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Author Topic: Best Practice for DEM "Resampling"  (Read 908 times)

SebastianFischer

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Best Practice for DEM "Resampling"
« on: August 12, 2020, 12:12:45 PM »
Hey there,

I am using Agisoft Metashape to create Digital Elevation Models. My data usually has a GSD in the low centimeter range but I want to export DEMs with an "even" cellsize (3cm instead of 2,1642cm for example). What is the best practice for the DEM "Resampling"?

1) Specify an "even" cellsize when exporting the DEM? (What resampling method is used?)
2) Specify an "even" cellsize when building the DEM? (What method is used for assigning a height value from the dense point cloud?)

Thanks in advance
Sebastian

CHiller

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Re: Best Practice for DEM "Resampling"
« Reply #1 on: August 27, 2020, 12:26:32 PM »
Hi all!

I'd like to extend Sebastians question by a further key point:

"What is the best practice for the DEM "Resampling"?"

3) Specify an "even" cellsize when transforming the DEM? (What method is used for aggregating the cell values?)

Best regards
Clemens

stihl

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Re: Best Practice for DEM "Resampling"
« Reply #2 on: August 27, 2020, 01:01:24 PM »
Hi to both.

The DEM resolution is inherently and directly tied to the Dense Cloud point density. For example - if your project was flown on 1 cm GSD and you process your dense cloud as Ultra High quality, this means that the resulting dense cloud will have an average point density (sampling resolution) of 1 cm (i.e. the points are on average 1 cm spaced from each other).

If you process your project on High quality (instead of Ultra High) then this point density goes down by a factor of 2. If you process it on Medium quality, the density goes down by a factor 4. Low is a factor 8 and Lowest is a factor 16.

The Build DEM step generates the highest possible sampled DEM based on the point density of the Dense point cloud. Meaning that if your project has a 1 cm GSD - if you process your project on Ultra High, the resulting DEM will have a default sampling resolution of 1 cm. High produces a DEM resolution of 2 cm, medium produces it on 4 cm, etc...

The next step is exporting the DEM from Metashape in any arbitrary resolution that you want. Such as 1cm, 2cm, 3cm, 4cm .. as long as it's a lower resolution than the DEM produced by Metshape. So if you have a 1 cm GSD project and wish to generate a 3 cm DEM - simply process a High quality dense cloud, generate a 2 cm DEM (default values) and export it as a 3 cm DEM geoTIFF.

SebastianFischer

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Re: Best Practice for DEM "Resampling"
« Reply #3 on: September 07, 2020, 10:35:54 AM »
Thanks for your input, but my question was rather aimed at the resampling method used to assign height values to the grid cells. I know how to specify the desired DEM resolution, what I am curious about is how the height value for a grid cell is computed when more than one point are within a grid cell and if this is different wether you do the downsampling during the Build DEM or the Export DEM step.

And since GSD is a measure for average point spacing there is also a possibility for empty grid cells at highest resolution, so there has to be an interpolation rule for empty cells as well!