Orthofoto with poor Geometry

[Suesseng]

Hello There,

i have a problem and questions concerning the Geometry of Orthofotos.

My Project:

1) I took fotos of a Mountaintop with a DJI Air 2s Drone to generate a Model and a Orthofoto for being able to draw some Walls that are up there and make a plan of them.

2) The Fotos are automatically georeferenced through the  drone.

3) I Processed the Fotos directly in Agisoft, since the Programm itself make the camera calibration/rectification. Or should i rectify the fotos before in Lightroom?

4) Also i did not make the "optimize Alignement" function in Agisoft after aligning the fotos, since the Alignement looked fine.

5) The coordinate system is WGS 84.

6) When i look at the finished model in Orthografic view, or at the DGM or at the Orthomosaic, some walls, which should be perfectly straight, are a bit crooked. To explain in other words: it looks like i can see one of the sides of the wall, but since it is an Orthofoto which is directly from above, i should just see the top of the Wall and not the sides. Also this is important to draw an exact Plan.

7) On the same Mountaintop there is kind of an corner of a room in the bedrock, with a perfectly angle of 90 degree and vertically walls (so it is documented in the field). In the Orthomosaic the angle is less than 90 degree and the Walls are not vertical but crooked. It seems like the model is not planar but a litlle bit tilted.

Is this because i did not use GCP´s? Or do i have to rectify the Images before processing? Or is it because it is an steep environment, so the objects that are close to the camera are bigger on the Fotographs than the Objects which are more far away?

9) Or do i need to choose another Coordinate System like UTM?

10) In another project where i used some terrestical Photographs additionaly, i saw the Problem increasing. there the vertical wall of a Tower was more crooked in the Orthofoto when i used the Terrestial Images additional to the drone pictures.

I hope i could explain my Problem understandable, if not please ask.

Do you have any Explanaition or better Solution for this Issue?

Thanks in advance,

Best Regards,

Suesseng

[Paulo]

Hello Suesseng,

definitely, I would optimize your cameras as there seems to be definitely image distortion. What is your exposure time?

If you can share a representative sample of photos (2 or 3 flight lines) we could have a look at it.

Do you use a Digital Surface or Terrain model to orthorectify? (DSM or DTM).

If you want a perfect  ortho then you would need at least 3 GCPs to correctly scale and level your 3d model...

[Suesseng]

Hello Paul,

Thanks for your Response!

okay, i will optimize the cameras and process it again.

The Exposure Time is between 1/2000 - 1/3000.

I use a DSM, so the height model of everything.

I will send you 2-3 flight lines!

Thanks for your Help!

Best Regards,

Suesseng

[Suesseng]

Hello There,

i processed again with optimized Cameras but it did not change anything.

Also i changed the coordinate System to UTM but the result was much worse then.

I don´t know what it is, it looks like one is looking not from above but a little bit from the side on the Orthofoto.

Am i the only person who recognised this?

Best regards,

thank you again Paul,

Suesseng

[Paulo]

Suesseng,

maybe you can share your processing report to have some more information....

a screen copy of your ortho where you see it to be tilted maybe, also

[Suesseng]

Hello Paul,
 
this is a good idea, since a picture can better explain than many Words!

So i attached a Screenshot of two Orthofotos with more ore less the same Problem.

Orthofoto 1: It shows the Tower. As you can see it looks like it is tilted because we can see the south side of the wall of it but not the North side of the Wall. Yet the Wall of the tower is vertical so in an orthofoto we should just see the top of the tower and not one of the walls.
As i was writing in my first post, this problem increases when i use terrestric fotos and the fotos of the drone.

Orthofoto 2: The corner in the bedrock should have an angle of 90 degree, but it shows a lesser angle. Additionally, since the carving in the bedrock is vertical we should not see the inner part of the wall.

It looks like an systematic failure since both of the orthofotos are tilted in the north, so we can see the southern side of the vertical walls.

I attached also the reports for both of the Orthofotos.

Thank you for your concern and help!

PS: Since i can not attach the files here because of the restriction of the size, here is the Wetransfer Link: https://we.tl/t-iioRiL9dV6