= Creating a DEM from a LIDAR point cloud = One of the most common uses of LiDAR data is to generate a high resolution Digital Elevation Model (DEM) raster. This page explains the steps used by NERC-ARF to generate DEMs from the point clouds provided as part of the standard delivery. == LiDAR only DEM == For LiDAR deliveries from 2016 onwards the LiDAR DEM is generated with the NERC-ARF DEM scripts (https://github.com/pmlrsg/arsf_dem_scripts) using a command similar to: {{{ las_to_dsm.py --method points2grid --projection UKBNG -r 2 \ -o dem/RG15_18-2015_290a-lidar-bng.dem \ flightlines/las1.2/*.LAS }}} Where the projection is given in the readme file. This command will merge all LAS files together using [http://www.cs.unc.edu/~isenburg/lastools/download/lasmerge_README.txt `lasmerge`], dropping any points with class 7 (noise). To generate to DEM the [http://www.opentopography.org/otsoftware/points2grid points2grid] program is called as part of the script with the following options: {{{ points2grid --exclude_class 7 --output_file_name /tmp/RG15_18-2015_290a-lidar-bng \ --search_radius 2 --output_format arc --resolution 2 \ --idw --fil_window_size 7 \ --first_return_only }}} Points to grid uses inverse distance weighted interpolation to generate the DEM, more details about the method used are available from http://www.opentopography.org/otsoftware/points2grid. As only first returns are considered the resulting raster is a Digital Surface Model (DSM), representing the top of canopy and buildings. To generate a LiDAR only DEM using the same method as deliveries prior to 2016 use the flag `--method GRASS`, this will take the average of all non-noise first return points within each grid cell. == Patched DEM for use in APL == For processing hyperspectral data in APL a second DEM is supplied with delivered data, reprojected to WGS84 Lat/Long and patched with another DEM (normally ASTER) to extend coverage and fill in any gaps. For deliveries after 2016 a command similar to the one below is used to generate a patched LiDAR and ASTER DEM: {{{ create_dem_from_lidar.py --in_projection UKBNG --out_projection WGS84LL \ --aster --gridded \ -o dem/RG15_18-2015_290a-lidar_ASTER-wgs84_latlong.dem \ RG15_18-2015_290a-lidar-bng.dem }}} For deliveries before 2016 the command was similar but a directory containing LAS files is passed in and the `--las` flag is used to indicate the LiDAR data should be gridded before patching. For more details on this command, and usage outside the NERC-ARF-DAN systems see the [https://github.com/pmlrsg/arsf_dem_scripts/blob/master/doc/source/tutorial_lidar.md#create-a-lidar--aster-dsm-for-use-in-apl tutorial].