2

I am working in Metashape Version 2.0.З. I have very basic skills.

I have two terrain survey results taken by an unknown UAV (the same UAV was used) in different years without reference points on the ground. The task is to measure the elevation changes that occurred during this period using a digital elevation model (DEM).

When creating the DEM and orthomosaic, the differences in elevation and horizontal shift between the two datasets are very noticeable.

I tried to apply the "Align chunks" tool, but the result is that the DEM can only be built for one of the images.

Is it possible in principle to eliminate this shift without reference points? (Absolute accuracy is not important).

enter image description here

1
  • 1
    Because these Questions are supposed to stand the test of time, using "latest version" isn't usefully descriptive. Please Edit the Question to contain the current version number.
    – Vince
    Nov 26, 2023 at 18:51

1 Answer 1

2

This approach is unlikely to provide you with useful information. The surfaces and orthomosaics built by Metashape would have both exploited the positional data of the drone at the time of image capture. Differences in GNSS signals at the time of image capture would introduce enough uncertainty in the two different outputs such that they do not align as you have stated. The lack of ground control points for the reconstructions ensures the resulting products only use the drone GNSS and IMU data. As you likely know, the x, y, and (especially) z values from consumer grade GNSS units have an uncertainty expressed in meters from the actual location ensuring that your surface will not align to the same vertical and horizontal datums - thus your calculations will never be 'accurate'.

Still, if you want to proceed I would suggest that you:

  1. Georeference one of the orthomosaics to the other.
  2. Save the resulting newly georeferenced orthomosaic as a TIFF with a world file (not a GeoTiff).
  3. Save the matching surface to a TIFF with a world file (not GeoTiffs).
  4. Delete the world file for that surface.
  5. Make a copy of the orthomosaic world file in the same directory as the surface Tiff but name that world files the same name as the surfaces.

This will apply the same shift to the surface as the newly georeferenced ortho and force the data to align.

Do your raster math with the understanding that the Z values will not match the same vertical datum because of GNSS error at the time of image capture.

EDIT: In addition to my answer above I wonder if your area of interest has existing imagery, LiDAR, and possibly roads? If so, create some faux GCPs using unaltered portions of roads, some road paintings like crosswalks or parking lines. Create GCPs based on these locations. Reprocess both flight data with these faux GCPs to align. Do your raster math.

4
  • Do I understand correctly that if I use the "Add markers" button on all photos with fixed objects (e.g. house corners), then re-processing all the data from the beginning will result in a smaller error in both plane and height than without such points? youtu.be/Q8edLfwNzxU?feature=shared&t=260 Nov 28, 2023 at 6:40
  • After adding markers, do I need to do any additional steps to link the chunks together? Each chunk is a separate orthophoto mosaic. Nov 28, 2023 at 6:43
  • 1
    adding markers is only going to align the reconstructions based on the images, the markers, and the GNSS data from that capture. Those markers are not GCPs so you will gain little through this approach.
    – GBG
    Dec 4, 2023 at 23:45
  • Here's how to create an orthomosaic using GCP and Metashape: youtube.com/watch?v=G09r5PXqhBc Dec 5, 2023 at 4:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.