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Author Topic: Merge Produces Errors in DEM  (Read 3174 times)

mwillis

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Merge Produces Errors in DEM
« on: December 01, 2016, 04:17:35 AM »
When merging adjacent chunks errors are being introduced into the DEM.  See below


When many chunks are merged the problem is even worse


I have noticed this problem in several UAV datasets.

mwillis

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Re: Merge Produces Errors in DEM
« Reply #1 on: December 01, 2016, 03:47:51 PM »
Here is what the data looks like when processed as one giant chunk.  The diagonal ridges you see in this data are real.  Those are furrows from plowing.  The problem with merging chunks is also present in version 1.3.0.




Alexey Pasumansky

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Re: Merge Produces Errors in DEM
« Reply #2 on: December 05, 2016, 01:05:15 PM »
Hello Mark,

During the chunk merging process the dense cloud points are being placed on the same grid, but due to the fact that each smaller dense cloud has different resolution, some approximation is required. And this approximation leads to the stripes on the DEM that can add the error up to (res / 4), where res is the DEM resolution that depends on the dense cloud quality in it's turn.

As a workaround I can suggest two solutions:
1) build mesh in the individual chunks, merge meshes and then build DEM based on mesh in the merged chunk.
2) build DEM in the individual chunks, export them and merged in the external applications. Then, if required, import back to the merged chunk for the orthophoto generation process.
Another method is modifying the internal resolution value for the dense clouds in the original chunks using Python before merging, but not sure if it may lead to some other unexpected issues.

Can you check my the profile measurement tool check the elevation difference of those ridges?
« Last Edit: December 05, 2016, 01:09:30 PM by Alexey Pasumansky »
Best regards,
Alexey Pasumansky,
Agisoft LLC