LidR catalog apply function not return dataframe

Question

Here is my code :

LASfile <- system.file("extdata", "Megaplot.laz", package="lidR")
ctg = readLAScatalog(LASfile)

myMetrics2 <- function(rnu, nor) {
  tab <- table(rnu, nor)
  return(tab)
}

g <- function(chunk) {
  las <- readLAS(chunk)
  if (is.empty(las)) {
    return(NULL)
  }
  metrics <- cloud_metrics(las, myMetrics2(ReturnNumber, NumberOfReturns))
  return(metrics)
}

opt_progress(ctg) <- FALSE
opt_chunk_buffer(ctg) <- 0
opt_chunk_size(ctg) <- 0
opt_output_files(ctg) <- ""

p_metrics <- catalog_apply(ctg, g)

It generate this error :

Error: Duplicated elements found. At least one of the metrics was not a number. Each metric should be a single number.

I don't understand why ?

Answer 1

Your metrics function return a vectors i.e. several values for a single metric while you are supposed to return a single value per metric.

When you have a single vector to return for the entire point cloud this may be confusing but consider the following case:

myMetrics2 <- function(rnu, nor) {
  tab <- table(rnu, nor)
  avg <- mean(rnu
  return(list(tab, avg))
}

pixel_metrics(las, ~myMetrics2(ReturnNumber, NumberOfReturns))

It is more obvious that this won't work.

As mentioned in the documentation you must return a named list

myMetrics2 <- function(rnu, nor) {
  tab <- table(rnu, nor)
  return(as.list(tab))
}