polygon metrics does not work with LAScatalog

[GregorDall]

Hi,

I am just adapting my workflow from using single LAS files to the LAScatalog processing workflow. Is it possible to use a LAScatalog with polygon_metrics or do I have to use catalog_apply?

temp <- polygon_metrics(las = mylas, func = ~list(
  minZ = min(Z),
  maxZ = max(Z),
  nZ = length(Z)),
  geometry = mygeometry)

#> Error in UseMethod("polygon_metrics", las):
#>   non-applicable method for 'polygon_metrics' applied to an object of class "LAScatalog"

Best Gregor

[Jean-Romain]

This is correct there is no polygon_metrics for LAScatalog. The reason is that it is impossible to handle such algorithm in the general case with the LAScatalog processing engine.

Assume you have a large and long polygon polygon that encompasses 3 tiles. This is impossible to handle with an processing engine that processes sequentially square regions.

If you are sure your polygons are relatively small and can fit in a chunk + its buffer, then, below are the steps of a custom function in catalog_apply:

1. Get the bbox of the chunk
2. Keep only polygons with the centroid inside the bbox
3. Compute polygon_metrics with these polygons

Process with a relatively large buffer to ensure that the polygons with a centroid inside the chunk are not beyond the chunk + buffer.

[GregorDall]

Thank you for the suggestion, I think that's the way I will move forward.