box.

[jan]

Hello Alexey,

thank you for the quick response;

sometimes Photoscan alligns a subchunk (a collection of photos) wrongly (at a wrong angle) with respect to the other subchunk. When this occurs, the bounding box will be oblique to both subchunks (even after deleting the wrongly alligned set of pictures). But as I understand I just have to rotate the bounding box then until it is more or less on the assumed horizontal plane, and then it will be ok;

(I take pictures with a balloon and a camera without gps receiver, so I have no gps or height info in advance.)

thanks,
Jan