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Author Topic: DEM elevation values -130 m below GCP elevations.  (Read 3966 times)

GBG1138

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DEM elevation values -130 m below GCP elevations.
« on: March 22, 2019, 08:33:19 PM »
Using a DJI P4P I captured 975 images.  Using a Trimble 7x with a centimeter kit I collected 16 GCPs across the flight area.  After processing the images following the tutorial for Photoscan with GCPs  I saw that the resulting DEM values were about -130 m below the GCP elevations.

Thinking that I must have incorrectly set some parameter I checked the coordinate reference in the reference pane and all z values of the input data.  I ensured that the cameras were all unchecked so that I was only using the GCP elevations values to generate the surface.  This second reconstruction returned the same surface with z values, -130 m below the GCP values. 

I decided the best course of action would be to upgrade to Metashape - which I did- and re-ran the reconstruction from scratch.  The resulting DEM returned the same erroneous surface. 

There are no images or GCPs with Z values at -130.
I have completed dozens of reconstructions with Agisoft using these same tools and never had this issue.
All CRS use WGS84 with z values in meters and x and y in long/lat.
Nothing looks odd with the cameras or bounding box in the model view.

Why are my resulting surfaces giving the wrong Z values?

Gerry


Alexey Pasumansky

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #1 on: March 23, 2019, 03:21:40 PM »
Hello Gerry,

Can you share the report file from the project and the screenshot of the source values tab for Cameras/Markers that include the altitude value?
Best regards,
Alexey Pasumansky,
Agisoft LLC

Dave Martin

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #2 on: March 24, 2019, 02:17:22 PM »
Gerry,
Almost certainly it is the long-running issue with DJI kit recording elevations as those above takeoff point, not above the ellipsoid or any other datum. If you search here on the forum  for, say, DJI height you can find other threads on this and solutions to correct the DJI data.
Dave

GBG1138

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #3 on: March 26, 2019, 11:36:49 PM »
Dave,
Thanks for the reply.  I definitely unchecked the images to ensure that the image elevations were not considered in the reconstruction.  How would a flaw with DJI elevation values affect the final surface model?

Jeremiah Harrington

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #4 on: March 27, 2019, 12:12:57 AM »
It sounds like your DEM hasn't been regenerated since adding the survey data. You need to regenerate the DEM/Ortho after integrating the survey data.

GBG1138

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #5 on: March 27, 2019, 02:19:12 AM »
Jerimeah, I generated and exported the DEM after adding the GCP values so I do not think that is the issue.  I am re-running the reconstruction just to make sure I did not make an ID10T error and will post the report when all is finished.

Alexey Pasumansky

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #6 on: March 27, 2019, 02:05:01 PM »
Hello Gerry,

If the issue persists, please send the report and project re-saved in PSZ format (with alignment results only) to support@agisoft.com for more detailed investigation.
Best regards,
Alexey Pasumansky,
Agisoft LLC

GBG1138

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #7 on: March 28, 2019, 07:35:37 PM »
Alexey,
I have added a copy of the report to my original post.  I have exported the psz file but this results in a 2.4GB file.  How do I only include the alignments in the PSZ?  I do not see this as an option.

Alexey Pasumansky

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #8 on: March 28, 2019, 07:57:34 PM »
Hello GBG1138,

To leave only alignment results in the project remove the following elements from the chunk contents in the Workspace pane:
- shapes,
- depth maps,
- dense cloud,
- 3D model,
- tiled model,
- DEM,
- Orthomosaic.
Also you can right-click on the Tie Points line and use "Remove KEY points" option, if it is active.

Then select Save As option in the File menu and choose "Metashape Archive" file type for saving the project under the different name.

P.S. I don't see any attachments in this thread.
Best regards,
Alexey Pasumansky,
Agisoft LLC

GBG1138

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #9 on: March 29, 2019, 12:30:03 AM »
OK, I emailed you the report file at support@agisoft.com.  I uploaded the PSZ to Dropbox.  The link is in the email.
Thanks for looking.

Alexey Pasumansky

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Re: DEM elevation values -130 m below GCP elevations.
« Reply #10 on: March 29, 2019, 01:41:09 PM »
Hello GBG1138,

Thank you for sending the project file and report.

You need to press Update button on the Reference pane after unchecking all cameras and loading the coordinate information for the placed markers. It will apply the reference information of checked on instances in the pane to the complete model.

To achieve better accuracy I recommend to use Optimize option prior to the dense cloud generation. Recommended set for the calibration parameters optimization is everything, except for P3, P4 and K4.
Note that optimization discards dense cloud, mesh and tiled model products, so they should be re-built. DEM and orthomosaic require to be re-generated as well, since the newly applied referencing wouldn't have any effect on them.
Best regards,
Alexey Pasumansky,
Agisoft LLC