Lidar

Day 1 of 30DayMapChallenge. Playing with points
R
spatial
datavisualization
30DayMapChallenge
Author

Michaël

Published

2024-11-01

Modified

2024-11-22

Aiguille du Midi

Aiguille du Midi – CC-BY-SA by Ivan Borisov

Day 1 of 30DayMapChallenge: « Points » (previously).

IGN is scanning France using LIDAR. More than half the country is available for downloading currently. Let’s play with points around the Aiguille du Midi

library(readr)
library(dplyr)
library(purrr)
library(fs)
library(lidR)

dir_create("tiles")

Selecting 4 squares on the selection map allows to generate a catalog of 1✕1 km tiles to download.

read_lines("liste_dalle.txt") |> 
  tibble(url = _) |> 
  mutate(file = path("tiles", path_file(url))) |> 
  pwalk(\(url, file) download.file(url, file))

Read and manipulate the LAZ files with the help of the {lidR} package. We only keep a region of interest 500 m around the peak.

ctg <- readLAScatalog("tiles")
roi <- clip_circle(ctg, x = 1001400, y = 6538400, radius = 500)

The point cloud has already been classified so we can extract only the ground (although at this altitude there is no vegetation).

ground <- filter_ground(roi)
plot(ground)

We display 38 millions points (50 points/m²)!

A 3d rendering of the Aiguille du Midi from LIDAR data
Figure 1: Aiguille du Midi in RGL

And then we can create a digital terrain model and a hillshade map…

dtm <- rasterize_terrain(roi, 2, tin(), pkg = "terra")
dtm_prod <- terra::terrain(dtm, v = c("slope", "aspect"), unit = "radians")
dtm_hillshade <- terra::shade(slope = dtm_prod$slope, aspect = dtm_prod$aspect)
plot(dtm_hillshade, col = gray(0:50/50), legend = FALSE)
A grayscale rendering of digital terrain model around Aiguille du Midi
Figure 2: DTM of Aiguille du Midi