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Author Topic: DEM from multiple flights - seam artifacts?  (Read 1527 times)

MatteaE

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DEM from multiple flights - seam artifacts?
« on: September 21, 2022, 04:09:52 PM »
Hello everybody,
I am struggling to generate a good DEM in mountain terrain from ~500 drone images taken over 4 flights (with a DJI Mavic 2 Pro flying at ~180 m AGL).
The photos appear to all align very well (sparse cloud after optimization: 399k / 2.43M points, RMS 0.212 px), but when I generate the DEM there is a big artifact at the seam line between flights (see attachments: a 2-4 m tall vertical misalignment).
Things that I have tried with no notable improvement:
- Unlimited tie points
- Adaptive camera model fitting
- Full rolling shutter compensation
I do not have GCPs.
Any ideas?
Thanks a lot in advance!
Enrico

Paulo

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Re: DEM from multiple flights - seam artifacts?
« Reply #1 on: September 21, 2022, 05:14:10 PM »
Hola MatteaE,

could you check if there are any common tie points between flights? If there are no or few tie points between flights then each alignement will be independent and the Z offset could be expected...

If there are no common points between flights, the solution I would see is aligning each flight in one chunk, and then place at least 3 common manual tpts in overlap area and then align chuns usings points or markers....

PS. If you can share the data set, we could have a look at it...
« Last Edit: September 21, 2022, 07:47:09 PM by Paulo »
Best Regards,
Paul Pelletier,
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aggieair

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Re: DEM from multiple flights - seam artifacts?
« Reply #2 on: September 22, 2022, 12:07:27 AM »
When we do projects where there are several flights to merge together, there has to be a ground control point in that overlapping area that is shared between FL1 and FL2. Otherwise we'll get this difference in elevations, just due to the error of the GPS on board the UAV.

Maybe you can make up a ground control point, using an available DEM and a high resolution basemap (other satellite imagery). It needs to be tied down.

Paulo

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Re: DEM from multiple flights - seam artifacts?
« Reply #3 on: September 22, 2022, 01:36:41 AM »
Actually,

using just 3 tie points (manual markers) placed in the overlap area between consecutive flights will do the trick for relative accurate result. For absolute accuracy, of course you need GCPs...
Best Regards,
Paul Pelletier,
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MatteaE

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Re: DEM from multiple flights - seam artifacts?
« Reply #4 on: September 27, 2022, 02:53:04 PM »
Hello all,
thanks a lot for your suggestions.

I have checked and indeed there are almost no tie points on the overlap area - not because of low overlap, but because all such tie points (except maybe 10 or so) were filtered out during the point cloud optimization. So that overlapping photos from two different flights have usually ~2000 matches with each other, but all marked as invalid.

I guess it makes sense that a tie point between two photos at different altitudes (and with some time delay) could have worse quality than between two consecutive photos in a same flight, so these get all filtered out.

With a less aggressive optimization, many more tie points are left in the overlap areas (I will have the DEM ready tomorrow to check the result).

So now I am wondering how this compares to the processing Paulo suggested: for an accurate relative alignment, is it better to place 3 markers in the overlap area, or let Metashape use its own tie points? I am not sure that I can manually place the markers with an RMS < 0.3 px (as is the accuracy of the sparse cloud after the more gentle optimization).

To share the data set, I have to ask permission, I'll let you know.
Thanks a lot!


Paulo

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Re: DEM from multiple flights - seam artifacts?
« Reply #5 on: September 27, 2022, 06:42:01 PM »
Ciao Enrico,

if you have a final RMS of 0.3 pix with still quite a lot of matching points between different level flights then Metashape will do great job creating model/DEM without artefacts...

In case where  you do not have any or very few matching points (because of quite different GSD), then placing manual tie points or markers between overlapping flights (usually with a RMS of around 0.5 pix) would be the soluzione,

Arrivederci

Per Enrico I suppose you are italian....
Best Regards,
Paul Pelletier,
Surveyor