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Abstract
This study focuses on the horizontal and vertical accuracy of point-clouds based on unmanned aerial vehicle (UAV) imagery. The DJI Phantom 3 Professional unmanned aerial vehicle and Agisoft PhotoScan Professional software were used for the evaluation. Three test sites with differing conditions (canopy openness, slope, terrain complexity, etc.) were used for comparison. The accuracy evaluation was aimed on positions of points placed on the ground. This is often disregarded under forest conditions as it is not possible to photogrammetrically reconstruct terrain that is covered by a fully-closed forest canopy. Therefore, such a measurement can only be conducted when there are gaps in the canopy or under leaf-off conditions in the case of deciduous forests. The reported sub-decimetre horizontal accuracy and vertical accuracy lower than 20 cm have proven that the method is applicable for survey, inventory, and various other tasks in forests. An analysis of ground control point (GCP) quantity and configuration showed that the quantity had only a minor effect on the accuracy in cases of plots with ~1-hectare area when using the aforementioned software. Therefore, methods increasing quality (precision, accuracy) of GCP positions should be preferred over the increase of quantity alone.
Highlights
Unmanned aerial vehicles (UAVs), known as unmanned aircraft systems (UAS) and remotely-piloted aircraft systems (RPAS, a subset of UAS excluding fully-autonomous systems), are a very promising technology for forestry practice
The main area of small UAV utilisation can be in determining highly-accurate digital elevation models (DEMs) in cases where the small area is not of concern and the required high density of points would be very time-consuming with the application of conventional surveying
This study presents the results of terrain point cloud accuracy evaluations under forest conditions with differing degrees of tree cover
Summary
Unmanned aerial vehicles (UAVs), known as unmanned aircraft systems (UAS) and remotely-piloted aircraft systems (RPAS, a subset of UAS excluding fully-autonomous systems), are a very promising technology for forestry practice. A review study of Torresan et al [1] focused on the use of UAVs within the European region in the field of forestry, reporting that RGB (red, green, blue) imaging is the most adopted technology used in combination with UAVs. The forest inventory and dendrometric parameters are primary research objects. In forestry applications, researchers are focusing on the estimation of forest inventory and crown parameters [2,3,4,5,6,7,8,9,10,11], real-time and post-forest fire monitoring and detection [12,13,14], health status and disease monitoring [15,16,17], individual trees and species detection [18,19,20], or surveying the current state of soil displacement [21]. Many other applications of UAVs have been described apart from forestry, e.g., agriculture [22,23], damage detection of buildings [24,25], landslide research [26], research of Forests 2017, 8, 151; doi:10.3390/f8050151 www.mdpi.com/journal/forests
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