Orientation of a wall for orthophoto

[Blackbyrde]

Hello,

Newbie here. I'm working on models of walls with relief decoration, and when I shot the images I didn't think ahead to define fixed points that I could use to set the orientation of my models (in other words, to make sure that in an orthophoto, I could get a "flat" view of the surface). Does anyone have any ideas for how to work around this? In other words, could I set up artificial coordinates that would allow the software to understand which view I want presented in an orthophoto? I would appreciate any suggestions, or pointers toward the right forum threads to help me figure this out.

Thanks!

[AdrienB]

Hi Blackbyrde

You have to use the marks of photoscan
Tools > marks > print marks

Then, at least on a A3, you print them ( 4 by page ), stich on the ground of your wall and take your pictures.

Marks are generated in vectpr .pdf, so with an app like illustrator, you take the distance between each point precisly.

Doing your process and after photo align or dense point cloud, select Tools > marks > detect marks

You'll have your 4 marks named by a number that you can see above your printed marks.

Then in your  references panel > marks you'll see columns X Y Z(m) ( you have to clic on the " see original values" button, just on the right of " refresh " ) Take one of your mark as reference X 0, Y 0, Z 0 and then write the distance from each other points to this reference point, the ones you took from illustrator. Then clic on refresh and your model will be aligned with this artificial coordinates.
Start your build orthophoto and select marks in your projection plan.

Tips :

- stich your A3 papier not on the " earth " ground but on your building ground like on the first step to improve your Z axis ( 90° from X axis to Z )
- your reference mark ( 0,0,0 ) could be the 90° angle of a rectangle triangle.
- make sure your XY XZ YZ marks axis following building axis


You have to improve yourself this workflow.
You can use Autocad to generate a .csv, using trilateration and improve your artificial coordinates.
If you want a perfect result, you'll have to use some tools like laser.

Good luck

[matt07lx]

If you already have a coordinate reference system in place in your model (from a total station, for example), there is another method you can use, detailed in this thread: http://www.agisoft.com/forum/index.php?topic=6377.msg33969#msg33969.

This works perfectly for me to make orthomosaics of walls, archaeological profiles, etc.

[Blackbyrde]

Thank you both! The problem I'm having is that I don't have any coordinates built into my photography at all, and I can't go back to the walls because they are in Egypt. Fortunately, there are a lot of right angles in Egyptian art, so I'm defining three points on a right angle in the design and placing them in a plane. This seems to be doing the trick; I'm just not sure I'm not introducing distortion based on the imprecision of the angle! I am interested in the idea of using CAD to set up coordinates, but I don't have any experience with that software. At any rate, I'll make a point of trying to anticipate this when I go back - maybe I could put one of those "L" shaped forensic rulers in the scene in such a way that it's parallel to the wall surface...

I appreciate the input!

[Yoann Courtois]

Hi everyone !

Firstly for AdrienB :

Your workflow looks OK but very approximate. I would advised you to search deeper in the forum to precise it

Then for Blackbyrde:

As you may have seen on Matt's link, I've worked a bit on wall orthophotos. Indeed, known-points (GCPs) or known-scales would be mandatory if you need a scaled orthophoto but, if you don't have any of them, you shouldn't need it ! Nevertheless, GCPs would give you the possibility to accurately correct your 3D modelling. Without them your model would be a bit indecisive.

As you know, you can define artificial points on your Egyptian wall. Then, you easily understood 3 points are needed to define the plane that while define your ortho-rectification. Fortunately, you don't need them to form a right angle ! Indeed, when you go to "Build Orthomosaic" panel, you can define:
- The type as Planar
- The projection plane based on Markers
- The two axis using your markers, for example horizontal from A to B and vertical from A to C.
- But you can also tick one of the two axis that will define your orientation plane. Your third point would only be used to orientate the plane around the axis, the angle will stay right !

In other words, you would only need two points at the same altitude (horizontal axis) and a third one, randomly place above your two first. Just be careful this third one is not too "behind" or "in front of" your wall to be sure your plane will be vertical !

I hope I was enough clear

Regards