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Author Topic: DEM negativel altitudes  (Read 1878 times)

Vania Assis

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DEM negativel altitudes
« on: January 29, 2021, 06:43:31 PM »
Dear,
I took a flight with 398 photos with the phnatom 4pro, at an altitude of 150 meters, but in the DEM there were negative altitudes.
Do you know what may have happened?
Attached flght log and DEM image.

Alexey Pasumansky

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Re: DEM negativel altitudes
« Reply #1 on: January 29, 2021, 07:08:23 PM »
Hello Vania,

Metashape expects that the altitude values in the Reference pane are estimated above WGS84 ellipsoid, when WGS84 (EPSG::4326) system is selected. Probably the altitude written to EXIF by DJI in your case is estimated as height above geoid (then WGS84+EGM96 system should be used) or above the take-off point.

See the related article, which describes, how you can correct the altitude values loaded to the Reference pane:
https://agisoft.freshdesk.com/support/solutions/articles/31000152491-working-with-dji-photos-altitude-problem-
Best regards,
Alexey Pasumansky,
Agisoft LLC

Vania Assis

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Re: DEM negativel altitudes
« Reply #2 on: February 07, 2021, 09:28:51 PM »
Dear Alexey Pasumansky,
thank you for the answer.
Another question: In the coastal areas it´s common the negative altitudes in the sea?
Look, please my DEM.
Thank you.
Sincerely,
 Vania Assis

Alexey Pasumansky

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Re: DEM negativel altitudes
« Reply #3 on: February 08, 2021, 09:09:41 PM »
Hello Vania,

The screenshot is of a low resolution, so it can be hardly seen anything, unfortunately.

Does the area on the right side (with blue-colored parts) represent the parts of the actual project, or whether this part is a result of interpolation? Maybe you can share the project with the alignment results and dense cloud with support@agisoft.com? To reduce the project copy size you can remove depth maps from the project contents.
Also please share the PDF report exported from the fully processed project.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Paulo

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Re: DEM negativel altitudes
« Reply #4 on: February 09, 2021, 01:50:01 AM »
Hi Vania,

your survey is over a coastal area with large amounts of water in many photos. This causes image matching and depth maps generation to be false over such water bodies. To get a better DEM, you should remove all dense cloud points over water or floating structures as docks or boats and redo the DEM from such edited dense cloud and result should be much better (assuming your alignment is correct).

As you can see in attachment screen copy of a coastal golf course,  dense cloud has many false points on the sea side and on lagoons
« Last Edit: February 09, 2021, 01:51:37 AM by Paulo »
Best Regards,

Paul Pelletier,
Surveyor

Vania Assis

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Re: DEM negativel altitudes
« Reply #5 on: February 09, 2021, 06:03:54 PM »
Hello.
Thank you for the answer.
I understood your explicaton.
But, how could I delete some points of dense cloud?
I send you by email, support@agisoft.com, my  report, my dense cloud, the DEM and the DEM image, in JPEG.
About my DEM I need only the ground, nothing in the sea.
Thank you, once more.
Vania Assis

Alexey Pasumansky

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Re: DEM negativel altitudes
« Reply #6 on: February 09, 2021, 06:59:21 PM »
Hello Vania,

You can manually remove isolated clusters of the dense point cloud related to the water bodies before building the DEM based on the dense cloud. Or assign "water" class to those points and exclude the class from the DEM generation process.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Paulo

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Re: DEM negativel altitudes
« Reply #7 on: February 09, 2021, 08:16:16 PM »
Hi Vania,

as Alexey pointed out, one way to select a lot of these erroneous floating water points is to filter dense cloud by confidence (assuming you generate cloud with Calculate point confidence option ticked). Use threshold 0 to 1 and many of these will be filtered , select them all and delete them or assign Water class to them.

Then with free form selection  tool select remaining erroneous water points and delete... From this edited dense cloud, you should get a much better DEM.

« Last Edit: February 09, 2021, 09:20:45 PM by Paulo »
Best Regards,

Paul Pelletier,
Surveyor