Ground Surveys Versus UAV Photography: The Comparison of Two Tree Crown Mapping Techniques

Abstract

The aim of this investigation was to compare tree crown area obtained by the classical ground survey methods (GS) and by the unmanned aerial vehicle (UAV) technology. The study was carried out in Prioksko-Terrasny Biosphere Natural Reserve (Moscow region, Russia) on permanent sampling plot of 1 ha. All trees with DBH at least of 5 cm (779 ind.) were mapped and measured during ground survey in 2016. For each alive tree the radii of crown horizontal projection in four cardinal directions were measured, and the total area of all projections was calculated. The aerial photography by a quadcopter DJI Phantom 4 was conducted in 2017 on August, 2 from altitude of 58 m and on October, 12 (68 m). Obtained two orthophotomaps were used for manual vectorization of visible crown projections by means of QGIS and the total area of all projections was calculated. The total area of tree crown projections and the number of trees obtained by GS and UAV methods were matched well. Coniferous trees clearly differed from deciduous trees in aerial photography, also pine was different from spruce. Broadleaf trees (lime and oak) were difficult to detect. For pine the individual crown area obtained by UAV methods was significantly higher and slightly lower for birch and oak. There were no differences for other species. Our results confirmed that UAV technology could be used to obtaining of spatial information on forest characteristics, but tree species identification is still challenging.