urban area orthophoto

[Wmthiab]

hello ..
Iam somehow between beginner and intermediate level in metashape
I have conduct a flight with my phantom 4 on altitude of 50 meter and overlap 80% angle 90 deg and single grid mission . The area is somehow urban and my main aim to have a true othrophoto i tried some options like to build medium dens cloud or to build mesh by sparse cloud and dens cloud but the building in orthophoto looks not straight and not that clear i will add photo in the next comment . Please kindly tell me what is the best setting and preferences to have a nice good looking true orthophoto

File attach

[dpitman]

Howdy,

What are the heights of the trees and buildings?  50m height may be too low.  You could share a sub-set of the images in a cloud sharing folder for others to take a look.

Dave

[Wmthiab]

Well the gorund it self is not flat . I used pix4d capture and set altitude on 50 m which i think it takes the takeoff gorund as level surface . And the trees i think they are 6 meter and bulding are varying between 9 meter and 20 meter and what iam trying to understand what is the relationship between the altitude and problem i showed above i mean low altitude doesnt mean much more details and in this way to over come the problem of the building modelling especially when they are some how close to each other

[dpitman]

The reason the height matters is simple. The overlap is calculated in reference to the ground level to the aircraft but when you are having subjects that are much higher than the ground level the overlap drops significantly resulting in degraded processing. You need to account for the tallest objects that you want to process when you set your height for capture.

[Wmthiab]

So you are saying that since there is tall objects in the area and altitude is relatively low . Although iam setting overlap 80% its actullay will not be 80%
.
Will is there any mathematical formula or a realtionship to use to find out the best altitude for the mission with respecet to highest object in the area?

[dpitman]

So you are saying that since there is tall objects in the area and altitude is relatively low . Although iam setting overlap 80% its actullay will not be 80%

Overlap will be 80% at the ground level but not on the objects that are higher than the ground.  Maybe a visual will help. (below)
.

Will is there any mathematical formula or a realtionship to use to find out the best altitude for the mission with respecet to highest object in the area?

https://support.pix4d.com/hc/en-us/articles/202560249-TOOLS-GSD-calculator

An easier workflow is to simply add the estimated height of the subject to what the planner is telling you the overlap will be.  So, in your case, if you want 80% overlap and Pix4d capture is telling you that you need to be at 50 meters to get that overlap.  But, you know that the buildings and/or terrain can be 10 meters taller, fly at 60 meters instead of 50.  It's not complicated once you understand.

I'm not saying that lack of overlap on the buildings is the reason for the poor reconstruction.  I am saying that it could be. 

[Wmthiab]

i see your point i will try to do one more mission for the same area to find out the problem , but just for knowing what else could be the reason for having this bad or weak reconstruction

[Alexey Pasumansky]

Hello Wmthiab,

You can try to generate the mesh model based on the depth maps (in High quality and Mild filtering) and then use the mesh surface for the orthomosaic generation, it may give you better results on the edges of the buildings.

Also make sure that the alignment is stable and georeferencing is correct, before proceeding to the surface reconstruction.