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Author Topic: Gap in Alignment of Orthophoto.  (Read 4814 times)

March Castle

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Gap in Alignment of Orthophoto.
« on: February 08, 2016, 01:07:21 PM »
Hello,

I am having problems eliminating a gap in my Orthophoto - I have repeated the alignment several times using different settings. This example requires the alignment of 681 images using High accuracy, Disabled Pairs with a 40,000 Key point limit and a 10,000 Tie point Limit. Are these settings suitable?

I calculated the flight to include the recommended amount of overlap and the GCP Locations are accurate to within 2cm accuracy.

The images appear in the photos pane but they do not appear in the model view after the alignment process - see screenshot below. The missing images do not appear to have any more blur or distortion than any of the others in the set.

I have attached the Log if that helps.

Has anyone got any suggestions?
Thanks, March.

HMArnold

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Re: Gap in Alignment of Orthophoto.
« Reply #1 on: February 08, 2016, 03:32:29 PM »
Without downloading the entire image set and giving it a spin, here are my initial thoughts.

1) bare ground agricultural missions are the worst to try and stitch together because since everything looks exactly the same. There is a place where almost every picture can be rotated and placed somewhere else with relatively good alignment. I have several missions where images of a field with crops would stitch, but the exact same mission in similar light on the same field a few months later when it was bare would not.

2) I am very surprised that Photoscan did as well as it did on a bare field without any geo-referencing information on the images. In my experience, even a rough lat/lon tremendously increases the likelihood that all pictures will be aligned, and if you add altitude and yaw, you will also get the images at the turns. If you add geo-referencing you can widen your mission legs significantly and still get complete orthos. Wider leg gaps means more acreage per battery.

3) I don't know this to be a fact but I have always assumed Photoscan gives higher alignment weights to images that occur in sequence by the file name, which I assume you're using - if not you might try getting the files named sequentially.

4) you might try manually making an import file that sets the altitude for each image to the altitude you know the mission was flown at. This will at least help Photoscan freeze the scale of each image.

2) it looks like you have included the diagonal images for either the first or last leg of the mission. In my experience these images also introduce the added mathematical variables of the unknown but changing altitude getting up to the first waypoint or descending from the last. The area of your gap seems to intersect that diagonal path. You might try deleting those images from the dataset.

3) As for settings, using the settings for each step as outlined in the Photoscan tutorial have always worked well for me.

4) I have successfully flown a bunch of agricultural imaging missions and never once used ground control points. I can think of no reason why GCPs would make the alignment process more difficult, but you might try not using them, or just in the corners if you're looking to generate a geo-referenced final ortho.

Let me know what I can do to help

HMArnold@msn.com
« Last Edit: February 08, 2016, 03:34:58 PM by HMArnold »

stihl

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Re: Gap in Alignment of Orthophoto.
« Reply #2 on: February 08, 2016, 03:37:08 PM »
To add to HMArnold's great story I'd suggest using "Generic" pair preselection instead of disabeld.


If you have a heavy duty computer I'd also suggest upping the key and tie point limit to 80k and 40k respectively.

James

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Re: Gap in Alignment of Orthophoto.
« Reply #3 on: February 08, 2016, 04:39:20 PM »
How many images were not aligned?

In the log file it suggests 670 out of 681 images were aligned, leaving 11 unaligned.

The gap in your screenshot is much bigger than 11 photos - more like 20-40, so either you don't have enough overlap in this area or several of your photos have been misaligned completely and placed elsewhere.

Or i misread the log file.

HMArnold

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Re: Gap in Alignment of Orthophoto.
« Reply #4 on: February 08, 2016, 04:44:20 PM »
Yes- if you're using "REFERENCE" without lat/lon it would probably confuse Photoscan no end - I missed that

Good catch, Stihl

And upon further review, I take back my suggestion about manually setting the altitude to the known mission height.

If you had lat/lon values, as in from EXIF, that would help, but since you don't, you would have to create an import file that includes lat/lon and altitude.

If there is a way to only set ALT and leave Lat/lon unknown I don't know how to do it.

Which means you would have to set the lat lon values to zero or some estimated value.

Having the lat/lon for every image set at 0, or even an estimated value would probably confuse Photoscan more than the good that you would get from having a set altitude value

From the shape of your turns it look like you're using some sort of fixed wing platform?

No hope of a flight log with Lat/Lon/Alt?

I recently came up with a way to extract the gimbal directions for every image based upon it's time code as compared to the associated values in the flight log file of a Phantom Pro 3. In addition to pitch and yaw, I also can get more accurate lat/lon/Alt values by interpolating between flight log entry lines than seem to come in from the P3P EXIF values.

Once you include pitch and yaw, as well as increase the accuracy of the Lat/Lon/Alt values, you don't have to delete the cross-grid first and last leg images anymore, and everything stitches every time.



James

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Re: Gap in Alignment of Orthophoto.
« Reply #5 on: February 08, 2016, 04:47:25 PM »
I can now see that its 45 images not aligned, but photoscan has aligned that 45 within their own group but was not able to align them with the other 636.

I suggest duplicating the chunk, resetting alignment for the aligned 636 in the duplicate, and selecting the 45 unaligned and aligning just those (using align selected cameras).

Then try aligning a few more adjacent cameras, and if that is successful you can merge the chunks together using 'align by cameras'.

HMArnold

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Re: Gap in Alignment of Orthophoto.
« Reply #6 on: February 08, 2016, 05:05:13 PM »
Am Reading the log file correctly that the alignment process took 49 hours to complete?

"Finished processing in 177696 sec (exit code 1)"

2 days?



stihl

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Re: Gap in Alignment of Orthophoto.
« Reply #7 on: February 08, 2016, 05:26:53 PM »
Two days for less than 700 images and only 40k key point limit is extraordinary long..

How much RAM do you have?

Alexey Pasumansky

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Re: Gap in Alignment of Orthophoto.
« Reply #8 on: February 08, 2016, 05:45:00 PM »
Hello March,

In addition to the recommendations given above, I can suggest to try aligning the cameras in the version 1.2, you don't to re-start the whole process, but just Reset camera alignment for all cameras and them run Align Selected for all the cameras at once.

And yes, for considerably big datasets it's better to use preselection (at least Generic, if the camera coordinates are not available) as it will reduce the processing time to a couple of hours for the dataset of the similar size.
Best regards,
Alexey Pasumansky,
Agisoft LLC