Same flight location - different elevation level in ReCap

[Anonymous]

Hi,

Wondering if anyone can help solve the mystery..

I carried out a drone flight (Phantom 4 Pro) and  the plan was to upload images into ReCap Photo to create model and then export a point cloud.  Flight was carried out in two parts as it was a railway bridge and we were not allowed to fly over the line for safety reasons.  So we took off on one side, carried out flight and landed, then walked to the opposite side of the bridge and took off, carried out flight and landed.
So essentially 2 flights but of the same object with the same drone - there was no recallibration or power down between flights.

There is good overlap of the photos and we made sure that multiple objects were captured on both sides to aid with the processing as we did not have GCPs for this flight.

When I upload all the images into ReCap Photo, for some reason the two sides do not match up in elevation.  They are in the correct location but just seem to be out of sync in height.

Images attached showing the elevation variation and also them in the correct location otherwise.

Any help or info would be great! Not sure if it's a ReCap Photo issue or issues with the photos themselves.

Aimee

[barmanr]

Hi @Anonymous ,

Were the heights of the 2 flight paths kept consistent? Since these are 2 separate captures and purely based on GPS info, there will be some error.

Could you send me a screenshot of the mesh in ReCap Photo showing the cameras by pressing the K key. Perhaps a side view to better gauge the camera heights.

Also, it would be even better if you are able to share this dataset with me so that I can run it directly with the engine and check further. If you are able to, you can PM me with the link.

Thanking You.

Warmest Regards,

Rishov Barman, Reality Capture

[Maksim.Melnikov.BY]

Greetings @Anonymous 
When performing photogrammetry from two flights, you must adhere to the following rules:
1. The overlap must be sufficient (> 70%)
2. There must be at least 2 control points in the break area.

I didn't understand if you have any breakpoints in your project at all.
Also keep in mind that the height of the photo is calculated from the height of the take-off point (Home point)
When you took off for the first time, the drone recorded the altitude of the takeoff point H1 with a navigation accuracy of +/- 10m. Then you took off to a height of 15m. and took a picture. Exif recorded height H1 + 15m
When you took off the second time, the drone recorded the altitude of the takeoff point H2 with a navigation accuracy of +/- 10m. Then you took off to a height of 15m. and took a picture. Exif recorded height H2 + 15m
That is, you would fly at the same altitude, but the difference between H1 and H2 can be 0-20m. Hence your mistake.
In order for the program to correctly combine the two departures, GCP is needed in the overlap zone.
Sorry for my english, I hope you can understand.

[Anonymous]

Hi Rishov,

Thanks for the reply.

The flight was free flight - so no set flight path.  

I am at home working on a different computer with no access to the model so can't send you the mesh image with camera views but I can forward you the images used if this helps?  What is the best way to share this with you?

I did recreate the model in a different program (Pix4D) and am having the same issues so it must be something to do with the GPS data, as you mentioned.

Aimee

[Anonymous · Accepted solution]

Hi Maksim,

Thanks for the reply and the information, it's been very useful. 

I think you are correct in the about the GPS error - it's not something I thought about so thank you!

We did not have GCPs as it was difficult due to the location of it being a railway line bridge - we cannot access the top of the bridge and it runs over a river so ground options are limited.  

By breakline do you mean the point at which the two sides/models overlap?  I'm just trying to think about ideal locations where we could possibly take GCPs.

Thank you,
Aimee

[Maksim.Melnikov.BY · Accepted solution]

Greetings @Anonymous 
I apologize for the incorrect translation.
You should have> 70% overlap between all photos in your project. This rule also applies for two flights.
You read that correctly. The GCP must be in the overlap zone of the two flights - a must. If you don't have ground access to this place, this will be a problem.
I would recommend that you shoot in one flight. BUT follow all precautions!
Also, if you are using a non-survey drone, you will need a few more GCPs on each side of the bridge.

[Anonymous]

Thank you Maksin,

This has been very useful for me.  I appreciate you taking the time to reply and provide me with suggestions.

I will look at taking GCP points where I can and retry.

[barmanr]

Hi @Anonymous ,

Yes, pure GPS image sets with 2 different captures will have errors. So as @Maksim.Melnikov.BY mentioned in order to constrain the scene, you need Control Points.

You can still send me the link to your images, perhaps PM me with a link to the upload. You could use one drive or wetransfer.

Warmest Regards,

Rishov