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Author Topic: Best way to handle blurry marker imagery as GCPs in photogrammertry?  (Read 1839 times)

jnk

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Hi there,

I've been running a fair share of low-altitude, high-GSD photorgrammerty projects so far with good results. Now I'm working on data taken from higher altitude. Many of the images are partly blurred or show some lens distortion at the edges, leading to issues when palcing markers, as the image quality of some GCPs is pretty bad.

GCPs were measured with a highly accurate DGPs system (~2cm).
GSD is at about only 5.4cm / pixel.

The markers used are way too reflective metal crosses (30x30cm, struts about 5cm wide) with a white sheet of paper. On bad images this ends up in a reflective mush (see pictures).


As there's no way to resurvey the area, I'll have to try to get the best results with the data I have. How would you recommend dealing with this?


Setting markers:

- Should I place markers on the blurry images, despite the obvious lack of accuracy due to image quality?

- Should I delete images/cameras that are partly blurry (blurry edges but otherwise fine) or mask the blurry areas?

- Should I just leave those blurry bits out and set no marker on these images?

- Or should I set all markers on not blurry images first, let photoscan set the others for me and just confirm the automatically set ones on blurry bits?


Setting accuracy:

- Should I set accuracy lower for blurred markers? If so, by how much?


So far I've been removing the worst images, not setting markers in images where they appear too blurred and setting overall marker accuracy to about 5cm[/b], which seems very low to me, given the manual recommends 0 for DGPS GCPs. How exactly does marker accuracy work? I would expect it to be not only RTK-error, but rather GSD-based, as in lower GSD leading to worse accuracy - if a single pixel is 5cm wide, is it even possible to come up with an accuracy better than 2.5cm (given I manually set the marker and it's set within one pixel=5cm)?


So, how would you recommend dealing with my blurry markers?

Sorry for the long post. I'm thankful for any info and will report back on the results.  ;)

Cheers.
« Last Edit: December 18, 2017, 01:15:47 AM by jnk »

Smallpoly

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Re: Best way to handle blurry marker imagery as GCPs in photogrammertry?
« Reply #1 on: December 18, 2017, 11:14:08 PM »
Blur can cause a lot of problems for sure, and messes up texture quality as well. I haven't found any magic bullet for getting useful results out of blurry images. It's slightly better than nothing but needs a lot more cleanup.

The next time you do a shoot like this I'd recommend both using larger markers, and overshooting - i.e. if your subject needs 200 photos for a good result, and you predict 50% will come out blurry under the conditions and gear, then try to shoot 400-600 so you'll have enough good ones left over after taking the blurry ones out to pasture.
« Last Edit: December 19, 2017, 01:12:01 AM by Smallpoly »

SAV

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Re: Best way to handle blurry marker imagery as GCPs in photogrammertry?
« Reply #2 on: December 19, 2017, 04:42:26 AM »
Hi jnk,

As mentioned by Smallpoly, there is not really a magic bullet for fixing blurry images (yet).

I understand that you cannot acquire the images again, that's why you are trying to make the best out of it. Depending on the amount of image overlap, you might want to disable/delete images that are very blurry/low quality. Use PhotoScan's ESTIMATE IMAGE QUALITY tool to quickly identify such images (right click on one of the pictures in the photos pane and choose ESTIMATE IMAGE QUALITY).

Regarding marker placement. I would not manually place markers on images that are really blurry/low quality. Depending on the fraction of blurry images, you will probably still need to place some markers in images that are not great (as visible in the 2nd screenshot that you shared). I think that's probably the best you can do.

Next time try to adjust your survey and camera settings to avoid image blur. For low-altitude surveys, you will probably have to reduce your horizontal flight speed in order to avoid motion blur or/and change your shutter speed. If possible, you could also use a different lens (shorter focal length) that would help to minimize motion blur issues. And last but not least, a 3-axis gimbal can also assist in reducing some motion blur and 'vibration blur' issues. I don't think that just shooting more pictures as suggested by Smallpoly is the best solution. It's better to change your settings & survey design in order to get reliable and consistent results.

All the best.... and Merry X-Mas.

Regards,
SAV

jnk

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Re: Best way to handle blurry marker imagery as GCPs in photogrammertry?
« Reply #3 on: December 19, 2017, 01:14:20 PM »
Thanks guys. The problem is that this is not my setup - the people who are using this are about to get a new drone and camera now and I can give them input on their markers, lens and flight height, but it won't change the data I have to deal with now. ;)

I think the issues are mainly due to the poor camera quality and lens, most blur seems to be due to chromatic abberation and false focus. SAV, I have used estimate image quality, but it seems to be good only for the really blurry ones - if an image is blurred to a gread amount bus still has a lot of distinguishable and contrasty interest points on it, the programme seems to classify it as okay - even if half of it is completely washed out.

Unfortunately that doesn't help with fixing the data I have now. As theres about 20 GCPs per scene with 20-something possible projections per GCP I have now set the best 12-ish projections per GCP, taken out the images which are just too blurry all over (about 3% of the cameras) and then calculated with the best 15 GCPs in the scene. Seems to go okay...

If there's any more input on how accuracy works, related to blur of marker images, I'd be very grateful for any info.

Cheers and happy holidays.
« Last Edit: December 19, 2017, 01:32:25 PM by jnk »