Georeferencing GeoTIFF "Current View"

[AliceA]

Hello everybody, 

I am currently using Photoscan to realise geological mapping, and I have questions about georeferencing.

I am using ArcGis values as ground control point, but I only have X and Y values (no altitude). For this reason I was using only 3 points on my model. I wanted to try to export it on google earth but the software still does not consider my model as georeferenced. Is this due to the fact I have only 3 points and no altitude? I will have better ground control points in the future, will it get better with a fully referenced model?

I can export orthophoto and DEM on ArcGis, and I can map things on it. But in a second time, I need to map the walls of my model. For mapping the area, I just need to export orthophotos from the “top XY” projection plane, and the georeferencing match with my pre-existing map.
But when it comes to the walls, I need to use the “current view” projection plane. How is the georeferencing made in this case? Because this time, ArcGis doesn’t find the positions of the pictures. I can map my wall without it but I need the orientation of the projection plane if I want to obtain 3D orientations of my structures. Is there any way to know what the orientation of this plane is?

(Sorry if I’m not clear, English is not my native language.  )
Thank you,
Alice

[Alexey Pasumansky]

Hello Alice,

To georeference the model in PhotoScan with GCPs you need to specify at least two projections on the aligned images for at least three markers. Do not forger to select corresponding coordinate system in the Ground Control settings dialog and press Update button when all this is done
In PhotoScan it is not possible to specify only two coordinates. If you input X and Y to the Ground Control pane Z is automatically set to zero.

When orthophoto is exported in Planar projection there is no referencing. Could you please specify the task you are tying to solve with ArcGIS?

[AliceA]

Thank you for your answer, Alexey.

My task is to use the DEM and the orthophoto to detect manually (and later automatically) faults, fractures or geological structures.

For now, I'm trying to determinate dip angles and directions of a plane (representative of a fault or a fracture). I can do this if I know the (X, Y, Z) coordinates of at least three points on this plane. With ArgGIS I can determinate the (X,Y) position of these points, and the DEM value on these points give me the Z coordinate.

But when I use the (X,Y) view, I loose information due to the distortion of the projection. What I need to do is to project the model, for example, on the (X,Z) plane. I can cut the model and obtain a DEM and an orthophoto from which I can determinate the position of my points from the previous method (the orthophoto gives me (X,Z) and the DEM Y).

Nevertheless, the distortion remains to important because my walls are not parallel to the projection plane. What I am doing now, is to use MATLAB to rotate my frame of reference about z axis. If the coordinates of my points are M(X,Y) before rotation and M'(X',Y') after rotation, I can rely them with the rotation matrix M'=RM and the previous method gives me the (X',Y',Z) coordinates. In the end, I can find the position of my points by inverting the rotation.

This method is long and there is still a distortion in the Z direction. This is why I want to use the "current view" projection. But if I want to recalculate the (X, Y, Z) coordinates of my points from the (X', Y', Z') coordinates given by the DEM and the orthophoto, I need to know the projection plane properties. Is there a simple way to extract this information from Photoscan ?

I hope I have been clearer,
Thank you,
Alice

[Alexey Pasumansky]

Hello Alice,

I think that the easier way is to use three markers to determine the projection plane (if the model is already referenced you can even get coordinates for these points in the Estimated tab of the Ground Control pane) and then in the Export Orthophoto dialog select Markers in the Projection plane combo-box.
In the next two lines (Horizontal and Vertical axis) you need to specify two vectors (for example, marker1 -> marker2 and marker1-> marker3), the radio-button between vertical and horizontal axis defines which vector will be used as the corresponding coordinate axis of the exported orthophoto.

Do you think such method is convenient for your task? Maybe some additional description/screenshots are required?

[AliceA]

Hello Alexey,

Thank you, this is the good solution for what I am trying to do.