Filtering data with more than 2 criteria in lidR

Question

I am just wondering whether lasfilter() is able to filter the las data with more than two criteria such as z value between 5 and 10 m and Number of point records greater than 1. I have tried the following code but it didn't work. I don't know how to filter the datasets with multiple criteria. There are 1173 files in my folder but when I used the following code it could only write 67 files because after 67th file there is another file which doesn't have point fitting with this criteria. So I want to write all those files which do meet the above criteria.

plotlist16 <- list.files(path = "./data/LidarData/2016_random_plot/AllTogether/normalized", pattern = ".las", full.names = T, recursive = FALSE)

for (i in 1:length(plotlist16))
{
   Rplot <- readLAS(plotlist16[i])
   ht_1.3_5m <- lasfilter(Rplot, Z>=1.3 & Z<5) # with this could only write 67 files 
   ht_1.3_5m <- lasfilter(Rplot, Z>=1.3, Z<5, Rplot@header@PHB$Number of point records>=1) # so want to filter with third criteria which has at least one point in this height range

   file_name <- gsub('.*/','',plotlist16[i])
   file_name <- gsub('.las','',file_name)
   file_name <- print(file_name)

   outname <- paste0('./data/LidarData/2016_random_plot/AllTogether/stratawise/strata_1.3_5m/', file_name, "_ht_1.3_5m.las")
   writeLAS(ht_1.3_5m, outname)
}

Answer 1

Many options here:

Use LAStools

lidR is good for some tasks but weaker for some other. This is a typical case where las2las from LAStools is more suitable for what you want to achieve.

Use streaming filter

If you want to work within R anyway, do not load the full point cloud in R. Read only the points of interest. Your filter is very simple here.

Rplot <- readLAS("file.las", filter = "-drop_z_below 1.5 -drop_z_above 5")

This way you don't need a full copy of your files. You only read what matters for you

Use LAScatalog capabilities

If you want a copy of the files anyway, use the LAScatalog capabilities. Don't use custom for loop

plotlist16 = readLAScatalog("./data/LidarData/2016_random_plot/AllTogether/normalized")
opt_filter(plotlist16) <- "-drop_z_below 1.5 -drop_z_above 5"
opt_chunk_buffer(plotlist16) <- 0
opt_chunk_size(plotlist16) <- 0
opt_output_files(plotlist16) <- "/new/folder/{ORIGINALFILENAME}_ht_1.3_5m"
plotlist16_ht <- catalog_retile(plotlist16)

Then you can easily select the files with the number of points you want.

plotlist16_ht <- plotlist16_ht[plotlist16_ht$Number.of.point.records > 1, ]

Notice that files with 0 points will be deleted automatically so you won't have files with 0 points