Digital Surface Model Algorithm
This function is made to be used in rasterize_canopy . It implements the pit-free algorithm developed by Khosravipour et al. (2014), which is based on the computation of a set of classical triangulations at different heights (see references). The subcircle tweak replaces each point with 8 points around the original one. This allows for virtual 'emulation' of the fact that a lidar point is not a point as such, but more realistically a disc. This tweak densifies the point cloud and the resulting canopy model is smoother and contains fewer 'pits' and empty pixels.
pitfree( thresholds = c(0, 2, 5, 10, 15), max_edge = c(0, 1), subcircle = 0, highest = TRUE )
thresholds: numeric. Set of height thresholds according to the Khosravipour et al. (2014) algorithm description (see references)max_edge: numeric. Maximum edge length of a triangle in the Delaunay triangulation. If a triangle has an edge length greater than this value it will be removed. The first number is the value for the classical triangulation (threshold = 0, see also dsmtin ), the second number is the value for the pit-free algorithm (for thresholds > 0). If max_edge = 0 no trimming is done (see examples).subcircle: numeric. radius of the circles. To obtain fewer empty pixels the algorithm can replace each return with a circle composed of 8 points (see details).highest: bool. By default it keeps only the highest point per pixel before to triangulate to decrease computation time. If highest = FALSE all first returns are used.LASfile <- system.file("extdata", "MixedConifer.laz", package="lidR") poi = "-drop_z_below 0 -inside 481280 3812940 481330 3812990" las <- readLAS(LASfile, filter = poi) col <- height.colors(50) # Basic triangulation and rasterization of first returns chm <- rasterize_canopy(las, res = 0.5, dsmtin()) plot(chm, col = col) # Khosravipour et al. pitfree algorithm chm <- rasterize_canopy(las, res = 0.5, pitfree(c(0,2,5,10,15), c(0, 1.5))) plot(chm, col = col) ## Not run: # Potentially complex concave subset of point cloud x = c(481340, 481340, 481280, 481300, 481280, 481340) y = c(3812940, 3813000, 3813000, 3812960, 3812940, 3812940) las2 = clip_polygon(las,x,y) plot(las2) # Because the TIN interpolation is done within the convex hull of the point cloud # dummy pixels are interpolated that are correct according to the interpolation # method used, but meaningless in our CHM chm <- rasterize_canopy(las2, res = 0.5, pitfree(max_edge = c(0, 1.5))) plot(chm, col = col) chm = rasterize_canopy(las2, res = 0.5, pitfree(max_edge = c(3, 1.5))) plot(chm, col = col) ## End(Not run)
Khosravipour, A., Skidmore, A. K., Isenburg, M., Wang, T., & Hussin, Y. A. (2014). Generating pit-free canopy height models from airborne lidar. Photogrammetric Engineering & Remote Sensing, 80(9), 863-872.
Other digital surface model algorithms: dsm_point2raster, dsm_tin