reimported DEM - introducing error in orthomosaic

[Robert Dzur]

Hello,

I'm trying to understand what I may be doing wrong or why I'm getting different results in terms of positional accuracy of an orthophoto mosaic when:

A) an orthomosaic is generated in a project (A) from a DEM developed from dense point cloud, compared to

B) the same orthomosaic is generated from DEM exported from project A) and reimported into a copy (project B) of the original project A)

The orthomosaic generated in project B) is using the DEM exported from project A, and appears to introduce about 11 meters offset in the resulting orthomosaic which happens to be the approximate resolution of the DEM exported from project A.

Has anyone encountered a similar issue and what recommendations might be available to replicate the original project A's orthomosaic positional accuracy in project B with the DEM reimported into project B?  I would expect that the orthomosaic from project B using the DEM from project A should be identical.

Thank you.

Robert
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[Alexey Pasumansky]

Hello Robert,

Can it be reproduced just by re-importing DEM to the same project?

Please also specify, the coordinate system used in the chunk and for the DEM export.

[Robert Dzur]

Hi Alexey,

Thank you for the response and clues to the issue.

I believe you helped get me on the right track for the root cause of this issue which is in fact coordinate system related.

The project coordinates are UTM.  The DEM is exported using the project UTM system which is a compound coordinate reference system (CRS) including vertical.  Although exporting the DEM using the defined project UTM system, the exported DEM is written with the standard UTM (2D) CRS.  Upon re-importing the DEM, the CRS is interpreted then as the default (2D) CRS.  Initially, I did not recognize that the DEM was re-imported using the default 2D UTM system instead of the compound CRS of the project.  When I changed the CRS on re-import to be the compound (horizontal+vertical) UTM system, thus matching the project system, the resulting orthomosaic now also matches the results before exporting and re-importing the DEM. 

Thanks again for your kind assistance with my issue of re-importing the exported DEM back into a copy of the project.

Best regards,

Robert
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