lidRalignment

license
license

lidRalignment enables automatic alignment of forest plot point clouds from different sources, such as ALS with TLS or MLS, or TLS with MLS, or ALS with ALS in a unified pipeline. The package is robust to large alignment differences (e.g., 180 degrees of misalignment), noise, and leaf-on/leaf-off conditions. A full description of the internal pipeline details is available 📖 here.

Features

  • Aligns forest plots from any combination of ALS, ULS, TLS, and MLS point clouds
  • Robust to large alignment differences (e.g., 180 degrees of misalignment)
  • Supports noisy and poor-quality data
  • Uses the same pipeline for all cases
  • Fast and efficient: alignment takes only a few seconds. Most of the computation time is spent reading the LAS or LAZ files

Installation

You need the latest version of lidR and lasR:

install.packages(c('lidR', 'lasR', 'lidRalignment'), repos = 'https://r-lidar.r-universe.dev')

Tutorial

The alignment pipeline consists of four stages, progressing from raw to extra fine:

  1. Raw: Roughly aligns the plot centers.
  2. Coarse: Uses the DTM and CHM to align the point clouds at low resolution. This stage can handle highly misaligned scenes.
  3. Fine: Applies an iterative closest point (ICP) approach to finely align the CHM and DTM.
  4. Extra Fine: Used only when aligning two ground-based point clouds. It extracts trees and aligns them to achieve centimeter-level accuracy.

A full description of the internal pipeline details is availailable 📖 here.

library(lidR)
library(lidRalignment)

fref = "als_file.las"
fmov = "mls_file.las"

# Setup the pipeline. It is important to tell the object
# what we are aligning in order to perform or not
# the last extra fine alignment
alignment = AlignmentScene$new(fref, fmov)
alignment$set_ref_is_ground_based(FALSE)
alignment$set_mov_is_ground_based(TRUE)

# Run the alignment pipeline
alignment$align()

# Visualize the different level of alignment
alignment$plot("raw")
alignment$plot("coarse")
alignment$plot("fine")
alignment$plot("extra", compare_to = "fine")

# Get the final transformation matrix to register the entire point cloud.
M = alignment$get_registration_matrix()

crs = sf::st_crs(readLASheader(fref))
ofile = transform_las(fmov, M, crs)
Rather than running the full alignment pipeline, it is possible to run it step by step (click to show). 
alignment = AlignmentScene$new(fref, fmov)
alignment$set_ref_is_ground_based(FALSE)
alignment$set_mov_is_ground_based(TRUE)

alignment$prepare()
alignment$plot("raw")

alignment$coarse_align()
alignment$plot("coarse")

alignment$fine_align()
alignment$plot("fine")

alignment$extra_fine_align()
alignment$plot("extra", compare_to = "fine")

lidRalignment has been sponsored by the University of Sherbrooke

Description