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Author Topic: Importing DEM to improve geomtry accuracy  (Read 2581 times)

3DWinter

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Importing DEM to improve geomtry accuracy
« on: February 13, 2018, 01:25:09 AM »
Not sure how long has Photoscan has the ability to import a DEM to improve geometry accuracy, but while reviewing the 1.4 manual, I discovered it this  option and I am very excited to explore its utility.

Yet, apart from mentioning the capability in a few words on page IV, I did not find further information in chapter 4,  Referencing and Camera Calibration.  Nevertheless, I did try to import an old lidar DEM .tif file, and aligned the pictures, but off the bat it does not seem the reference DEM was accounted for when inspecting the sparse point cloud. What am I missing?
It would be nice to know the limitations, of importing a reference DEM to the final model. For example, if a reference DEM has much coarser resolution, and not necessarily includes all the features capture from SfM.
Thanks

Alexey Pasumansky

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Re: Importing DEM to improve geomtry accuracy
« Reply #1 on: February 13, 2018, 03:52:58 PM »
Hello 3DWinter,

Imported DEM doesn't have any effect on the alignment operation and wouldn't improve the accuracy of the alignment. It is just a surface that can be used to project the images from the estimated camera positions on to. So it is important to have the alignment and DEM accurately referenced in the same coordinate system, otherwise the images would be projected to the incorrect area.
Best regards,
Alexey Pasumansky,
Agisoft LLC

3DWinter

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Re: Importing DEM to improve geomtry accuracy
« Reply #2 on: February 13, 2018, 09:58:17 PM »
Interesting, you said the DEM can be used to project the images from the estimated camera positions. When does that step happen? Would there not be a conflict between the dense point cloud and the DEM, how is that resolved?
I am still missing the steps to incorporate the DEM. Should I import the DEM before I align the pictures, or at a later stage? Can you please describe the steps?
thanks for your response.

stihl

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Re: Importing DEM to improve geomtry accuracy
« Reply #3 on: February 14, 2018, 03:15:14 PM »
The DEM overwrites the DEM that's already generated in the report or is simply being used as the DEM to project the images onto. That step happens at  Build Orthomosaic, where you can choose which Surface to use (Mesh or DEM).

The order of steps is;
1. Align images in proper coordinate system
2. Geo-reference the project either by geotagged images or GCPs
3. Optimize the tie-points with the info from step 2
4. Import external DEM in the same coordinate system
5. Build Orthomosaic with Surface set to DEM