R: LAScatalog drivers

lidR-LAScatalog-drivers {lidR} | R Documentation

## LAScatalog drivers

### Description

This document explains how objects are written on disk when processing a LAScatalog. As mentioned in LAScatalog-class, users can set a templated filename to store the outputs on disk instead of in R memory. By defaut `LAS` objects are stored in .las files with writeLAS, `Raster*` objects are stored in .tif files with writeRaster, `Spatial*` objects are stored in .shp files with st_write, `data.frame` objects are stored in .csv files with fwrite, and other objects are not supported. However, users can modify all these default settings and even add new drivers. This manual page explain how. One may also refer to some unofficial documentation here or here.

### Generic form of a driver

A driver is stored in the `@output_options` slot of a LAScatalog. It is a list that contains:

write

  A function that receives an object and a path, and writes the object into a file using the path. The function can also have extra options.

extension

  A string that gives the file extension.

object

  A string that gives the name of the argument used to pass the object to write in the function used to write the object.

path

  A string that gives the name of the argument used to pass the path of the file to write in the function used to write the object.

param

  A labelled list of extra parameters for the function used to write the object

For example, the driver to write a `Raster*` is

    list(
     write = raster::writeRaster,
     extension = ".tif",
     object = "x",
     path = "filename",
     param = list(format = "GTiff"))

And the driver to write a `LAS` is

    list(
     write = lidR::writeLAS,
     extension = ".las",
     object = "las",
     path = "file",
     param = list())

### Modify a driver (1/2)

Users can modify the drivers to write different file types than the default. For example, to write in GeoPackage instead of shapefile, one must change the `Spatial` driver:

    ctg@output_options$drivers$Spatial$extension <- ".gpkg"

To write in .grd files instead of .tif files one must change the `Raster` driver:

    ctg@output_options$drivers$Raster$extension <- ".grd"
    ctg@output_options$drivers$Raster$param$format <- "raster"

To write in .laz files instead of .las files one must change the `LAS` driver:

    ctg@output_options$drivers$LAS$extension <- ".laz"

### Add a new driver

The drivers allow `LAS`, `Spatial*`, `Raster*` and `data.frame` objects to be written. When using the engine (catalog_apply) to build new tools, users may need to be able to write other objects such as a `list`. To do that users need to add a `list` element into the `output_options`:

    ctg@output_options$drivers$list = list(
     write = base::saveRDS,
     object = "object",
     path = "file",
     extension = ".rds",
     param = list(compress = TRUE))

The `LAScatalog` now has a new driver capable of writing a `list`.

### Modify a driver (2/2)

It is also possible to completely overwrite an existing driver. By default `SpatialPointsDataFrame` objects are written into ESRI shapefiles with st_write. `writeOGR` can write into other file types, such as GeoPackage or GeoJSON and even as SQLlite database objects. But it cannot add data into an existing SQLlite database. Let's create our own driver for a `SpatialPointsDataFrame`. First we need a function able to write and append a `SpatialPointsDataFrame` into a SQLlite database from the object and the path.

    dbWrite_SpatialPointsDataFrame = function(x, path, name)
    {
     x <- as.data.frame(x)
     con <- RSQLite::dbConnect(RSQLite::SQLite(), path)
     RSQLite::dbWriteTable(con, name, x, append = TRUE)
     RSQLite::dbDisconnect(con)
    }

Then we create the driver. User-defined drivers supersede default drivers:

    ctg@output_options$drivers$SpatialPointsDataFrame = list(
     write = dbWrite_SpatialPointsDataFrame,
     extension = ".sqlite",
     object = "x",
     path = "path",
     param = list(name = "layername"))

Then to be sure that we do not write several .sqlite files, we don't use templated filename.

    opt_output_files(ctg) <- paste0(tempdir(), "/mysqlitefile")

And all the `SpatialPointsDataFrame` will be appended in a single database.

[Package lidR version 3.2.2 Index]