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Author Topic: aligning forest images  (Read 5748 times)

dowallin

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aligning forest images
« on: August 22, 2016, 11:22:50 PM »
I'm having difficulty aligning images of forest in Washington state. I'm flying at 400' AGL and using a Canon S100. I am using the CHDK script UAV_KAP to control the camera. Images are taken every 2 seconds with a minumum shutter speed of 1/1000 sec.  I am using the recommended 60% sidelap and 80% frontlap. The entire flight is mostly over a braided river channel with exposed gravel and shrubs and these images align just fine. Portions of the flight are entirely over forest and the trees are quite tall (100-150') with a continuous canopy cover. I suspect that there may be two issues:
1. relative lack of key features to use for alignment (just a green canopy)
2. subtle movement of the canopy (between images) in the light breeze

Maybe there is nothing that I can do about this but I'm hoping for some suggestions.

And this may be a dumb question but is there any chance that the Agisoft Lens program would help? I'm unclear on what this program does. Based on the manual, I realize that it provides lens calibration information, but my images look quite good (at least to my eye).  In looking at the report that Agisoft generates, I get camera calibration information (see attached) but I"m not sure how to interpret it.

Any suggestions would be greatly appreciated!

Regards,





ekbmuts

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Re: aligning forest images
« Reply #1 on: August 23, 2016, 12:00:25 AM »
dowallin,  I don't do a lot of aerial work.  I mainly shoot and model inanimate objects.  But for what it's worth, whenever I shoot anything that has no distinguishable texture, repeating patterns or is moving even slightly, it doesn't work.  Meaning, the model doesn't come out as an accurate rendition of the object itself.  My main indicator of this is that the cameras are all piled on top of one another after alignment.  Then I know that I have a texture issue.

So seeing as the program doesn't know whether it's looking at a forest or a baseball, I'd be willing to bet that what you think is wrong is what is wrong.  I think your analysis is correct.

If you're using the professional version you can try placing markers and see if any of them take, that is, get picked up and repeated by the program itself when it recognizes in other photos where you have created a marker in the first photo.

And hopefully someone who has a bit of experience in doing aerial work can give you some more pointers.  Someone like stihl or Mehoo.

Jon

dowallin

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Re: aligning forest images
« Reply #2 on: August 23, 2016, 01:05:39 AM »
Jon,
All of the images are geotagged so they all are properly given a tentative position in space by Agisoft. After alignment, some of the images simply don't have any tie points. I'm attaching the overview, showing aligned and unaligned images. I am also attaching two sample images. One is aligned and mosaiced with adjoining images and many tie points appear on the image. The second image is adjacent to the first but no tie points show up. But just by eye-balling it, you can clearly see that it should align just fine.

I realize that I can manually go in and define tie points between these two images (and all of the other unaligned images in my project). I've done this before but is a HUGE time sink and I've found that the result is often less than satisfying.

Any suggestions would be greatly appreciated!

David

Paulo

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Re: aligning forest images
« Reply #3 on: August 23, 2016, 01:09:45 AM »
Dowallin,

forest  is one of the hardest things to have pictures align as everything looks the same. Especially if your tree height is considerable wrt flying height. In your case, it is so that overlap on tree canopy will be much less than over ground and will further complicate matters.

I would sugest flying higher (750 ft) and increasing side overlap to 75% and 75% front lap to have a better chance of finding overlapping feature points. Even doing perpendicular lines would help but of course will increase your flight time quite a bit.

Hope this helps,
Best Regards,
Paul Pelletier,
Surveyor

SuperNewb

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Re: aligning forest images
« Reply #4 on: August 23, 2016, 05:27:38 AM »
I agree with pap1956,

When I am shooting heavy canopy areas, I increase my overlap by as much as 90% to get photoscan to build.  It is overkill in many situations but it saves me from having to re-mobilize to the scan site again.

ekbmuts

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Re: aligning forest images
« Reply #5 on: August 23, 2016, 05:33:39 AM »
I'd go with what these gentlemen are saying.  I'm a little out of my depth at this point...

Jon

dowallin

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Re: aligning forest images
« Reply #6 on: August 23, 2016, 07:46:34 AM »
pap1956

Great point RE how tree height influences overlap! Not sure why I didn't think of this. I will definitely pursue this option. Of course the flying higher than 400' AGL is problematic given the FAA regs but I'll try increasing sidelap and frontlap and/or fly perpendicular flight lines.

Thanks for the input!

David

frank.stremke

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Re: aligning forest images
« Reply #7 on: August 24, 2016, 02:16:06 PM »
with moving things like trees in the wind it is worth trying lower allignment settings like medium
frank