writeRaster for LAScatalog

[taeyoonlee87]

I got an error message “Error in (function (classes, fdef, mtable) : unable to find an inherited method for function ‘writeRaster’ for signature ‘"LAScatalog", "character" “..

It seems like I cannot use writeRaster for LAScatalog. but this error makes me confused.

Here is my code.

setwd("E:/Lidar_2022/R/Data_R")
las_cat<-readLAScatalog("E:/Forest_LiDAR/226776/tiles")
opt_output_files(las_cat) <- "E:/Lidar_2022/R/Data_R2/dtm_{XLEFT}_{YBOTTOM}"
dtm <- grid_terrain(las_cat, res = 2, algorithm = knnidw(k=10, p=2, rmax=50), keep_lowest = FALSE)
writeRaster(dtm, "E:/Lidar_2022/R/Data_R/dtm.tif", overwrite=TRUE)                                                              **<- no problem in here**

opt_output_files(las_cat) <-"E:/Lidar_2022/R/Data_R2/norm_{XLEFT}_{YBOTTOM}"
MyNorm <- normalize_height(las_cat, dtm) 
writeRaster(MyNorm, "E:/Lidar_2022/R/Data_R/chm.tif", overwrite=TRUE)                                                          **<- error** 
#> Error in (function (classes, fdef, mtable) : unable to find an inherited method for function ‘writeRaster’ for signature ‘"LAScatalog", "character" 

The outcome from grid_terrain was able to be written as raster (.tif file) but the outcome from normalize_height was not able to be used by writeRaster

what is the difference between the two outcomes? I believe both are LAScatalog.

And how can I bring back the "dtm" (LAS catalog) from raster (dtm.tif) without re-running grid_terrain?

When I used a function of raster, it seems like RasterLayer instead of las file or LAScatalog

Thanks in advance!

[Jean-Romain]

• grid_terrain (deprecated, use rasterize_terrain) produces a DTM i.e. a raster. A raster can indeed be written in a raster format
• normalize_height produces a point cloud. A point cloud is not a raster and cannot be written in a raster format
• considering you used opt_output_file to produced the DTM you do not need to use writeRaster. The raster is already saved on disk.
• A LAScatalog is an object handling a collection of las file on disk.
• rasterize_canopy produces a CHM.

[taeyoonlee87]

Thank you for replying to me.
It makes me clear!

By the way, I am still wondering how I can get LAS file from raster (*.tif) without running "grid_terrain"?

[Jean-Romain]

I don't understand your question. A tif file is a raster. A las file is a point cloud. You can't have a point cloud from a raster.
Well actually you can consider each pixel like point and have a regularly spaced point cloud but lidR does not offer such tool out of the box.
So you can't "get a LAS file from a raster"

[taeyoonlee87]

Ah,,, so the outcome(dtm) from “grid-terrain” is a LAScatalog.
Then, is there anyway to save LAS catalog as a single las file so that I can save it and retrieve whenever I need.
I hope my question is makes sense as I am a newbie in analyzing LiDAR.

[Jean-Romain]

Ah, so the outcome(dtm) from “grid-terrain” is a LAScatalog.

No it is a raster. grid_terrain rasterizes the point cloud to produce a raster (here a dtm)

Then, is there anyway to save LAS catalog as a single las file so that I can save it and retrieve whenever I need.

A LAScatalog is an object pointing to multiple LAS files. You could merge all your LAS files into one huge file but what would be the purpose?

[taeyoonlee87]

I see now,

I really appreciate your response.