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Author Topic: Difficulty stitching corn field  (Read 4933 times)

jeremyeastwood

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Difficulty stitching corn field
« on: October 03, 2014, 11:07:40 PM »
I'm running into difficulties stitching large corn fields from aerial imagery, as shown here:

https://www.dropbox.com/s/fazxwnvjxwqvwun/Screenshot%202014-10-03%2012.35.30.png?dl=0

Photoscan doesn't seem to be able to align the photos in the center of the field (see https://www.dropbox.com/s/oaw9qdza4l646ya/Screenshot%202014-10-03%2012.36.19.png?dl=0), and the parts it does align are done so poorly (e.g. broken / non-straight corn rows), so I was hoping I could get some advice on ways to improve the stitch quality.

The images were taken with a Sony NEX5 (although were downsampled to 2MP), have 60% front and sidelap, and the overall flight path was flown in a crosshatch pattern, such that the entire area was covered twice, with a 90deg rotation between each full coverage (to ensure at least one flight path is not aligned with the corn rows).  Each image was tagged with GPS info when captured, from a GPS devie with +/-3m accuracy.  A sample image can be viewed / downloaded here:

https://www.dropbox.com/s/pbdud431ouldyfq/pict20140923_160305_0.jpg?dl=0

The Photoscan workflow used high accuracy settings in each stage, photo alignment was optimized after the align stage, reprojection error was limited to 1 and the bounding box was fitted to the exact area of interest.  The report can be downloaded here:

https://www.dropbox.com/s/qohjnrsx8r2rbvg/report.pdf?dl=0

I have experimented with different aerial capture approaches - higher overlap, single flight paths (rather than the crosshatch pattern), higher altitudes, etc., and have had some improvements, but still run into the same issues in the center of large homogenous areas such as corn fields, so was hoping there might be improvements I could make to my photoscan workflow?

Any advice / suggestions would be greatly appreciated, and let me know if you want more details on any aspect of the image acquisition / photoscan workflow I've used.

Thanks

aggieair

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Re: Difficulty stitching corn field
« Reply #1 on: October 07, 2014, 11:37:44 PM »
I am interested in this as I will surely have a corn field project in the future!

I recently completed a vineyard project that went very smoothly.  It was flow at 400m AGL however.  Maybe the roads on the sides assisted in this case.  We did three flight lines going north to south.  Our GPS on the plane is even coarser than 3m.


David Cockey

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Re: Difficulty stitching corn field
« Reply #2 on: October 08, 2014, 06:00:17 PM »
Image size of 1616 x 1080 is 1.7 megapixels. Probably not enough resolution given the scale of the corn rows in your photos.

Any particular reason downsampling? If you have the images in the original 14.2 megapixels resolution then try using them.

jeremyeastwood

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Re: Difficulty stitching corn field
« Reply #3 on: October 08, 2014, 07:56:24 PM »
Not other than speeding up file transfer and processing time.  I agree that the full size images would be better (also increasing overlap and shutter speed), however I've run other projects with full size images and still run into difficulties stitching the center of large homogenous areas, which is why I was hoping there might be some other options / settings in photoscan which might improve the results.  I've heard that pix4d has special modes ('alternative processing') built specifically for agricultural mapping which often gives improved results, so was hoping there might be something similar (better?) in photoscan I could try (either a specific mode or a different group of parameter settings etc.)?  Any ideas would be really appreciated.

Mick Carney

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Re: Difficulty stitching corn field
« Reply #4 on: October 14, 2014, 07:23:37 AM »
Did you make any improvements with this? I am having same problem with tree tops in canopy uniform canopy. Even with 60% overlap still some areas un aligned. I can upload sample data.