deformed buildings in Photogrammetry

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deformed buildings in Photogrammetry

Post by max72 »

I would like your opinion about photogrammetry software.
By the way.. would it be possible to add a forum sections?
I did a phantom 4 scan of a set of buildings, 3 cm GSD (about 65-70 meters from ground).
I did a test with photoscan personal, Pix4d and reality capture demo, but the buildings are inclined on one side.
The buildings are about 15 meters high. The effect is systematic, and I cannot figure why it happens and how to fix it.
Any insight or suggestion is welcome.
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Re: deformed buildings in Photogrammetry

Post by pedropcsena »

Maybe it is due to the marker and camera position accuracy that you set?
If you set the values too low, the software tries to make your parameters fit therefore causing distortion.
When I worked with Photoscan I set my camera position accuracy to 10 meters and the ground marker to 0.05, even though my ground control network was way better than that - I set it with total station and performed a least squares adjustment.

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Pedro
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Re: deformed buildings in Photogrammetry

Post by max72 »

Thanks for the reply.
This is before inserting the GCP (with photomodeler basic GCP are not available by the way).
I'm not sure I would have any advantage when using them, mainly because my set of GPS surveyed points lies at floor level.
I'm checking with Altizure too..
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Re: deformed buildings in Photogrammetry

Post by tbwester »

Any reason to believe the buildings are not slanted? the stacks of material on the ground do not look distorted.

Cant think of any other reason... Could try a quick trial of pix4d. I've had better luck with pix4d with drone data.
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Re: deformed buildings in Photogrammetry

Post by max72 »

Thanks.
The mid image is the Pix4D trial.
The last is reality capture trial.
The three show the same behaviour, more or less.
I'm confident the buildings are not slanted (This is just one of the many buildings, all slanted in the same way..).
It looks like distortion increases along z.
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Re: deformed buildings in Photogrammetry

Post by pedropcsena »

I am not sure then... But as a heads up, when you bring the GPS coordinates you might have issues with the Z values. The software uses the ellipsoid height and not the geoid height. It is necessary to download the geoid file in order to have it working properly.
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Re: deformed buildings in Photogrammetry

Post by max72 »

This is a strange issue..
photoscan basic is not using any king of geotagging nor GCP and is offering a slightly better result.
Altizure looks even better. I have to download the data and check.
For a 200 meters project the geoid-ellissoid offset can be assumed as constant, moreover I used GCP in Pix4D in local coordinates in the past, but no buildings. Anyway they worked fine.
Phantom used to place a relative height in the picture tag, now it's more like an ellipsoid.
I can transform my GPS measurements to sea level, but that cannot be the issue. It's just a constant offest for such a small patch.
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Re: deformed buildings in Photogrammetry

Post by 3DForensics »

I am wondering if something about how you flew the flight path and the rolling shutter on the camera. Did you happen to have the camera pan sideways??


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Re: deformed buildings in Photogrammetry

Post by max72 »

I was thinking about the rolling shutter issue, but I flew a grid mission, so going back and forth they should compensate, I would say. Moreover recent Pix4D versions should be able to compensate also the rolling shutter.
Altizure output is much better, by the way.. (no GCP).
I veryfied the orthophoto too, I had to scale it a little bit, but the result is quite nice and the deviation is reasonably limited.
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Re: deformed buildings in Photogrammetry

Post by max72 »

A short update:
I tried some of the on line services, and I still have the same issue. The only notable exception is Altizure.
I flew the Altizure app, but as far as I know it's not reasonable to consider the possibility for the app to tamper with data (DJI sdk has many limitations). Maybe I'l cross check and try to fly the same mission with different apps, but I'm confident data logging cannot be modified. I'm also confident the building is not slanted, and in any case it cannot be THIS slanted.. (I laser scanned it, by the way)

I'm attaching some othophotos of the tallest building, using Altizure, Maps made easy, Drone deploy and reality capture.

Massimo
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