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Here, we used ray tracing on a high-resolution voxel grid to quantify sampling variation in a temperate mountain forest in the southwest Czech Republic. We decoupled the impact of pulse density and scan-angle range on the likelihood of generating a return using spatially and temporally coincident TLS data. We show three ways that a return can fail to be generated in the presence of vegetation: first, voxels could be searched without producing a return, even when vegetation is present; second, voxels could be shadowed (occluded) by other material in the beam path, preventing a pulse from searching a given voxel; and third, some voxels were unsearched because no pulse was fired in that direction. We found that all three types existed, and that the proportion of each of them varied with pulse density and scan-angle range throughout the canopy height profile. Across the entire data set, 98.1% of voxels known to contain vegetation from a combination of coincident drone lidar and TLS data were searched by high-density drone lidar, and 81.8% of voxels that were occupied by vegetation generated at least one return. By decoupling the impacts of pulse density and scan angle range, we found that sampling completeness was more sensitive to pulse density than to scan-angle range. There are important differences in the causes of sampling variation that change with pulse density, scan-angle range, and canopy height. Our findings demonstrate the value of ray tracing to quantifying sampling completeness in drone lidar.<\/jats:p>","DOI":"10.3390\/rs16152774","type":"journal-article","created":{"date-parts":[[2024,7,29]],"date-time":"2024-07-29T16:37:17Z","timestamp":1722271037000},"page":"2774","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Near-Complete Sampling of Forest Structure from High-Density Drone Lidar Demonstrated by Ray Tracing"],"prefix":"10.3390","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2975-1679","authenticated-orcid":false,"given":"Dafeng","family":"Zhang","sequence":"first","affiliation":[{"name":"Department of Ecology, Evolution and Organismal Biology, Brown University, Providence, RI 02912, USA"},{"name":"Institute at Brown for Environment and Society, Brown University, Providence, RI 02912, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3848-2119","authenticated-orcid":false,"given":"Kamil","family":"Kr\u00e1l","sequence":"additional","affiliation":[{"name":"Department of Forest Ecology, The Silva Tarouca Research Institute, 60200 Brno, Czech Republic"}]},{"given":"Martin","family":"Kr\u016f\u010dek","sequence":"additional","affiliation":[{"name":"Department of Forest Ecology, The Silva Tarouca Research Institute, 60200 Brno, Czech Republic"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3464-1151","authenticated-orcid":false,"given":"K. C.","family":"Cushman","sequence":"additional","affiliation":[{"name":"Department of Ecology, Evolution and Organismal Biology, Brown University, Providence, RI 02912, USA"},{"name":"Institute at Brown for Environment and Society, Brown University, Providence, RI 02912, USA"},{"name":"Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA"}]},{"given":"James R.","family":"Kellner","sequence":"additional","affiliation":[{"name":"Department of Ecology, Evolution and Organismal Biology, Brown University, Providence, RI 02912, USA"},{"name":"Institute at Brown for Environment and Society, Brown University, Providence, RI 02912, USA"}]}],"member":"1968","published-online":{"date-parts":[[2024,7,29]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"834","DOI":"10.1126\/science.1184984","article-title":"Terrestrial Gross Carbon Dioxide Uptake: Global Distribution and Covariation with Climate","volume":"329","author":"Beer","year":"2010","journal-title":"Science"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"593","DOI":"10.1146\/annurev-ecolsys-110512-135914","article-title":"The Structure, Distribution, and Biomass of the World\u2019s Forests","volume":"44","author":"Pan","year":"2013","journal-title":"Annu. 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