Résumé
Terrestrial laser scanning (TLS) was conducted in October 2021 by G. Vincent and J-L Smock (IRD) using a Riegl VZ-400. Scans were acquired at locations on a 10 m Cartesian grid. Capturing a complete sample of the scene at each location requires two scans (upright and tilted), owing to a 100° field of view. 249 scans in total were collected. The angular resolution between sequentially fired pulses was 0.04°, resulting in approximately 22.4 million emitted pulses per scan (i.e., 5.42 billion per ha). Up to four targets can be resolved per pulse, with a nominal ranging accuracy of 5 mm. The laser itself is characterised by a beam divergence of 0.35 mrad, and the diameter of the beam at emission is 7 mm (e.g., the diameter of the beam at a range of 50 m, would be 21 mm). The pulse repetition rate was 300 kHz, therefore, each scan took approximately 3 minutes to complete. To generate a plot-level point cloud from individual scans, all scans were co-registered and projected to a standard geographical coordinate system (epsg 2972). To this end, 5 identifiable targets with known X,Y,Z coordinates (plot corners + plot centre) were positioned using a total station.
Once co-registered using RiScanPro software, individual scans were exported in las extrabyte format (including deviation) using LidarFomartConverter v.1.2.(AMAP code based on RivLib). Reflectance range was set to -30dB to +5dB and stored in the Intensity field as a long integer. Echoes outside this reflectance range were discarded. Coordinate precisions were set to 0.001 m. The full point cloud (all 249 scans) was then cropped to 1.4 ha plot (+10m buffer around 100x100m plot), and tiled per 20 x 20m (no buffer). Cropping and tiling were done with LAStools software. Scan position number was stored as flight line to allow selection of scans if needed. In particular, distant scans which contribute little more than noise could be deleted. LiDAR data were acquired without the “reflectance optimization filter”. In order to keep only returns with reflectance above -20dB (equivalent to setting reflectance optimization filter) all returns with Intensity below 18724 were dropped.