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Author Topic: Artifacts in DEMs derived from scanned air photos  (Read 1097 times)

ian.madin@oregon.gov

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Artifacts in DEMs derived from scanned air photos
« on: November 18, 2020, 03:58:29 AM »
I have been using metashape to build DEMs from scanned airphotos for several years. I follow the USGS procedures for editing and optimizing the sparse point cloud and georeference using control points. Usually I get good (albeit noisy) results, but sometimes get a characteristic patchy artifact where there are zones that step up or down in elevation abruptly.  Any advice on the cause of this? I have attached a sample.

Arie

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #1 on: November 18, 2020, 12:05:35 PM »
I've had this before and it was due to a slight misalignment of the camera pose. For me, disabling generic preselection solved this issue. Of course, the alignment time was a looot longer.
Cheers.

ian.madin@oregon.gov

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #2 on: November 19, 2020, 08:39:38 PM »
Thanks, I will pay more attention to getting better alignment.

Alexey Pasumansky

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #3 on: November 21, 2020, 01:57:43 PM »
Hello Ian,

Such artifacts are usually caused by the alignment problems.

If the scanned images do have fiducial marks, you need to enable the corresponding option in the Camera Calibration dialog, put all the images to the same calibration group and place the fiducial marks on each image.
Best regards,
Alexey Pasumansky,
Agisoft LLC

ian.madin@oregon.gov

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #4 on: November 25, 2020, 03:06:28 AM »
I have tried using fiducials on several sets of photos, and it never seems to make much difference in the final product, but can take up a lot of time. Many of the older photos I am using do not work with the automatic detection.

PROBERT1968

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #5 on: November 25, 2020, 06:39:53 PM »
Ian,

I've been doing a lot of 1930's forest service aerial photos and a lot of them do not have fiducial marks and have been working on them with PhotoScan/Metashape the last 3 years .  At first it was hard for me because I have never experience with these Photogrammetry skills  ( I am GIS Specialist) and work with my co-worker ( He is going to retire at this end of year) and we work together the last few years and made some suggest to me on some of the areas I have learned.

It is learning curve for me but I always work on and make efforts to  make it work. A lot of my photos are mostly on the mountains ( I am in Colorado ) and in some areas are flat areas.... What my co worker found that a few I have worked on them and he found a lot of my work excellent . He was very satisfied with my work...  The hardest part is finding the feature on the old photos vs the today's photos.

It can be very frustrate at first but the more you work the better understand then you will be very good at it.  I have learned a lot over the years now.

So without the fiducial marks, it still can work unless if there is some bad aerial photos I have faced with it...

So maybe if you could show us your original scanned aerial photos you have

Here is an example of my work of the same area Before and After.

The Before Orthos had some black markers is because of the aerial photos I have and the After Othos, I did clean it up and the black markers disappears.

PROBERT1968

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Re: Artifacts in DEMs derived from scanned air photos
« Reply #6 on: November 25, 2020, 06:47:36 PM »
A few examples of bad aerial photos that I shared with my co-worker and I had to let him know what I am dealing with

BTW, if you need help and feedback  you can go to my post and send me a private message if you want

I work at the US Forest Service  Pike and San Isabel National Forest in Pueblo, Colorado