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Author Topic: Missing elevation points in DEM  (Read 975 times)

Vaishhhh

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Missing elevation points in DEM
« on: August 25, 2020, 01:01:42 AM »
Hello!

I generated DEM with dense cloud for multispectral images collected with the RedEdge 3 sensor mounted on a Matrice 100 UAV. The attached images show that the DEM does not have the elevation for few points. It appears as though those places are empty, but actually there are plants at those locations. When I checked the dense cloud model, it did not have the correct elevation values for those points as well.


I have processed several similar datasets, but didn't have this issue with any of them. I am not sure, what could be causing this issue. Can some one suggest how I retrieve the height points in the model?

Thank you,

Vaishali

stihl

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Re: Missing elevation points in DEM
« Reply #1 on: August 27, 2020, 01:04:04 PM »
It appears that the dense cloud algorithm was unable to find any features on those specific plants. Perhaps due to wind that the branches and leaves moved too much in overlapping consecutive images. Does this issue appear in other regions of your model as well?

Vaishhhh

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Re: Missing elevation points in DEM
« Reply #2 on: August 27, 2020, 09:22:36 PM »
Hello stihl, thanks for responding. I had this issue in other places too. You are probably correct, there was some wind on these days. But is there anything that i could do to improve the tiepoints in these locations?

Vaishhhh

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Re: Missing elevation points in DEM
« Reply #3 on: August 27, 2020, 09:24:50 PM »
Also, I noticed that most of the locations with missing elevation points belong to smaller plants. I have a mixed trial where some plants are shorter than the others. I would assume the shorter plants sway lesser due to the wind. Could there be other issues affecting the dense cloud generations?

stihl

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Re: Missing elevation points in DEM
« Reply #4 on: August 28, 2020, 10:44:54 AM »
This specific issue is separate from the tie-points or the alignment. The features (i.e. the branches and leaves) of these specific plants are too difficult for the dense cloud algorithm to match.
What you can try is reposition the bounding box so that it envelops the region where the vegetation is problematic and re-run that specific region with a lower quality dense cloud. Reducing the quality level effectively means that Metashape down-samples the original images before running the dense cloud algorithm. Because the resolution is lowered, small features such as leaves and branches might come out better in the dense cloud because small (pixel level) deviation between the leaves and branches in consecutive images becomes smaller and thus increases the chances of successfully reconstructing the plant, albeit in a lower level of detail, but a lower level of detail is always better than no detail. :-)

Vaishhhh

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Re: Missing elevation points in DEM
« Reply #5 on: August 31, 2020, 08:28:19 PM »
Thank you! I tried building the dense cloud with lower accuracy settings, but that resulted in a much flatter surface. I also tried reducing the accuracy of the alignment process and building point clouds with medium and low settings. I was getting flat surfaces from those too. It seems like the best reconstructions were obtained only when the accuracy settings were set to the highest. I am not very sure what other methods to test.