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Author Topic: What does Metashape do with fiducial marks?  (Read 1220 times)

adehecq

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What does Metashape do with fiducial marks?
« on: June 24, 2021, 04:40:06 PM »
Hi,

Sorry if this has been answered before, but I could not find the information on the forum.
I was wondering what does Metashape exactly with fiducial marks when working with film camera? What sort of transformation does it apply to the images to correct for scanner/film distortion?

The reason I'm asking is that I'm trying to process a set or aerial images from the 1960s. This is not an easy data set as the images where scanned in two different periods with scanners of different resolution and quality. Even after providing the fiducial markers positions, the alignment didn't work so well.
In the end, I warped the images externally (using a Python script) using the 4 fiducial marks positions so that the markers in all my images were perfectly aligned and the images had the same dimensions. That's when I realized that an affine transformation was not sufficient to make the alignment of the markers consistent. Instead I had to use a projective transform. Only then I got really good results in Metashape.
My assumption is that Metashape only corrects for an affine transformation. If so, would it be possible to enable projective transformations to be corrected?

I was also wondering, is Metashape able to handle images of different resolutions? (one set is 14 microns, the other 21). Although the tie point matching is scale independent, I'm not sure it's the case for the depth maps calculation.

Thanks a lot for your feedback!

 

PROBERT1968

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Re: What does Metashape do with fiducial marks?
« Reply #1 on: June 26, 2021, 08:11:47 PM »
Hi,

I work a lot of 1930's aerial photos on my National Forests here in Colorado and they do not have fiducial marks on mine ! I have a lot of them over 1,000 of them. So far I have done around almost 200. 

All I can say is you can do it but it can take a lot of time to get it right. 

You just have to find similar  features on old aerial photos and the images you see today's.  It is tough but I can manage it.

I don't know what software are you planning to use with what ?  I use ArcGIS Pro to compare with Metashape basemap. I wish they would have the same as ArcGIS Pro but they use different basemap.


Btw, where are you going to work on ? Are you from USA ?

Let me know if I can help you if you ran out of options.

P.S.  Hey Alexy, Can you tell him the steps how to do this for him ?



« Last Edit: June 27, 2021, 04:56:15 AM by PROBERT1968 »

adehecq

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Re: What does Metashape do with fiducial marks?
« Reply #2 on: July 23, 2021, 09:55:24 AM »
Hi PRobert1968,

Sorry for the slow reply, but I don't get any notifications when I get a reply on the forum and I don't come often.
I think what you are talking about (finding tie points between aerial images and modern basemap) is different from the issue I raised. In my case, I have 4 fiducial markers, that I can use to correct distortions of the film introduced during the acquisition and scanning. Usually an affine transformation is sufficient, but in that specific case, I had to use a projective transformation. It does not seem to be handled well by Metashape and I was wondering if this is something that could be improved.

I work on Bolivian aerial images. I'm based in Europe.

Best,
Amaury