prepare_tile: Import and prepare a LAS tile for further processing
In mbedward/CERMBlidar: A collection of functions to process aerial LIDAR data

prepare_tile

R Documentation

Import and prepare a LAS tile for further processing

Description

This function imports data from a LAS file and prepares it for further processing. Point heights are normalized to relative height above ground, either by fitting a surface model to ground points (plus water points by default) or by taking ground elevations from a provided raster DEM. Height normalization can be disabled if desired. Flight lines are identified based on GPS times for points and, optionally, any flight lines with less than a threshold number of points are discarded. The flightlineID column is added to the LAS header so that it will persist if the object is written to file.

Usage

prepare_tile(
  path,
  normalize.heights = FALSE,
  treat.as.ground = c(2, 9),
  drop.negative = TRUE,
  drop.negative.threshold = 0,
  drop.missing.coords = TRUE,
  fields = NULL,
  classes = NULL,
  min.points = 1000,
  flight.gap = 60,
  unzip.dir = NULL
)

Arguments

`path`	Path to the LAS or LAZ format file to process. If the file extension is '.zip' it is assumed to be a compressed LAS file that will be unzipped before processing (see the `unzip.dir` parameter below). A compressed file should contain only one LAS file (identified by having a 'las' or 'LAS' file extension) although it can also contain other files (e.g. HTML or XML documents).
`normalize.heights`	Whether, and how, to normalize point heights to ground level. Can be one of the following: A logical value If `TRUE`, relative ground level is estimated from an elevation surface fitted by triangulation to ground points in the LAS file. This is equivalent to calling the `normalize` function directly with the argument `algorithm = tin()`. If `FALSE`, point heights will not be normalized. An algorithm name as a character string Point heights will be normalized using the specified algorithm. Must be one of: `'tin', 'knnidw', 'kriging'` which correspond to the algorithm functions provided by the `lidR` package. A raster layer If a raster layer (a terra package `SpatRast` object or a raster package `RasterLayer` object) is provided, the cell values will be used as ground elevation to normalize point heights. The layer should have the same or greater extent as the LAS file. A raster filename Any character string that does not match one of the supported algorithm names will be treated as the path to a raster file. Supported formats are GeoTIFF ('.tif') and ESRI ASCII ('.asc'). If the file extension is '.zip' it is assumed to be a compressed file that will be unzipped before processing (see the `unzip.dir` parameter below). A compressed file should contain only one raster file (identified by having a '.tif' or '.asc' file extension) although it can also contain other files (e.g. HTML or XML documents). NULL Same as `FALSE`, ie. point heights will not be normalized. The default value is `FALSE` to leave point heights unchanged. If point heights are normalized, the original values are copied to a new data table column `'Zref'`, but this is not added to the LAS header by default so if the object is written to file the original heights will be lost.
`treat.as.ground`	Point classes to treat as ground points when normalizing point heights by interpolation using one of the lidR package algorithms (`'tin', 'knnidw', 'kriging'`). If point heights are being normalized from a raster DEM, any classes specified by this argument will have their normalized heights set to zero. The default value `c(2,9)` specifies ground and water classes.
`drop.negative`	If `TRUE` AND point heights are being normalized, any points whose heights are more than `drop.negative.threshold` below ground level will be discarded.
`drop.negative.threshold`	If `drop.negative` is `TRUE` AND point heights are being normalized, any heights more than this threshold value below ground level will be discarded. Default value is zero.
`drop.missing.coords`	If `TRUE` (default), discard any points that have missing values for the X, Y or Z coordinates. If `FALSE`, retain the point but issue a warning message.
`fields`	Either `NULL` (default) to include all data fields, or a character string containing single-letter abbreviations for selected fields. See `readLAS` for details of the available, single-letter field abbreviations.
`classes`	Point classes to include or exclude. The default (`NULL`) is to include classes 2 (ground), 3-5 (vegetation), 6 (buildings) and 9 (water). Specify a subset of classes as a vector of integers, e.g. `classes = 2:6` would include ground (2), vegetation (3, 4, 5) and building (6) points. Negative values can be used to exclude selected classes, e.g. `classes = -6` would include all classes except those classified as building points. Note that overlap points (class 12) are always excluded unless an explicit integer `classes` vector is provided that includes the value 12.
`min.points`	The minimum number of points in a flight line for it to be retained in the imported tile. The default value (1000) is intended to exclude flight lines that only appear at the margins of the tile.
`flight.gap`	The minimum time gap (seconds) to use when assigning points to flight lines.
`unzip.dir`	The directory in which to uncompress a compressed LAS file (identified by a '.zip' extension). If `NULL` (default) a temporary directory will be used. After processing, the uncompressed file will be deleted.