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Author Topic: Aligning Aerial Photos of Forest Canopy  (Read 1675 times)

JamesHolland

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Aligning Aerial Photos of Forest Canopy
« on: October 22, 2019, 11:20:37 PM »
Hi.  I'm trying to align approximately 800 aerial photos with 75+ side and forward overlap for a forested property.  The majority of the photos align but two large areas will not, regardless of the parameters used.  It is not clear from the spacing of the photos or the composition of the forest why the number of tie-points should be any different than elsewhere on the ortho.  The source of the imagery is a DJI Phantom, geotagged photos taken at 75m. 

The processing parameters used for the full dataset are: 
-high accuracy
-reference preselection
-120,000 key point limit
-0 tie point limit
-adaptive camera model fitting

I've tried pulling out the unaligned photos into a separate chunk and adjusting the parameters.  I've also added additional photos from another flight taken one week earlier, albeit with quite different lighting and leaf conditions.   Nothing has been able to align more than a fraction of these remaining images. 

Any suggestions or ideas would be greatly appreciated.   Currently, these areas appear distorted in the final orthophoto because the underlying DEM is filling gaps in the point clouds.   

Thanks,
James

stihl

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Re: Aligning Aerial Photos of Forest Canopy
« Reply #1 on: October 25, 2019, 05:06:58 PM »
Hi James,
I've always gotten much better results with dense vegetated areas when using downsampled images. This is due to there being less pixel variance on lower resolutions on vegetation which causes Photoscan to find more reliable tie-points and thus is able to align the images. This also accounts for the densified point cloud.
If you change the accuracy parameter (from High to Medium or even Low) then Photoscan will first downsample the input images before finding tie-points.
In the case that this does not give suficient results then it's also worth while to manually reduce the size further in an external application and then using these lower res images again (with high, medium and or low parameter tries).

I've gotten very good results from a nearly useless data set in Nigeria by downsampling the input images in Photoshop before bringing them back in to Photoscan and running it on Low quality dense cloud.

JamesHolland

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Re: Aligning Aerial Photos of Forest Canopy
« Reply #2 on: October 28, 2019, 03:10:19 AM »
Thanks stihl for the suggestion.  I've tried different accuracy parameters, downsampled the images, and played with all the other parameters, but I'm not getting better results.  There must not be enough tie-points to align the photos and the only thing I can think of is increasing the overlap next time.