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Messages - MatteaE

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1
General / Best way to align two UAV surveys with different GNSS base
« on: September 11, 2024, 07:22:56 PM »
Hi all,

I have data from two surveys of a slow-moving landslide, taken at 1 year interval and featuring a lot of stable terrain and some moving areas: photos were taken from a same commercial, standard-GPS drone (50-100 images each), but ground control points were measured with two different methods - the first survey was done with RTK over mobile connection, so that absolute positioning of the GCPs is decent (3-5 cm), while the second survey was done with a local base, so that absolute positioning of the GCPs is off by a few meters. The first survey has 10 GCPs, the second only 5 (they are mobile GCPs, not the same between the two surveys).
What would be the best practice to process these data, with the goal of measuring surface height change (and ideally obtaining data with good absolute positioning)? I am considering these options:

1- process the two surveys in two different chunks, then align these, finally produce aligned DSMs

2- process the two surveys independently, export (misaligned) DSMs, co-register the second DSM to the first with an external tool

3- "co-alignment" approach: process the two surveys in a same chunk (using two camera calibration groups), marking only the (accurate) GCPs of the first survey and relying on sparse cloud matches over stable terrain to align the two datasets

Any input and other ideas are greatly appreciated!

Thanks a lot,
Enrico

2
General / Re: DEM from multiple flights - seam artifacts?
« 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!


3
General / 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

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